Foam in the middle – Will it stand the test of time?
I’ve done a lot of looking/reading and I must not be searching with the right key words. I can’t find anyone else that has built this style of wall/roof (layer of foam in the middle) but I’m sure someone has.
I’m building a house using passive solar that should reduce my heat need by over 90% (time will tell). My original plan for the berm style home is to have a double wall and roof with 3″ of EPS foam (R12) in the center. I live in zone 5A (Iowa). I’m aiming for true R35+ walls and R65+ roof and < 0.6 ACH blower door test. Based on the hours of reading that I have done this should allow for drying to both inside and outside. The walls would be layered from inside-out… drywall, 2×4 with fiberglass, 3″ EPS foam, another 2×4 wall with enough gap for loose insulation to hit my R35+ goal, plywood, Tyvek type housewrap, siding. The advertised R-value would be R42+ The roof from inside-out… drywall, 12″ I-joist with fiberglass, 3″ EPS foam, another 12″ I-joist with loose insulation, plywood, roofing underlayment, steel roof. Roof would not be vented. The advertised R-value would be R84+ I plan to use the EPS foam as the primary air barrier but the house wrap and other layers will help as well. My question is what kind of moisture/condensation problems will I run into if any? My expectation is that what little if any moisture that gets in the wall/roof will dry either in or out depending on location of the H2O. Thank you for your feedback.
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Carl,
You are suggesting a fairly complicated and expensive way to build a high-R wall assembly and a high-R roof assembly. Why reinvent the wheel? What makes your approach any better than more common (and cheaper) methods?
Just for starters -- to get your roof assembly to dry to the exterior (so that all of the materials on the exterior side of the EPS have a drying path to the exterior) won't be easy or cheap.
If you're building a double wall, why not skip the foam and fiberglass batts and just fill the entire wall cavity with cheap cellulose? If you want a dedicated interior air barrier, You could use an interior layer of plywood (some OSB is not airtight) positioned in the outer facing of your inner wall.
Using 3" foil faced polyiso in the middle tight to the studs on both sides would get you to R35 whole-wall. without the gap or need for loose fill. With R15 fiberglass or rock wool batts the outer studs w/sheathing is good for about R9.5, the inner studs w/ half-inch wallboard and R15 batts is good for another R9.5, and the 3" of polyiso is good for R15.5-R18.4"
Alternatively you could go with 4" EPS for a slightly thicker wall. But it's nearly impossible to do a good job of loose fill with the gap.
A 2x6/ R21-R23 batt 24" o.c. stud spacing wall with 2" of polyiso on the exterior of the sheathing + 2" of EPS on the exterior of the polyiso (to keep the polyiso warm enough to perform well) would also be R35+, and would have huge dew point margin for the structural sheathing. It's fewer boards to cut, and fewer board feet of lumber, and the foam would also thermally break the stud plates & band joists, unlike the doubled 2x4 center-foam solution.
The double 12" TJI roof assembly is out of sight expensive, and without venting the middle 3" of EPS becomes problem for the roof deck. A single 16" TJI with 1.5" of vent channel with cut'n'cobbled half-inch fiberboard sheathing for the exterior air-barrier for the fiberglass tacked to the interior side of the TJI, and 14" of 1.8lbs dense packed Spider or Optima blown fiberglass, 3" of EPS on the interior side under the wall board would get you to R65 whole-assembly (R75+ center-cavity) for less money and dramatically less moisture risk. That's a really long sentence- the roof stackup would be:
shingles
roofing felt
structural sheathing
1.5" air space formed by TJI flange
0.5" fiberboard sheathing
14" of 1.8lb blown fiberglass
3" EPS
1x furring through-screwed to the TJI
1/2" gypsum board
At ~12-20 perms the fiberboard has plenty of drying capacity into the vent channel, and the ~1-1.5 perm vapor retardency of the 3" EPS is just fine on the interior side. It won't accumulate or trap moisture. With the unvented 12" "TJI x 2 with the 1-1.5 perm EPS in the middle the drying rate of the structural roof deck is impeded a bit- it might work, but it's far less resilient than the vented approach.
Excellent feedback.
I was able to get a decent deal on the EPS which is maybe why thoughts of the cost is high? The double wall is only one side of the house. The rest of the house is concrete walls with 6" of EPS on the exterior (I'm using the concrete as thermal storage).
Martin,
Actually I've looked at other options for the roof including what I've seen others use. The cost I've calculated is less than others. An example is using a single but deeper 2x4 floor truss. The cost of an 18" 2x4 floor truss is over $115 for 30' but two 12" I-joist are less than $90 and give me 25% more room for insulation. Roof trusses which don't work for the house design are about the same as the floor truss. I liked the idea of the foam in the middle so that I use bat insulation in the lower part which would make any future remodel or maintenance cleaner. I'm open to ideas and appreciate your input/questions. Is there a cheaper option that I didn't mention for the structure? If not is there a different order you would layer the same material. The labor isn't a major issue since I'm doing most of the work and this house is for myself.
Nate,
The foam in the center of the wall will give me a complete thermal break where my windows will be located. I could use plywood to bridge between the walls for the window openings but like the idea of a solid thermal break. Another option would be to put the foam on the outside and use a 9-1/2" I-joist for wall with cellulose but I thought the framing would be easier with a double 2x4 wall.
Dana,
Thank you for the detail. I'll check out the costing on your layering recommendation on the roof. The one problem I see with the foam on the ceiling is the number of holes to seal from the lights.
The wall with just fiberglass (R15) would get me to R31 (might be close enough... need to double check).
So the EPS isn't an issue in the center of the wall but it is on the roof? I'll just mark that up as one I still need get my head wrapped around.
Martin and Dana,
Would using a single 12" I-Joist with 6" EPS under unvented structural sheathing be a good option as well? I'd be a little short of R65 but I'd be close and it would be easier than dealing with venting.
Thank you again for all the feedback. It's great help!
With thermal bridging an 2x4/R15 wall is really R9.5, so a pair of those gets you to R19, not R31.
Walls are vented to the exterior behind the siding,(even if it's a tiny vent space) and the sheathing can dry toward the exterior. Walls do not get a lot of direct wetting or carry loads of snow either, both of which blocks drying to the exterior on roofs.
Steel roofing tight to the underlayment it cannot dry to the exterior unless there is a vented space under the roof deck- it can't dry through steel, which is a true vapor barrier.
If you had venting ABOVE the roof deck- a vented air space between the roof deck and steel roofing and nothing tighter than #30 felt on the roof deck it can sort-of-work in climate zone 5, but it's better to use vapor permeable fiberboard rather than plywood or OSB for the roof sheathing if going to go that way. If the steel roof is going on 2x purlins you can probably get there with your stackup.
Hanging 6" of EPS on the underside of joists would be very difficult to pull off. It's doable above the roof deck, but R24 foam is not sufficient dew point control for more than R36 of cavity fill in zone 5A. The exterior R needs to be at least 40% of the total center-cavity R to keep the roof deck warm enough. With some really junky low-density fiberglass in the 12" TJIs you might be able to go that route. Installing 9" of EPS (R36) above the roof deck is pretty awkward (I don't know anybody who has even tried), but with that much foam it would be fine to install 12" of 1.8lb Optima or similar in the TJIs in your climate.
I know it's an unusual stack but feel I get the best air seal and thermal performance with the foam in the center and so far the cost with what I can actually get locally isn't bad.
Thanks Dana, I wasn't clear on the wall. You are correct with double 2x4 but I had R12 foam in the center to get to R31.
I checked with Menards on the 18" TJIs but they wouldn't even special order them for me so I'll need to check a couple other local lumberyards. The best they could do was 16" TJIs but 10' short of the length I need.
I agree on the EPS under joist and really don't like the idea of putting it under at all in any amount.
30# felt is what I was planning on under the steel but had not planned on the purlins... I do understand what you are saying. Would you be comfortable using the fiberboard w/o the purlins? I think the steel will be quieter and more durable directly against the felt and fiberboard vs on top of purlins.
Just to be sure, this link below is what what you are talking about when you mention fiberboard. I recall using it for exterior wall sheathing on some duplexs we built.
fiberboard link
Thanks again for help. It is truly appreciated!
On a side note I just got my Broan ERV with ECM motors today (it was on sale) and I'm testing it's function. So far so good :)
Carl - I think your idea for the walls would work fine with some modifications. Dana's assessment on the watered down R-value may be a bit conservative, and I think that with high density batts and thought out framing you could get to R-11 or 12 per wall. Using 2X6's on the outside wall on 24"centers may be more advantageous. The purpose of the foam is an insulated air barrier, and as such it doesn't need to be so thick. 2X6 outer wall, 2"EPS and 2X4 inner wall may be a good stack up, but I admittedly like minimizing the foam, even EPS, and maximizing insulations with less embodied energy.Your inside foam would presumably thermally break the plates, and could even thermally break the rim joists with some creative framing(having the inner wall hold up the ceiling). I like the idea of the foam inside where ants would have less access to it, and windows could be detailed conventionally to solid sheathing.
Your ceiling/roof stack up seems too complicated, and I agree with Martin and Dana that there may be simpler ways to do it. Are you proposing a cathedral type ceiling?I'd feel better with a vented assembly there. Best of luck to you on your project. This is a great site for running ideas. Martin's thoughts are almost always useful. Dana's analyses are almost always useful as well, and other contributors can be useful as well.
Hi Kevin,
The 3" EPS has already been purchased as part of the order for the 6" EPS used in the new TF System I'm using. I do plan on the internal wall being the support for the roof and running the center foam in the wall to the foam in the roof for a non-stop full foam thermal break. If I properly detail the foam and keep the holes in it to a minimum it should make for a very tight air barrier.
I agree the roof is complicated but wanted the foam layer for various reasons. I'm struggling with keeping the cost down on other options which seems to be counter to the experience of others. I'll be checking with other sources on the option Dana outlined. I looked into the 18" 2x4 floor framed floor joist for the roof because I seen a passive house had used them but the cost was more.
I'm at least getting a better grasp on my options and I'll keep the door open for more until I get down to the time I need to get product on order.
I agree on the useful advice on this site. I've seen a lot of posts from Dana and Martin and have enjoyed reading them as well as others.
Carl,
If you want to integrate a layer of rigid foam into your roof assembly, you can. The way this is typically done is to install one or more layers of rigid foam above the roof sheathing, followed by a second layer of roof sheathing. Unlike the roof assembly that you are proposing, you don't need two layers of I-joist rafters with this method.
For more information on what I'm talking about, see How to Install Rigid Foam On Top of Roof Sheathing.
Dana or Martin or..., by stapling insulation net a couple inches down from the top of the I-joist that would create a vent from the eve to the ridge. Would that be enough of a vent to make the stack work?
I reread the how-install-cellulose-insulation article and some related posts but I still feel confused if dense pack would be workable. Seems like from what I read the venting with the insulation net would be better.
Martin, the extra layer of EPS needed will cost more than what I can get the extra I-joist and insulation for and I'll have higher R-value with the extra I-joist.
Thanks again for all the feedback. I think I'm getting close to a workable solution.
Carl,
Yes, you need a ventilation channel between the top of the insulation layer and the underside of the roof sheathing if you are installing cellulose insulation between your rafters. This is a code requirement as well as a good idea.
No, insulation netting is not a good material for a ventilation baffle, because insulation netting is air-permeable. What you want in this location is an air barrier. For more information on this issue, see Site-Built Ventilation Baffles for Roofs.