Cathedral Ceiling Insulation Details
So I’ve been searching through the archives and have not been able to find an answer to my issue.
I have a 6:12 cathedral ceiling that is conventional framed using 2×12 rafters and already plywooded and shingled outside. My insulation plan had always been to simply run Durovent air baffles and FG batts to get an R-38-ish rating.
Sometime during construction a few things changed the plans. For one, I now have 3 skylights in three non-consecutive rafter bays. And for two, I now have a large pile of leftover 4’x8’x2″ EPS; enough to do one layer in each bay. Also, I already bought all the foam baffles but haven’t yet bought any of the FG batts.
It should be noted that money constraints prevent me from using spray foam, dense pack, or any other method that needs to be hired out. The cheaper the better.
I definitely believe in and want a cold roof so now I am considering returning the Durovent air baffles and instead ripping some 1x and the EPS to create 3/4″ air channels then installing R-30 faced batts for a total of R-40-ish, but I cannot decide how to keep the roof cold around the skylights. In retrospect, I would have added strapping/purlins to the outside of the roof but I am now well passed that option. I thought about drilling holes near the tops of the rafters above and below the skylights but was afraid it would create a structural issue (although the span is only 8.5 ft.).
Any thoughts or details I could refer to?
Thanks.
DC
Replies
I don't like FG much anymore, but...
First notes: seal the foam board to the rafters with caulk or Great Stuff. Infiltration is your main enemy, and FG won't stop that. The EPS will be on the wrong side for a vapor barrier, however. Make sure you don't add a vapor barrier to the interior side or you will trap moisture in the bays. Alternatively, maybe you could keep the cold side leaky, then seal up the warm side *really* well.
As for keeping the top side of the skylights cold... How about laying down your 1x strips first. Drill smallish holes in the mid-section of the rafters just outside the skylight blocking. Add a layer or two of XPS from the hole to the inside edge of the rafter (seal in place to prevent air leakage). Add blocking against the XPS from just shy of inner side to your channel. (I'm suggesting XPS since you only have half the thickness and coincidentally, it has twice the R-value of EPS, but you thermal bridging will probably swamp that. Not going all the way to the inner edge of the rafter will give a bit of thermal break.)
You might even be able to do this by splaying the blocking above and below the skylight rather than being perpendicular to the rafters, which brings in a lot more light. (That is, make the finished opening larger than the skylight.) You then can add perpendicular blocking against the inner edges of the splayed blocking, and drill through at the triangles.
If the channel above the skylight gets warm, it can draw cool air from the neighboring rafter bays, and likewise, the channel below can leak to its neighbors. Small holes in the middle of the rafter shouldn't affect strength as much as notching would.
good luck,
---mike...
Edited 9/21/2009 7:09 pm ET by MadisonRenovations
No matter what you do, you will not have a vented cold roof at the skylights.
Given the restrictions you spell out, I would install the vent strips you have, then the FG batts, un-faced if you have not bought them yet, then apply the eps to the bottoms of the rafters for a continuous thermal break and VB. Run strapping over them attaching with long screws into the rafters, then the finish surface
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
So, in essence what both of you are saying is that I pretty much painted myself in a corner on this one?I hadn't thought about the VB rating of the EPS being an issue. But it makes sense.I don't understand why I would need the strapping on the underside?I also don't know what the difference is between EPS and XPS, maybe just a nomenclature thing? I have 20 sheets of OwensCorning Formular 250 that I need to use up; is that EPS or XPS to you guys?thanksDC
I think that's the pink stuff and if so, that's XPS (extruded polystyrene, I think). EPS is 'expanded;' it's like the white coffee cups. EPS is lets more vapor through, is compressible, and has lower R-value, if I recall correctly. XPS doesn't compress.---mike...
The Formular 250 is extruded. Extruded is better than expanded for insulative and vapor retarding considerations.I agree with I think it was piffin that mentioned installing the foam on the faces of the rafters. I usually use polyiso, when I use that I gap the 4' by 8' sheets when I hang them on the rafter faces. Leave about a 3/8" gap between the edge of one wheet and it's neighbor. The sheets ends don't have to break on a rafter, they can break mid-span. Especially 2" thick foam. I then peek in the gaps and mark the centerline of the rafters on the face of the foam sheet with a sharpie marker. Just a little tick mark. I then use canned foam to seal the gaps. Gives a nice tight seal. You can tape the seams too. You can always omit the gaps, set the sheets tight, and just tape the seams. Your choice.I then run furring strips across the ceiling, using the tick marks to locate the rafters. I screw through the furring strip, through the foam, and into the rafter edge.Drywall gets screwed to the furring per normal.You could omit the furring strips and drywall right over the foam, but then you'd be trying to drive 4" screws through drywall, through 2" of foam, and into the 1-1/2" wide edges of the rafters. Sort of like blind mans bluff. Plus, when you install rock right over foam, it's easy to compress the foam a bit when you drive the screws home. Over time the foam can push back, causing the sheetrock to pop of the screw. Furring strips are inexpensive. You can buy them cheap or rip 3/4" ply into 1-1/2" to 2" wide strips. The ripping and installing is easier than trying to screw drywall through 2" of foam. And you'll get a better installation.
"I don't understand why I would need the strapping on the underside?"It is a little harder to use 3-1/2" sheetrock screws to hit the framing thru 2" of foam.Actually a lot harder
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
I'm getting materials rounded up to insulate this ceiling now and I ran into another issue...If I run 2" strapping on the ceiling perpendicular to the rafters then later will I need to install the sheets of drywall running up the slope?DC
I would, but don't have too.
Either way, it is not an 'issue', just a simple decision.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
wow, you sure deleted that one quick!There are many, many advantages to foam, nit just the one you mentioned.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Yeah Piffin,
I deleted my post after some other quick second thoughts...
I realized that advantech on a cathedral ceiling would not be very user friendly... and not-so-cheap.
Advantech with Zip tape could make a nice air barrier though.
oh, yeah. I forgot the next bay will have insulation, not the channel. How 'bout a conduit from the hole to the channel?Personally, I high density spray foamed the whole thing. No vapor or air leakage, lots of insulation, quiet, stronger roof. A couple other people who've done it are also pleased, even when cost was an initial concern.---mike...
foam sprayed is the best choice, but the man already said that is out.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
thanks guys, I would love to go with sprayed foam, but I just cannot afford it. It makes me mad just thinking about it.... Why do the best home insulators have to be the most expensive? If it were the cheapest option then everyone would use it and we would all use less energy. A win/win all around...<grrr>So, I like the idea of the XPS on the bottom (sorry about getting the acronym wrong earlier). What I was thinking when I asked about why the strapping was a bit different. I was thinking of ripping 2x to 2" then installing that at the XPS joints, sealed with Great Stuff. Then I would install the DW to the ripped strips. I could even tape over the strips to create a better VB and the only thermal bridge would be 1.5" x 1.5" every 2'.To me, this just creates a stiffer substrate for the DW and less likelihood of screw pops or seam breaks. But, I will still use the Durovents and un-faced R-38 in the bays as suggested.I am still plotting out a way to vent around the skylights using a couple of holes and air channels. I am past the idea that holes in the tops of the rafters will make much of a difference in their performance. I recall the use of 2x12's was for the insulation space not the span rating.I realize I will not be able to make a drafting roof at this point but at least I can alleviate some of the temperature difference by connecting bays and letting the air cool by convection. I know this isn't exactly what you guys were directing me to do, but does is sound like a good alternative? It's still giving me R-48-ish at this point. I like the idea of being slightly experimental (to me at least). I bought this house to do and learn things I never yet got a chance to in the field. It acts as a home for my family, a learning tool for my design/build skills, and a show piece for my business.Thanks all.DC
Dreamcatcher wrote:
> Then I would install the DW to the ripped strips.ROFL. "DW" has a particular meaning on these sorts of forums. I had a vision of her hanging from the ceiling as part of this project... :-)---mike...
Piffin noted:
> foam sprayed is the best choice, but the man already said that is out.Yup. I just said what I did, in case anyone else is reading and considering roof insulation. One of my friends was really concerned about cost, but in the end decided that although it was well beyond his original budget, it was worth doing, and he's glad he did.Proper rafter insulation gives some of the best paybacks because that's where hot air rises. Hot air hits the cold ceiling and cools, then drops creating a draft and allowing more warm air to move into place to be cooled. The initial costs are high, but the payback for spray foam is quick, then it's saving money from that point on. Do a bad job on a cold roof and you have discomfort, energy waste (fuel costs), and mold.---mike...
Mike, I feel as though someday I will have a house that I want to live in forever but this house is being remodeled to sell. Although I do catch a lot of flak from friends and family over the investment I am putting into a "flip" house, I figure I don't need to go to the expense of spray foam to be able to declare to prospective buyers that the insulation is well above and beyond the required standard.In the same sense, I am trying to develop a method of building that gives the most bang for the buck to prove that, with some effort, one can achieve a higher standard for a much lower cost.DC
DC: I guess if you're remodeling to sell, you are hoping not to reach the payback period for spray foam. :-)Just be a good sport and really seal off the FG. It's okay without air movement, but is worse than useless if not sealed. (mold)---mike...
Good way to put it; In this market, all I have is hope that I won't be living here forever.But, that's an [uneducated] experiment too: trying to flip a house to entice an older and more knowledgeable crowd, possibly a 2nd/3rd time homebuyer. FIL who used to do lightning fast flips thinks I'm crazy. Maybe.For as much wet/moldy FG I've already SCOOPED out of this place, you better believe I am doing what I can to keep it dry this time. Thanks again for the heads up on the placement of the vapor barrier.DC
If you are going to use any of the Great Stuff foam, do yourself a huge favor and buy the professional application gun . I tried using the throwaway cans for years, and they are a royal pain. I bought the gun about a year ago and love it! You can use part of a can and walk away for an hour or a month and still be able to use the rest. You can adjust the bead size from tiny to "whoa baby!" Its a snap to clean with the screw on can and you can get either the regular or low expanding foam cans. I no longer get the residue all over my trigger finger, and you use 100% of the can every time. Its worth the money, absolutely and with no reservations.
I bought mine at a Menards box store, but after some coaxing and showing the benefits, my local lumber yard now orders it for me. And it is the "Great Stuff" brand, so they have no problem trusting it. Oh, and the cleaner comes with a slip on spray nozzle, so if you do get it on you, its easy to get it off before it cures.
Good luck with your project!
Actually, I have a Great Stuff gun I got for $20 a long time ago but no local source for cans. No "local lumber yard" around here besides Home Depot. They can get it for me but I have to buy it by the case and it is actually more expensive (per oz) than the one time use cans. I wish those stupid companies would wise up and make the cans work both ways.FWIW, I have found that if you remove the nozzle from the throw away cans after each use then you can use the whole can at your leisure. I have many leftover nozzles from all the times I have used several cans and the same nozzle. They clean easily with a wire and thinner too.How much are you getting the gun cans for?DC
clean better with acetone if done immediately
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Our small chain lumber yard had them on for $8 a can in their catalog, good thru Dec. My biggest complaint about the disposables has always been having to hold them just right, and getting all of the product to dispense. And I always wind up with some on my hands, especially around the nozzle.
When I showed my store how well it worked, they were willing to put it on the shelf, so no case requirement anymore. Too bad H.D. won't do the same, its a great product (of course, they wouldn't sell as many throw-aways then!).
Hi Dreamcatcher,
I apologize if I've missed something you've detailed already....didn't see anything as I ran through the previous posts.....
You're insisting on a cold roof but I don't really see the overall point with all the info on hot roofs particularly in your region....I might rethink that with a shingled roof in FL/TX area....but even then with all the data I'm not sure I'd be concerned there.
At this point for max thermal resistance and some of the materials you now have: I'd open cell spray the bottom side of the roof sheathing just to seal up air infiltration at a budget minded cost, fill the rafter bays with unfaced fiberglass batting, use the 2" XPS on the bottom of the rafters creating a thermal break.
Once your shingles are worn out, and it's proven not to shave but a few months life off of them with a hot roof, but anyhow, once it's time to replace.....strip it and put on a metal roof as your last roof maintenance ever.
View Image