Insulation facing direction-which way?
Hi, after reading the posts from the discussion on gable vents and ice dams in this forum I’ve decided that I definitely need to insulate the space behind the kneewalls upstairs in my Cape Cod-style house.
Which side should the facing on the insulation rolls face? I was also wondering if I should also use rigid board insulation in addition to the rolls of figerglas insulation. Do they make that rigid insulation with a radiant barrier backer (like foil) and , if so, which side should that face? In the summer it gets really hot up there on the second floor. I do have a ridge vent but there are only soffit vents on one side of the house. The other side doesn’t have much of an overhang to allow soffit vents there for some reason. I’ve looked at the roof trusses and they seemed to have been placed on the house favored to one side only, leaving an overhang on one side and hardly no overhang on the other side, it’s about only a 3″ overhang there (strange?), and I haven’t figured how to get vents in there, it’s tight. I’m also concerned about condensation and want to do this right. I want this kneewall space to be cold to prevent ice damming.
Replies
At the top level of the decision tree, there are 2 ways to treat a kneewall space. You can make the kneewall an 'outside' space or an 'inside' space.
If you treat a kneewall as outside the weather boundary, you have to seal and insulate the floor as well, which can get very complicated.
Consider bringing the kneewall 'inside' by insulating and sealing off the rafter bays.
Reflective side of panels face inwards in a heating climate, though they do not do very much.
Check out our old friend Fred @:
http://www.taunton.com/finehomebuilding/pages/h00021.asp
also
http://library.energyguide.com/EnergyLibraryTopic.asp?bid=nstar&prd=10&TID=13940&SubjectID=7835
and
http://www.southface.org/web/resources&services/publications/factsheets/25_insulateceilings_4pdf.pdf
Thanks so much for the info!!
I was considering making these spaces "cold" to prevent any ice damming problems.
Will bringing the attic space behind the kneewall into the conditioned space cause ice dam problems?
By the way, I had some A/C ducts installed this past summer in these spaces and there is an access door to get to the space that is allowing heat to get in there. I will be creating insulated door access panels to help seal the heat out of these areas. I know I need to do this quickly before the snow flies here to prevent the ice damming. I can see where the frost melts on the roof in the mornings where the heat is getting now into the kneewall attic space. This is why I'm trying to find out all I can to properly do this right.
Thanks.
"By the way, I had some A/C ducts installed this past summer in these spaces and there is an access door to get to the space that is allowing heat to get in there."
Another argument for bring the kneewall into the conditioned space. Kneewalls are popular for mechanicals and storage. Making exterior doors in kneewall is a challenge.
Just 'thermal insulate' the interior of the door. (no challenge what-so-ever.
I have doors in my kneewall to an unconditioned attic space.
How should I thermal insulate the doors? They are hollow core interior doors.
Simply attach your insulation to the cold side of the door. No big deal.
I was just on Owens Corning website and also Johns Manville's website , looking up info on insulation installation. Tell me if the following is correct:
For discussion lets consider the kneewall area (in section view, a side view in other words) shaped like a right triangle. With the vertical leg of the triangle representing the kneewall, the horizontal leg represents the "floor" of the space behind the kneewall, and the slanted side of the triangle represents the roof.
What I got from those websites is that I should place the kraft-facing of the insulation against the backside of the drywall that covers the kneewall (so it's toward the "warm in winter" living area as they said).
For the "floor" of the area behind the kneewall I should place the kraft-facing against the top-side of the drywall that is essentially the ceiling of the first floor below it (so it too is on the "warm-in-winter" side)?
For the roof side of this triangle I should place the kraft-facing away from the roof sheathing allowing for the 1" minimum of space for the venting from the eaves?
So now if I look into this space behind the kneewall, I should only see the kraft-facing on the roof side of this triangle formed by the kneewall space? The other two sides of the triangle I should see the fiberglas only?
After all this is done can I place rigid foam boards on top of that? Like the FORMULAR stuff that Owens Corning makes?
Thanks in advance for you help.
"For the roof side of this triangle I should place the kraft-facing away from the roof sheathing allowing for the 1" minimum of space for the venting from the eaves?"
It would make no sense to insulate all three edges of the 'triangle'. Either bring the kneewall space "in" by insulating and sealing the sloped roof, or put the kneewall space "out" by insulating and sealing the kneewall and the floor.
If you put it out, do not rely upon fiberglass to airseal. Place blocks (or use 'fiberglass bagging') between floor joists (at bottom of kneewall), rafter bays (at top of kneewall), and seal all doors and penetrations. Then you can insulate.
Doesn't placing insualtion blocks between the rafter bays at the top of the kneewall interfere with the ventilation route that needs to be maintained from the soffit vents to the ridge vent?
"Doesn't placing insualtion blocks between the rafter bays at the top of the kneewall interfere with the ventilation route that needs to be maintained from the soffit vents to the ridge vent?"
Well, it depends upon your 'religion'. There is this heretical theory that venting causes more problems than it solves if convective losses are truly controlled. If there is no warm moist air to vent, what are the vents doing?
Here is a picture of someone with a lot of experience (and some noteriety) in these matters stuffing the rafter bays tight.
View Image
See:
http://www.taunton.com/finehomebuilding/pages/h00021.asp
FWIW, two thirds of my attic is finished. I dense-packed those rafter bays with cellulose, and have no venting. The remaining third of my attic is, shall we say, cold storage. Neither side ice dams. However, come those cold moringings, the frost is noticeably thicker on the 2/3 with the unvented roof. Stays nice and cool in the summer, too. Andy Engel
Senior editor, Fine Woodworking magazine
Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Jeez, stuffing those rafter bays goes against all of the articles I've just researched. Now I'm a bit at a loss on really what to do.
The previous owner of my house put insulation on all 3 sides of the triangle I mentioned above. And with the kraft facing AWAY from the warm sides, so all I see is kraft facing on all 3 sides when I look in to the attic space behind the kneewall. And even some of the soffit vents are blocked by the insulation. That is why I want to get in there to fix it. But do it correctly. I haven't decided on whether I want this space "conditioned" or not "in" or "out" side of the conditioned space.
Check out buildingscience.com.Andy Engel
Senior editor, Fine Woodworking magazine
Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Edit: Oops, shuld have been to pizza, not Andy...anyhow:
Now we have pictures of Fred popping up and Andy advocating no-vent roofs, all in the same thread?.
"This thread will self-destruct in five seconds..."
Pizza, for what it's worth, I advocate that you bring the volume behind the kneewall into your building envelope. Run insulation from one end of the rafter to the other...plate to ridge. when you hit the kneewall, bring the insulation up and over it to prevent heat/air loss at the top plate of the knee.
If you want an airspace for venting, feel free. Do you need it? Probably not. If you want to stuff your rafter bays tight, have at it as well. More details should you want them.
For attics, use whatever you want in the rafter bays. If you want an air space, then leave one and use FG batts. Personally, I'm with Andy and prefer cellulose.
Regardless of FG or cells, I button nail 4' by 8' sheets of foil-faced polyisocyanate insulation on the faces of the rafters. I leave about 3/8ths of an inch between the sheets, a gap on all four sides. The sheets can end mid-span, between two rafters. Peeking between the gaps, I find the hidden rafters and I use a sharpie to put a tick mark on the foil face, showing the centerline of the soon to be hidden rafter.
Using canned foam, I then foam the gaps between the sheets. The foam will give you a primo air seal.
I then run horizontal furring strips over the face of the foil-faced polyiso. 16" oc, fastened through the polyiso and into the rafters. Use the tick marks to show you where the rafters are so you can hit them with your fasteners.
Drywall per usual, attaching it to the furring strips with 1-1/4" screws.
Going back a bit, at the kneewall I bring the polyiso up and over the top plate of the kneewall, foaming pieces together (after peeling off the foil) so that polyiso completely separates the attic space from the rafter bays.
Bringing the kneewall space into your thermal envelope wil make it a more suitable place for your air conditioning equipment and ducting, it'll also make it more suitable for storage.
Now go into your basement and use the canned foam to seal any penetrations in your basement ceiling to prevent basement air from entering the living space of your house. Shoot plumbing gaps, wiring holes, around HVAC ducting, etc. Essentially, isolate your basement from your first floor.
I did the same technique in my own house. My attic is now finished space, one large room about 850 sqft. It has no heat of it's own but stays warm throughout the winter. With a few bodies up there and some lights on, it almost gets too warm.
Summertime, my previously FG insulated attic would roast. The day I started putting up the polyiso, the temp at the ridge was 127 degrees. The next day, after the polyiso, it maxed out at 77 degrees. Now, in summertime it holds in the mid-70s regardless of how sunny it is outside, or what the outside temp is. Not having the hot attic was noticed within days as our second floor bedrooms cooled off quite a bit.
One question to ask yourself before you vent is...what are you venting? And why?
Edited 12/1/2004 1:01 am ET by Mongo
Sound advice.
Andy Engel
Senior editor, Fine Woodworking magazine
Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Thanks so much for that. Really appreciate all of the information!! Based on that I will try to get that space into the conditioned space. And I will still try to keep a vent path going from the soffit to the eaves.
You asked what am I venting and why. Well, after reading and researching all of this I still am afraid that if I don't have that cold vent air running from the soffit to the ridge vents on the underside of the roof sheathing then I might get ice dams. Even when I realize your point that the heat is now not getting to the underside since I'll be insulated now by your system. I used to live in Minnesota and know the havoc those ice dams can cause.
Thanks, to all again, profusely.
Vent, if you must.
If you use the Mongo method, venting will likely not cause any new problems. The rigid foam becomes the air barrier, and the venting happens behind it. Just be sure the soffit vents are completely boxed off from the conditioned space at the bottom.
The venting problem then becomes; what is in the finished ceiling? Does it have foam vent chutes already? If not, your vent path is through the FG batts. Not good.
That pic only indicates how ignorant some installers can be...
VENTILATING is the necessary key to preventing many problems
I respecfully disagree that venting is necessarily going to solve anything. It does very little to reduce shingle temperature, and where I live in New England, ventilation air is frequently wet enough to add moisture to framing cavities.
The history of roof venting and the building code goes back to the 40's, when it was suggested as a way to deal with moisture coming up from crawlspaces in the south. It's since become codified, and part of building lore. It never worked very well then, and now we have materials and methods that provide for better moisture control and air sealing than were available in 1947. You may note that the 2003 IRC provides for unvented roof assemblies. You might also spend some time searching around at http://www.buildingscience.com.Andy Engel
Senior editor, Fine Woodworking magazine
Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
I'll stick to the old methods that have beenproven ok for 50 years or more.
The bldg. sci. IMO has not been around too long to have any proven record.
VENTILATION is the key.
Hube, I understand your point-of-view, and I'm sure you know a helluva lot about building. But for the sake of debate, lemme throw some points out there for others new to subject to consider:
I'll tack on a few paragraphs (get a pillow) to address what I asked pizza...
"What are you venting?"
Older houses were moisture-producing machines. Moisture from basements, moisture generated from living (cooking, bathing, humidifiers). Fortunately, older houses also leaked like seives, so often times that moisture exited the house without causing too much trouble.
We then started building tighter houses.
But we still generated gallons of moisture.
Builders thought they were building better...adding vapor barriers, tightening up windows, sealing this and sealing that, etc...but were they just doing things differently, or were they indeed doing things better? Were they building in consideration of the science behind the structure? In a sense it was like treating the entrance wound of a gunshot victim while ignoring the exit wound. All looks great on the facade...but why is the patient still dying?
Today's construction methods do a bettter job of dealing with moisture when and where it's generated. We have under-slab drainage, perimeter footing drains, vapor barriers under the basement slab, poured foundations wth sub-grade waterproofing. Basements are tighter and drier.
We have exhaust fans in bathrooms. Exhaust fans over cooktops.
Instead of stand-alone humidfiers pumping gallons of moisture into the house 24/7, we have humidfiers installed in the HVAC duct, adding moisture only on an on-demand basis. Or, since our better built houses aren't leaking like they used to, we don't even have a humidifier.
We just don't have as much moisture in houses as we used to. So what are we trying to vent?
Some will seal the attic rafter bays like I've written above in my previous post...but still say they need and air channel between the rafter bay insulation and the roof sheathing "to vent." That's fine, and I consider it a freebie. But in essence, they're not venting anything. They're theoretically doing nothing more than cooling the roof and lowering the temp of the shingles by a few degrees. That can be good for the shingles...but what are we giving up in the process?
They're doing that by bringing outside air into the rafter bays and running it in a channel between the insulation (usually FG) aand the sheathing, then up and out the ridge. The risk is in washing that air over their (usually FG) insulation. What is the moisture content of that outside air? That's up to Mother Nature. If we end up with a bunch of humid air in that channel, and the sun goes down and the roof cools...what happens if the underside of the sheathing reaches dew point temp? Will the moisture from that outside air now condense and drip into the rafter insulation? Maybe...maybe not. We really can't say, because it's not something we control. We've left that in the hands of Mother Nature.
Back to my version of "better building". Since we're building better basements ans exhaust much of the generated moisture at the source, we have minimal moisture in the house, thus less moisture that can possibly end up in the attic. With the polyiso acting as both an air and vapor barrier, no moisture that makes it to the attic is going to get into the rafter bays. Assuming some does, if cells were used the cells will absorb, then dole out the moisture as time ges by. Even FG can handle that. By knowing where and controlling where the dew point occurs in the roof cross-section, moisture vapor just isn't going to be a problem. Opening up the roof and allowing outside air into our roof takes that certainty out of the equation. We don't really know, with 100% certainty, what's going on up inside the rafter bays.
One thing that you're absolutely correct on, and one thing that people need to understand, is that you can't do these things in a half-azzed manner.
You either need to control the sources of the moisture, then tighten up the house as a whole...or you leave all the moisture generators alone and maintain a leaky, vented house. What you don't want to do is seal the lid and leave the bottom of the box open. That can indeed be a recipe for disaster.
Still, for a poor analogy, I look at today's construction methods as us nicking ourselves with a knife, then putting a band-aid over the cut to control the bleeding. All we need to do is to put the darn knife down. That'll stop the bleeding and no first aid will be required.
Ice dams? We all know that's from heat from the living space leaking into the rafter bays and melting the snow on the roof. The melt then flows down the shingles towards the eaves, and with the underside of the eaves (overhang) not heated but open to freezing outside air, the roof there has a lower temp, so the melt refreezes. Repeat the cycle and the dam grows, working up the roof and possibly under the shinges.
Again, instead of all of us sitting around the table and agreeing to not let the heat hit the underside of the roof, we instead end up with an ice membrane on the sheathing. Now don't get me wrong...I certainly think ice membrane is a great product, and I use it, and would even if it weren't mandated by code...but it's a band-aid solution to a broken bone problem. Let's simply stop breaking bones by not allowing warm air from the living space to infiltrate up and into the rafter bays.
Remove the heat by installing insulation. (duh!)
But it has to be insulation that works. (Ding!)
My opinion is that the even bigger and more important goal is to remove the ability of the air to move into the rafter bays by sealing the attic rafters with polyiso or another RFBI. Stop the air infiltration and there goes 99% of your problems.
I'd venture that a roof wth nothing more than 2" of foil-faced polyiso, foam-sealed on the faces of the rafters, and with NO insulation in the rafter bays (R-14) would out-perform a roof with no polyiso, but with the rafter bays stuffed with R-whatever of FG batts (R-50?).
To me, R-value is almost of less a factor than air movement.
Why? Because when air moves through insulation...which it does quite easily through FG batts...it renders the published R-value for the FG as virtually worthless. When you wash air across the face of FG batts...it renders the published R-value of the FG batts as virtually worthless. When the ambient temp of the FG batts drops down low...it renders the published R-value of the FG batts as virtually worthless.
You ended your previous post with "VENTILATION is the key."
Indeed it is...if you have something to ventilate. But my opinion is that if a builder doesn't know WHAT they're venting or WHY they're venting, they can actually make things worse.
Edit: for a slew of spelling goobers
Edited 12/1/2004 2:20 pm ET by Mongo
I've read Mongo's prescription several times, and the idea of putting polyiso panels up as baffles is tempting (I now have an air handler in my attic), though I might conservatively still vent air between soffit and ridge vents behind the iso... But I do have a concern about the weight of all that drywall hanging from the rafters.... How bad is it to do without DW? Suppose I've already got sprayed poly foam on the second story ceilings, but my understanding is that a thermal barrier is not required here since it's not in the conditoned space.... The idea is not to bring the attic into the conditioned space, but just partially condition it so the handler is not such a weak point in the insulation....BTW I've got a walk-up attic so air-sealing it is a bit of a challenge...
A walk-up makes birnging the idea of bringing the attic into the thermal envelope of the house all the more worthwhile.
If you have foam, wisdom dictates that it really should be covered with drywall for fire protection.
I've never seen a case where rafters couldn't suport the weight of drywall. Sure, the sheetes themselves are heavy, but on a per sq ft basis...now trusses, I've never framed with trusses, so there may be load restrictions there. But trusses are out of my league.
I do know people that "box" in air handlers that are in unconditioned attic spaces. It's certainly not a "one size fits all" fix, though...depends on first-hand inspection and analysis of the situation.
"To me, R-value is almost of less a factor than air movement."
Ignore this heretic. The world is flat.
Trouble is, in many cases, these methods have not been working. At least half the new houses I see, with traditionally vented roofs, suffer from ice dams. Venting can work, but there's almost always an energy penalty.Andy Engel
Senior editor, Fine Woodworking magazine
Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
That's why you're the editor and I'm the writer.
You: Lucid, concise, to the point.
Me: Ramblin' Man
Nah, I think it's just that you've got more time to kill.
AndyAndy Engel
Senior editor, Fine Woodworking magazine
Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
You been talking to my wife again?<g>
Okay, bye bye to Taunton and hello to sawdust...but first I gotta crank up some Joplin (Janis, not Scott)...REAL LOUD.
Another idea,
Fill the rafter bays and knee walls with Icynene and get one or two good direct vent fans. If you do collect moisture in the cavity the fans ventilate from the inside.
Vapor barriers always face to the heated side.
Insulate vertical kneewall and horizontal (floor)
do not insulate the sloped part of this small triangle.
Make sure the soffit vents are open right up to the high or ridge vents.
Edited 11/30/2004 12:01 pm ET by Hube
Vapor barriers always face to the heated side.
always?
Always. Thats where the vapor is ,in the house. if installed onthe on the wronge side, then the insulation will asorb the vapor and eventually loses its R value by being wet.
Get it?
"Vapor barriers always face to the heated side.""Always. Thats where the vapor is ,in the house. if installed onthe on the wronge side, then the insulation will asorb the vapor and eventually loses its R value by being wet."I can (sorta) agree with the first statement.But there are lots and lots of problems with the remainding comments.The "vapor" is not always on the inside of the house.The insulation can (and will) "absorb" vapor without gettting wet or loses it's R-value.The water vapor does not cause any problems until it condenses. The important thing is to know the moisture content of the air and the tempature of any potential condensing surfaces.
Well ya, I didn't want to run the risk of war.
letting it be
For better or worse, I found some pics. Unfortunately, it's not a step-by-step, just three shots as the attic progressed.
First one shows the rafters already polyiso'd. I only use a few bottons on each panel, just enough to secure it. It'll get locked down completely by the furring strips. Drywall is going up behind the knees. The polyiso that shows hasn't had the gaps foamed yet. The ceilings got 2" polyiso, the walls 1". In the kneewalls, more studs were knocked out and headers installed for storage access, there are a total of five kneewall access doors on each side of the attic.
Pic 2 shows the drywall up and faux beams being installed. Beams hide the wiring, break up the large plane of ceiling, and divide the kneewalls into sectors. As to wiring, I don't allow any penetrations (can lights, junction boxes, etc) through the polyiso.
Pic three shows the almost finished attic. Beams are primed and painted (a dark, deep, almost black green). The wainscotting around the walls, the full-length window seats (hides ducting and provides storage), and the shelving that surround the stairway are just primed, with gray-tinted primer.
Floors are bamboo.
Sorry to not have more in the series. Some pics showing the furring strips might have been helpful.
Thinking about your case again, the airhandler is yet another argument for bringing the kneewall inside the envelope. The sealing and insulation on the ductwork is yet another potential weak link in the system, particulary if used for heating. Could add a lot of heat to the cavity.
Thanks. My duct work is only for my A/C unit. I have hotwater baseboard heat throughout the house. After all of this (great discussion by the way!), I think I will bring the area behind the kneewalls "inside" the envelope. I will seal all potential air routes and then insulate with the foil backed polyiso stuff. But I will still make a path from the soffit vents to the ridge vents. I know, I know what you've said. Thanks to everyone!! I will try to get some pics up here if and when I get to this project. I'm going to make the insulated access panels first.
I think you're wise to bring this area into the envelope. While the ducts are only used for AC, I'll bet you breakfast at Tiffany's that there are convective currents passing through them all year long. Ever feel any cold drafts at the registers nearest those attic ducts? Cold air sinks. Consequently, I have no doubt that they cost you heating dollars in the winter, and could contribute to ice damming. As to venting, it's generally harmless, despite my opposition to it.Andy Engel
Senior editor, Fine Woodworking magazine
Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig