Insulation Debate: Cathedral Ceiling
Hi All:
Fascinated by the debate about insulation I see here from time to time. So I thought I’d let the debaters take a swing at helping me choose the insulating methods for my planned addition.
Here are the details:
Climate: Toronto, Ontario. Cold winters and hot, humid summers.
Existing house: we’ve done the best we can to tighten it up without tearing it down or breaking the bank. New low e/argon windows, mineral fibre batt insulation and VB in exterior walls, ~ 8″ of FG under the existing roof (1/2 of which will be covered by new roof framing). But the house is far from airtight, and has a crappy hole of a 5’10” basement, uninsulated and kept cold, where my miserable present shop is located.
HVAC: radiant floor heat to supplement rads in the existing house. Central A/C to be added, but probably only used for ~ 1 month per year (and not continuously at that).
Main roof: 4/12 shed-roof “cathedral ceiling”, 2×10 rafters 16″ O.C. with sheathing on top and VB + drywall underneath. Similar construction in the attached garage which will contain my glorious new shop (no more sore back from stooping over- no more sore head from bashing it on the basement beam- hurray!). Asphalt shingles. Planned continuous soffit vent and continuous ridge vent. No skylights. No ceiling-mounted lighting fixtures, ducting etc.- lots of other, less risky places to put that stuff without puncturing the ceiling VB.
Walls: planned poly VB, 2×6 studs sheathed w/plywood or OSB, w/1″ foam thermal break, + furring strips and Hardie lap siding. Lots of wire and fixtures but NO plumbing or ducts running in exterior wals (duh?!).
New basement will be ICFs w/foam under heated slab- that should be good enough, I hope! (to be finished out with furring strips and drywall).
Originally planned mineral fibre batt insulation (Roxul) because I’m familiar with it, can do it myself and it’s cheap (and better performance than FG)- but you guys have got me thinking…
I’ll be doing the work myself (unless it’s sprayed foam of course). Never done blown cellulose but figure if I pay someone to do the roof cavities I might be able to figure out how to do the walls (or am I nuts?). Heard (here) that some people will rent/lend you the blower if you buy their cellulose.
Questions:
1) Blown-in cellulose w/staple-up vent baffles: is this even feasible? Can you reliably blow in cellulose to the right density in this kind of geometry before the drywall goes up? Wouldn’t the VB sag like hell and make drywall installation a PITA?
b) Or two layers of mineral fibre batt insulation (Roxul) with a 1″ venting gap for the ceiling? Yeah, I know how bad batt insulation is supposed to be, but this is pretty simple geometry and there’s no reason for big insulation-defeating air gaps. Won’t it do the job?
Can’t afford spray foam to R-30 thickness (plus I don’t like isocyanates much). My fear about blowing the rafter bays right full of cellulose without venting is the durability of my asphalt shingles if I don’t provide sheathing cooling by means of under-roof ventilation. But I’ve also heard enough horror stories about moisture problems in cathedral ceilings and want to avoid same as much as possible.
2) Wall cavity insulation: same questions, plus:
a) if I get over my isocyanate fear and spring for sprayed foam instead of cellulose or the mineral fibre batts, can I get enough thermal benefit to eliminate the need for the thermal break foam layer and all that PITA furring to support the Hardie siding? Or would staggered 2x4s on 2×6 plates be the best option?
b) which is better/cheaper/less work: staggered 2x4s on 2×6 plates with cellulose insulation and no external foam for thermal break, or conventional 2×6 construction with foam insulation and no external foam thermal break?
If it were your place and you were doing the work, which would you choose?
I look forward to your informed opinions! If you need any more details, let me know and I’ll fill you in. Thanks in advance…
Replies
What, did I scare you away with too long a post? Sorry, guys! I'll be brief:
If you use mineral fibre batt insulation in simple geometry (ie. a wall or ceiling with no boxes, wires or plumbing), does it STILL suck that badly relative to cellulose?
Is spray foam with regular 2x6 studs, or dense cellulose with staggered 2x4 studs on 2x6 plates, so much better because of its air sealing that I can do away with a sheet foam thermal break between my plywood wall sheathing and my Hardie siding? Hate the idea of those furring strips on every stud...
Any worries about using cellulose in a (half) cathedral ceiling? Does the advantage go away if I want to vent the underside of the sheathing to keep my shingles cooler?
Geez, why couldn't I have been that brief in the first place?
If you need more details, they're in the previous post.
Thanks in advance!
Cellulose Fibre has the highest RSI value per unit thickness of all the loose fill insulations. Cellulose is 3.4 to the inch. Glass fibre is 3.0 to the inch.
heh heh
molten,
'bout the best thing to do right now is search thru the old posts here on BT under venting and cathedral to study the various opinions regarding such.
There are just a handful of regular pros here with any real knowledge from practical experience on this subject that can give you qualified advice.
This topic is one of continual debate and small wars have arisen offending and defending various viewpoints in the past.
A certain air of civility has prevailed of late regarding the same and perhaps that is why so far there has been silence regarding your quest.
OK enough public relation crap.
I'll give you my opinion for what it's worth as just an uncertified non-experienced homeowner. IMHO...
You want to do as much yourself as possible? It's an addition and not a whole house under construction? I'd forget staple-up chutes, sprays, and shooters for your insulation in the cathedral roof.
Sheets of foamboard insulation get the rap of being expensive, but what you can get is a durable, airtight and longlasting insulation you can work with easily yourself at your pace once you get the process down with little mess. It's your place and not commercial or rental the extra effort is negligible..
1 1/2 inch airspace beneath your sheathing from soffit to ridgevent with spacers at sides of rafters. Cut foam tight to fit. Some guys use canned foam for edges to ease the need for cutting exactness.
1/2inch foilback celotex foamboard across the rafters beneath the drywall with taped seams for a vaporbarrier.
"sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",
Jack Williamson, The Legion of Time
rez:
An air of civility is prevailing? Geez! There goes my hope of stirring the hornet's nest again!
I've done the search but the debate is so lively there are a lot of posts to dig through- and most only deal partially with my particulars.
It's my own place, I've no intention to move or rent it, and I want to do it right- but I'm also anxious to avoid those nasty furring strips under my siding, and I don't want to blow the budget completely with spray foam. Rigid foam board's pretty expensive per unit of R relative to mineral fibre- if you can trust its R rating. And I need R30 in my roof- that's a lot, even more in Canadian dollars...
Thanks for your advice- I do appreciate every reply, and I also try to help out where my knowledge is sufficient to be a help rather than a hindrance.
So you'd suggest an interior thermal break on the rafters, using the 1/2" celotex for that purpose and as a vapour barrier? Drywall goes up ok over that stuff?
(as to your signature, I like Tom Waits' take on that one: "I'd rather have a bottle in front of me than a frontal lobotomy!")
I'm reading and wondering? the ceiling is? 10"=9.5"? and your looking to get ?R-30
The walls are???? and you want to get ?R
When working with the Iso you can get real good cutting it, just like sheet rock score and snap. When it gets to thicker then 2" it needs to be scored, and sawn through to get a nice straight cut. You can use thinner Iso foil faced on the top layer then use paper faced for the fill which is cheaper. Roofing supply houses always carry the Iso in .5" up to 4". If you go that route you stagger the joints. It does a nice job and lets you space the vent easily. It is a little slower but it is your time so for the quality and R it shouldn't be a problem.
Clay:
The ceiling's 2x10 which is 9 1/4". With a 1" air gap, I'd be just above R-30 (the insulation, not the ceiling assembly) with mineral fibre batts. There'd be no convenient way for me to combine normal thicknesses of batts to make up the space- 5.5" + 3.5" = no venting...So I'd be looking at a 5.5" layer of mineral fibre plus a 2.5" layer of something else...Or 2x12s. My back is hurting already, thinking about lugging those 2x12s up there...Or TJIs, I guess...
The walls will have 2x6 plates (whether I go with 2x6 studs or staggered 2x4s is up for debate yet). I'll have the usual wire and boxes so I'm still leaning toward cellulose, but 5.5" of mineral fibre gives you R21.5 if I remember correctly (neglecting the air gaps etc.). I know that 5.5" of fibreglass with 2x6 studs and no external thermal break is good enough to meet code in our area, and the mineral fibre is a bit better than that.
By iso, do you mean polyisocyanurate, the pinkish stuff that's kind of crumbly and dents easily if you press it with your thumb? Since you're familiar with it, any idea how much R per inch you get, and roughly how much it goes for per sq. ft. per inch of thickness?
The Pink stuff is extruded poly styrene, better r then the white stuff styrene. The Isocyenurate sp?has been down ratted with the new E friendly process to a little over R-6 per inch. I would look to using a 2" iso top layer,tack small furring strip/spacers to get your vent space 1.5 or 2" and then use fiberglass bats R-19. It's my theory that the major defect comes in the installation of insulation so pay close attention to detail and pack fluff and fill the cavities. Is there a better way? Yes but you can do all this yourself and make it perfect. Which in many cases beats a better theory with a pourer application.
What's the venting? cause this will require a ridge vent.
The Iso is available at HD for about 20$ It's foil faced on both sides, the same thing with a felt face from the roofing suppler is 11.50. I can get the foil faced cheaper thru the supply yd but they don't stock it and I have to buy it by the bunk. But i kind of like the foil for your application to get the reflective benefit.
Plus what size of the roof? I'm thinking you said it was a small addition? If that's the case then a little extra expence on matterials will be offset by your labor if you are doing it all your self.
Clay:
I'm going to look into the polyiso stuff- never seen it in HD up here, but maybe I haven't been looking. If I stick with the 2x10s for rafters I like your option pretty well, except I'll stick with the mineral fibre batts instead of the fibreglass. Roxul's good stuff and readily available up here- made in Milton, Ontario.
The rafter span's pretty short- 12' 6" horizontal span from centre of bearing to centre of bearing- but the rafter extends beyond the wall at the peak to support some in-fill framing between the old house and the addition, so I need 18 footers. Sticking with 2x8s, which are more than good enough structurally, is appealing for that reason. The option of putting the rigid foam under the rafters and then furring to apply the drywall (Mike's option) is looking pretty good right now, even though installing the furring will be some extra work.
Plus your going to use 5/8s rock so nail pops arn't a problem. I hope
'Course now's the time to think about sweetening up the ceiling with some t&g."sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
...an excellent suggestion! Would look WAY better. I can probably pay for the extra materials cost with the money I'll save by not getting someone to do the taping and mudding of the drywall for me. I do such a crap job of taping and mudding drywall that it's one thing I'll be paying someone to do for me! (that's my excuse and I'm sticking to it- and if anyone tells my wife that it's just because I'm too damned lazy to do it on a job this big, I'll smack 'em!)
Also if your looking to keep the rafters as skinny as possible you could use all Iso say 4" and a 2" for R-36. Figure out the most economical for least waste, you may need to cut it in the 4' runs instead of 8's and stagger joints.
Another favorite of mine that he said was "Reality is for those people who can't face drugs."
http://forums.taunton.com/tp-breaktime/messages?msg=26654.31 for the Mooney wall.
Plusses for the 1/2 under the drywall for the added R on the rafters and the vapor barrier issue. Only question was the possibility of screw pops later on which I haven't seen in 3 years since I did it with thicker foamboard screwing directly thru drywall and foamboard into the rafters.
Some guys nail furring strips to the rafters and then screw to that. My concern was maximizing headspace in a storey and and half and was counting those quarter inches:o)
Seems like maxxing insulation techniques is full of grey areas with overated R values that change and vent/no vent issues. Hard to find clear reliable uncontested data to rely on.
Think half of it is just staying out of a bad situation to get you by ok. What you questioned earlier in another post about sagging the v-barrier when pumping insulation was addressed someplace here this week by applying the drywall in increments and pumping the bays full then adding another run of drywall and repeating. Sounded sensible.
"sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
I question the wisdom of a foam thermal break on the outside in a mostly heating climate. Plus tricky nailing for siding and trim.
When considering staggered studs for a thermal break, also consider the option of a continuous foamboard layer under the drywall instead.
csnow:
Is the concern about external foam on the wall sheathing related to moving the condensation point somewhere it shouldn't be? Or is it with having something that walks and talks like a vapour barrier on the exterior of the wall? Guess it would do that for sure, eh? Not good...not nearly as breathable as Tyvek. Sh*t! That was a benefit of using the foam- no need for Tyvek!
If I have to go with internal foam plus furring (I don't trust my installation skill to avoid screw pops in the drywall over foam without furring), I think I might give the Mooney wall a try, provided Mike tells me what this insul-mesh stuff is. Trouble is, inspectors and plan reviewers around here are used to seeing plain 2x6 exterior stud walls, and they already give DIYs like me a hard enough time without me having to teach them something new. But I definitely want to get rid of that thermal bridge one way or another- seems just plain stupid to go to a 2x6 wall to fit more insulation if your studs still end up acting as a fin to conduct heat out of your house.
But relative to staggered 2x4 studs on 2x6 plates, with thermal-bridging 2x6s only at openings like windows and doors (for ease of installation and trimming more than anything else), is the Mooney wall all that much better if you blow in cellulose in either case? I guess one downside would be all those extra studs- you'd need to have minimum 24" O.C. on both the interior and exterior, which means a stud every 12" instead of every 16"...Or would you need the interior studs on 16" centres to support this insul-mesh so that the cellulose doesn't cause it to bulge out and ruin your drywall application?
This is exactly the kind of discussion I wanted- it helps me understand what's involved, and what the pros and cons are of each choice so I can make the best overall decision for my specifics. Thanks again csnow!
My concern with the exterior foam is that it would be a VB on the wrong side in your climate.
I designed a large home in New England with 2 inch of foam board under the drywall on all exterior walls and ceilings. The idea was that the foam layer would be airtight, similar conceptually to the "airtight drywall approach", but with a thermal break. Thermal performance is amazing. Despite its size and 10 ft ceilings it sips fuel. My old 'work in progress' home at about 1/3 the size uses more fuel than the big house. Solar gain does much of the work. 7 years later, the drywall shows no pops or cracks. The rockers and finish carpenters and electricians did not seem to have much trouble with the foam layer. No complaints (that I heard about), at least...
Due to local skill limitations, it was not possible to use cellulose in the walls, so this home has FG batts. It does have about 3 feet of cells in the attic. I expect performance would have been somewhat better with cells in the walls.
<I designed a large home in New England with 2 inch of foam board under the drywall on all exterior walls and ceilings. The idea was that the foam layer would be airtight, similar conceptually to the "airtight drywall approach", but with a thermal break. Thermal performance is amazing. Despite its size and 10 ft ceilings it sips fuel.>
Despite its size and 10 ft ceilings it sips fuel.
csnow- you just plain rock man.
hat tips to you."sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
What type of foam did you use? and what was it's facing?
"What type of foam did you use? and what was it's facing?"
This particular home has XPS foam with foil facing.
In my opinion though, EPS is about as good, and cheaper.
The XPS has been found to provide less than rated R-value after it off-gasses over time, and comes in about the same as EPS. I doubt the foil facing is worth much in this application. It's really the air sealing and thermal break that provides most of the benefit in this design anyways.
If you go to a fabric store it's called Pelon. (SP?) It's used for stiffening in lapels and stuff.
It's a non woven fabric, white stiff fibers stuck together. Looks like fiberglass mat but thin.
Joe H
Wow- who'da known that you could use "interfacing" as a structural material in your house! My wife makes theatre and film costumes for a living- I'll have to ask her how much that stuff is per yd. Not the fusible stuff, mind you- doubt the glue's good enough for me to be able to iron it on to my studs!
Was a little concerned about it from a flamespread point of view, but doubt it matters behind drywall.
Thanks, JoeH for the tip. Mike- is that the stuff you meant?
yes... here's a good explantion and source..
http://www.regalind.com/Mike Smith Rhode Island : Design / Build / Repair / Restore
That's what my wife said it was.
8' rolls from Regal can be UPSed, or it comes in 4' rolls too. It's cheap from Regal, I don't know what it would cost from a fabric supply, but if you need only a small amount it might be cheaper that way.
Joe H
exactly... and some of the latex glues for gluing fabric to walls will do the trick for the glue..
or you can order evrything from Regal Industries.. or get most of it locally at an insulation jobber or supply house... ( all of the insulation contractors buy from a distributor near you )Mike Smith Rhode Island : Design / Build / Repair / Restore
walls .... 2x4 Mooney walls with dens-pak cells behind insul-mesh stapled and glued..
cathedral... 2" EPS (1lb/cf density) nailed to the bottom of the rafters and furred so you can hang your gypsum.... for thermal break plus R10.. denspak rafter hot roof if your building inspector will wllow a hot roof.. 9" x 3.5 = R31.. net cathedral section say.......R38 - R40..
furring will also reinforce the EPS so you can blow your dens-pak right thru 2" holes , then plug the holes and rock the ceiling..
any flat attic/ceiling areas blow 24" so you get a settled density of about 20" ( apx. R60)
if the BI insists on a vented roof... follow the same thing only with the propa -vents and a plywood dam at the plate and ridge
Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike: thanks for that. The Mooney wall looks attractive- I have read these threads before. A few questions:
1) It's been a while since those threads were posted, and you're a busy guy. Have you done a lot of Mooney walls since then? Have they worked out well for you?
2) Pardon my ignorance, but what's this "insul mesh"? I realize it's the structural membrane which allows you to blow in your dense-pack cellulose before you install your drywall, but what physically does it look like? The only reference I could find to "insul-mesh" is a steel wire product. Right or wrong, in our mostly heating climate I doubt I'll be able to get away without a poly vapour barrier on the interior. Would the insul-mesh go over the poly VB?
3) I'm still thinking that I want venting under my sheathing, to keep the shingles cooler and to permit a radiant barrier so I don't have to use A/C so much in the summer. Yes, I know that these are competing interests, that a radiant barrier will reflect heat back toward the sheathing, but I still figure the sheathing will run colder if I have a vent space under it than if I don't. There'll be plenty of convective driving force on a roof with that pitch for the hot air to rise and flow out the continuous ridge vent, and that's all heat that won't make it into my living space. Any reason that I should reconsider this point of view?
I like your idea of 2" of EPS under the rafters with furring to install the drywall. It'll get me down to 2x8s from 2x10s which are only that deep to fit sufficient insulation. With the claimed r21.5 for mineral fibre batts, I'd have a nice R31.5 vented ceiling with a thermal break, without having to worry about getting someone in to blow in cellulose. What would you recommend for furring- do I need to go to 2x3s?
As if you need another suggestion....
SIPs for the roof is the best detail. If the span isn't too long, you can even eliminate all the rafters and save a little money. No insulation labor. You can get them with prefinished T&G on the ceiling. No venting.
No can lights is the main drawback.
SIPs are appealing except for my need to support the infill roof framing between the existing house and the addition. There's no doubt a way to do it, but I'm sure the planning dept guys will say, "Sure- get an engineer to stamp it and we're good to go! And my buddy who does structural eng for residential is so stacked up with work that he can't even answer simple questions for me right now. SIPs are very appealing otherwise, since I'm giving up on pot lights anyway.
The process we have to go through to get permits up here forces us to make so many decisions so early- it's a pain in the behind! But I'm glad for this forum- helps me ground-truth a few of the decisions before I hand in the plans. Once they're in, I'll be more or less pinned to doing things a certain way or risk delay and further cost.
Too true about the administrative hassles with SIPs. Those hassles have cost me three months and thousands of dollars on my new house. Bleeding edge technology for sure.
Okay, I've thought about this a little more, considered everybody's opinions and this is the way I'm currently leaning:
Walls: 2x6 wall, dense-pack cellulose, 1" exterior foam, 1x3 furring and Hardi siding, or 2x6 plates, staggered 2x4 studs (except at window/door openings), and no foam. Leaning toward the former.
Reasoning: water problems in a mostly heating climate are mostly due to air leakage from warm side into wall cavity. Poly VB (properly detailed) helps reduce this and is code required in my area, and cells help even more by filling all voids. Extra cost of cellulose relative to batts is clearly worth it over the long term. Foam on the exterior will act as a VB, but so will my plywood sheathing to some degree- it's not all that vapour-permeable either, so the key is to keep moisture out of the wall cavity. Plus furring will leave an air/drainage gap between the siding and the foam, which is desirable to keep the wall assembly dry from the outside in. Plus, fewer problems for an amateur like me to install drywall, trim out windows and doors etc. than if I used the foam on the inside, and it keeps the wall studs within the heated space and puts the thermal break on the outside where it belongs in my climate.
Ceilings: 2x8 cathedral ceiling rafters, 5.5" mineral fibre batts R21.5, leaving 1.75" vent space under roof sheathing, maybe with reflective radiation barrier on top of batts (1/2" foil-faced rigid foam?), plus 2" rigid foam insulation w/taped seams installed under rafters. May need poly VB to make code inspector guys happy- if so, install to underside of rafters. Special attention to sealing at the joint between the addition wall and the peak of the ceiling. Finish out with T&G without furring. No roof penetrations except for plumbing vent stack, and that will be well flashed and sealed with canned foam from underside.
Reasoning: good R value without deeper roofing members, thermal break (wrong side, but not a big deal because of 2" of foam), vented roof = longer shingle life and happy code inspectors, and less leakage worry because there's no wires etc. so no need for extra bother/cost of cellulose. 1/2" foam as radiant barrier = slightly less A/C in summer, plus reduces air leakage around edges of batts.
Any flaws in my logic?
Look into airtight drywall approach and building gaskets. Wall assembly is going to have difficulty drying to the outside (or inside) with this design, so air sealing is even more important.
The FG in the cathedral ceiling is not going to do much if it's vented, though the interior foam would make up for a lot. Avoid pot lights.
Yup. No penetrations in the cathedral ceiling other than the one plumbing vent stack, which I'll seal like crazy.
I'm not sure I entirely buy this "drying to the inside or outside" logic, though I have been to the BuildingScience site several times and read everything I could here on the topic. Think it matters much more when you've got significantly longer A/C periods than we do up here and the VB ends up on the wrong side of the wall. Air movement and leakage (both from the interior AND exterior) is minimized in a properly-designed wall cavity, and the diffusion rate through plywood sheathing is pretty minimal. If you've got a water leak into the cavity, it's going to take a long time to dry, period. We're required to have an exterior air barrier, which eliminates the effect of the seams in the sheathing and reduces the vapour diffusion rate further- and we're code-required to have an interior VB. I doubt there'd be much drying in an exterior wall cavity whether I put the exterior foam on it or not. I'll be working as hard as I can to eliminate any sources of moisture to the wall cavity in the first place, and figure that's about the best you can do without actually punching holes in your air barrier to permit actual convective circulation of outside air in the stud bays to keep it dry (i.e defeating some of the purpose of your insulation).
I take it you agree with the other poster and me that the top layer of 1/2" foil-faced foam in the cathedral ceiling is a good idea, to reduce air circulation in the rafter bays insulated with mineral fibre batts? Figure it would act as a sort of "false sheathing" layer below the vented section. Do you think I should go to the extent of sealing the edges of the foam to the sides of the rafters with caulking or spray foam?
I've been calling local insulation contractors and none of them so far do dense-packed cellulose. Not a good sign. One guy is offering icynene foam, apparently the full 5.5" deep, for about $2 Canadian (~$1.50 US) per square foot of wall- for reference the mineral fibre is $0.50 CDN per square foot without the installation labour. The difference in installation cost assuming I do the mineral fibre myself and pay myself nothing for the bother, is about a year's present heating bills. I can't even guess what the installed cost for dense-packed cellulose is because I can't even find someone who installs it, but presume it's cheaper than 5.5" of foam. Any idea what this stuff costs? Even a rough range?
"I take it you agree with the other poster and me that the top layer of 1/2" foil-faced foam in the cathedral ceiling is a good idea, to reduce air circulation in the rafter bays insulated with mineral fibre batts? Figure it would act as a sort of "false sheathing" layer below the vented section. Do you think I should go to the extent of sealing the edges of the foam to the sides of the rafters with caulking or spray foam?"
Not sure I see the benefit of that part. Do you have to vent it? Circulating air in a cathedral invites trouble.
Here we go..."sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
I have two choices:
1) Do battle with the local building code guys, unarmed because I'm a know-nothing DIY to them.
2) Shut up and vent the ceiling.
Given those choices I'm going to vent the ceiling. The question is how best to do it. With a continuous VB and two inches of foam beneath the rafters and only one ceiling penetration, do I really have that much to worry about from venting the space between the batts and the sheathing?
Will a 1/2" foil-faced layer on top of the batts be a help or a hindrance in some way I haven't yet imagined? From a cost and effort standpoint it's not a big deal, but if it's pointless I won't do it.
You probably can't beat the Inspector unarmed, but the guy spraying the foam might help.
Ask him about spraying the roof without the vents.
Seems like a no brainer at $1.50 a foot, you will have one warm room in your house.
Joe H
Keeping the air currents from hitting your batts will give the batt insul better performance.
What are you going to use to keep the batts from pushing up into your vent space? You have to pay for the popin foam channels and about zero R factor. The 1/2 foamboard will at least give you a theoretical 2 to 3 for the money. ELBH"sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
If I go with the mineral fibre batts, spacer blocks between the 1/2" foam and the sheathing was my intention to maintain the venting space, rather than the pop-in baffles. I think the 1/2" foam's worth the extra $$.
The foam guy could say whatever he liked, but the plan review guy will do his job, point to the code book and say, "Where in here does it tell you/me that it's okay for you to not vent your roof if you use foam or cells instead of batts?". Which will be greeted by a blank stare from me- not very convincing. The code sections I've read don't give permission to avoid venting, but by no means am I a code expert! The foam guy probably breaks code all the time- they do principally renos, most of which don't bother with a permit because there's not much activity visible from the street.
As to using spray foam with the (cardboard?) vent baffles in the cath. ceiling, it's a possibility but it's also a lot of extra expense. The walls will be costly enough at $1.50/sq. ft., and there I agree that the foam's a big benefit with all the wiring, boxes etc. to act as air leakage points. If I could justify eliminating the sheet foam under the rafters or reducing that to 1" from 2" it might be worthwhile, but I still feel a need for a thermal break there. No thermal break just plain offends my commonsense.
Still looking for even a rough guesstimate at the cost of cells per sq. ft.- anybody venture even a rough guess? Should I bother to keep looking into cells vs icynene at $1.50/sq. ft US?
1) Do battle with the local building code guys, unarmed because I'm a know-nothing DIY to them.
2) Shut up and vent the ceiling.
Why not do both? Install the foil back sheathing on top of the rafters (and you get to use full depth insulation), then furr with 1x over (same direction as the rafters) and install the roof deck over that. Vent the soffit over the foilback and under the 'true' roof deck, and vent above as "normal." (You could use 2x furring to get an 1 1/2" vent space if they balk.
Ok, more work and more materials. But, you get to properly seal the insulation in--and separate it from external air for venting the roof surface. Occupational hazard of my occupation not being around (sorry Bubba)
I'm not the guy who asked the question, but I like your answer the best--elegant solution!
elegant solution
Don't know from elegant, this is Texas, we want to separate insulation from hot (very hot) roof surfaces whenever possible. Even better to have a reflective surface that only shades the "real" insulation.
And, we have our fair share of municipalities where "you gotta go by the book." Necessity and the mother of invention, as the old saw goes . . .
The "flying" roof does have a gotcha, if it ever leaks you have the devil's time figuring out "where," as a leak will find the weak spot in your insulation vb skin, or just rot the soffit off. Neither are good things.
This is why I prefer to "hang" and internal cathedral ceiling below the roof structure (around 'one less' in 12 pitch to the roof). That, and you can get IC cans in, or ductwork, or snake a borescope into to see where the #$%#& leak is . . .
Which goes back to it being Texas, too. The rafters just carry the roof, the ceiling joists get sized for the insulation--not near as much conflict that way. But that's mostly my opinion, too.Occupational hazard of my occupation not being around (sorry Bubba)
Yeah, that's why I think if you're really concerned about heat, you should have a reflective or light colored metal roof.
Yup. Fortunately got that one figured out long ago- light-coloured shingles last longer because they stay cooler. The current roofing is the lightest coloured asphalt shingle that looked good with the house, and the addition will be shingled with the same thing. The radiant barrier, whichever way I go about it, will help too.
Ya, new construction and that's the ticket.
Go molten!
How'd you decide on that handle anyhow? From Quasimodo?"sobriety is the root cause of dementia.", rez,2004
"Geodesics have an infinite proliferation of possible branches, at the whim of subatomic indeterminism.",Jack Williamson, The Legion of Time
CapnMac: Are you saying to basically sheath the roof with rigid foam, then furr it in the same direction as the rafters, then sheath it with plywood? Sounds great- puts the thermal break on the cold side, which I like, plus it keeps the roof venting while separating the cath ceiling insulation from the vented space completely. But isn't the foam bearing the weight of the snow load, plus the dead load of the asphalt roofing, concentrated in the narrow areas between the furring strips and the tops of the rafters? Not so sure I like the prospect of that foam crushing between the rafters and the sheathing, even if it could only crush 1/2". If I were using I-joists for roofing members, the wider top flanges would make me a lot more comfortable, but now that I'm down to 2x8s again I have no intention of going to I-joists.
FYI- the"moltenmetal" handle is a reference to one of my more unusual hobbies. I built a propane-fired furnace which I use to melt old bronze plumbing fittings and scrap to make sand castings, mostly rough hardware for my woodworking projects. The furnace is basically an insulating firebrick box with a hard firebrick tile for a bottom, wrapped with a batt of Roxul mineral fibre household insulation. I can have the furnace up from room temperature to 1000 C in about 15 minutes.
But isn't the foam bearing the weight of the snow load
Hit the design idea square on.
I hate forgetting about snow loading. I was also thinking of the foil-faced rigid sheathing (and can't put a name to the product--it's a CRS morning).Occupational hazard of my occupation not being around (sorry Bubba)
The stuff I've seen is by Celotex and is called "R-Max". I think I've also heard it called Thermmax and Tuff-R unless I'm confusing it with something else. It's something like polyisocyanutare (spelled?). It's pretty sturdy, but Moltenmetal may have a point about snow load....
Celotex and is called "R-Max".
I may have some early 80's product stuck in my head, too.
I suppose a person could use a spacer to protext the celotex. Be a pain, though. I'll keep noofling on it.Occupational hazard of my occupation not being around (sorry Bubba)
Ok. Doesn't matter whether I believe it or not, it would appear that the Ontario Building Code believes in csnow's point about having a relatively air- and moisture-impermeable layer such as rigid foam insulation on the exterior (cold side) of an insulated wall. OBC section 9.25.1.2 says that unless I put on exterior foam to a thickness such that the thermal resistance of the foam and everything outboard of it is no less than 20% of the thermal resistance of everything inboard of the foam, then you'd need to drain and vent the wall cavities or provide sheathing gaps that more or less do the same thing.
So: with R20 foam in the wall and 1" of polystyrene on the exterior, the ratio would be R5/R20 (neglecting the insulating value of the air gap behind my sheathing and the drywall) which is 0.25, which is good enough. But this has me a bit worried now- maybe I should follow csnow's advice and just not bother with the foam!
OBC also says that there shall be a vent space under the sheathing "unless it can be proven to be unnecessary". Oh boy, that means a dumb-#### DIY proving stuff to a know-it-all building department reviewer- ferget it!
So, csnow- I agree with you that it's risky business to put the foam on the outside- especially if you use batt insulation. Just because the OBC will permit me to put a 1" layer of foam on the outside for my number of degree-days doesn't mean that there isn't an underlying moisture condensation problem to deal with there. Boy, you'd never conclude that reading the stuff on Dow's website... Still figure it's much less of a problem with cells or spray foam as the main cavity insulation as discussed before.
So- let's imagine for a minute that I want to put 1" of foam on the inside of my walls, without furring. With the firm polystyrene foam and 5/8" drywall I think I can get away without furring- but how the heck would I trim out a room with an inch of foam on the interior? Box extensions on all the boxes are no problem, but the rest will require a little imagination. Any tips?
Here's another thought: can I use icynene foam with 2x4 staggered studs on 2x6 plates? That would eliminate over 75% of the direct thermal bridging in my place (i.e. everywhere except at window and door openings) without the bother of exterior/interior foam layers. Icynene is a high-expanding product- if you spray it behind the interior row of studs in a staggered-stud wall, would it bow those studs out? Geez, I'd hate to find that out by experiment...
"So, csnow- I agree with you that it's risky business to put the foam on the outside- especially if you use batt insulation. Just because the OBC will permit me to put a 1" layer of foam on the outside for my number of degree-days doesn't mean that there isn't an underlying moisture condensation problem to deal with there. Boy, you'd never conclude that reading the stuff on Dow's website... Still figure it's much less of a problem with cells or spray foam as the main cavity insulation as discussed before."
Foam on the outside is great for a cooling climate. It would probably even work fine in a heating climate in a perfect world if there were zero air leakage into the wall cavities, no convective currents....
"So- let's imagine for a minute that I want to put 1" of foam on the inside of my walls, without furring. With the firm polystyrene foam and 5/8" drywall I think I can get away without furring- but how the heck would I trim out a room with an inch of foam on the interior? Box extensions on all the boxes are no problem, but the rest will require a little imagination. Any tips?"
I do not see any advantage to furring. Just an awkward void. The foam plane should be sealed air tight, with particular attention paid around the perimeter of the openings, and electrical boxes. Trimming amounts to extensions around the openings, and finish head screws for the flat trim. You could probably still use nails through the foam, but I like the screws.
"Here's another thought: can I use icynene foam with 2x4 staggered studs on 2x6 plates? That would eliminate over 75% of the direct thermal bridging in my place (i.e. everywhere except at window and door openings) without the bother of exterior/interior foam layers. Icynene is a high-expanding product- if you spray it behind the interior row of studs in a staggered-stud wall, would it bow those studs out? Geez, I'd hate to find that out by experiment..."
I'm sure that Icynene stuff works fine (no direct experience), and I think the thermal break would be helpful. It's just expensive. I expect it would not bow the studs, since the path of least resistance on an open wall would be to expand around them, but I do not really know.
Thanks for that. I do appreciate your advice and that of the rest of the crew- it's been extremely valuable to me!
The icynene isn't cheap- but as I said, I can't even get a GUESS out of anyone with respect to doing dense-packed cellulose. None of the insulation contractors I've spoken with actually do it. I'd really appreciate an approximate per sq ft cost from one of you guys in the NE US who have done a lot of dense-pack cells.