Exterior wall assembly with EPS
Hello,
I am working on final details for the envelope on a new build in British Columbia, Climate Zone 6 [https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/construction-industry/building-codes-and-standards/guides/climatezone5-7a.pdf ]
I am looking for opinions on wall assembly, I am hoping to use an EPS (right now I am planning to use SilveRboard [https://www.amvicsystem.com/products/hd-foam/silverboard/]) but have some concerns/questions.
Wall assembly plan is as follows:
Westform ProLok or Duraclad metal siding
1×4 strapping
3 layers of 1.5″ SilveRboard EPS taped seams
1/2″ OSB w/ taped joints
2×6 framing @ 16″o.c
1/2″ drywall
Latex paint
We are just finishing up the roof framing and will be getting the envelope closed up next. Before ordering any products I want to ensure that this assembly will perform as I expect. We are wanting to avoid stud cavity insulation altogether. R20-22 is our goal while trying to achieve as airtight as possible. Every precaution has been taken up to this point to plan for airtightness.
I know 3 layers of 1.5″ seems silly. But it is the cheapest option I can find in my area. We are fairly remote and delivery charges or special orders become very expensive very quickly. My local hardware stores (Home Hardware and Timber Mart have been very little help so I have been looking to neighboring communities.
This type of building assembly is fairly new in my area and not many people (building officials included) have much knowledge on the practices. The main concern I have is with permeability/breathability of the assembly. I know more layers of EPS will affect the perm rating but what I don’t know is if this should really be of any concern. I’ve been told by an Amvic technical advisor as well as a Beaver Plastics advisor that the OSB does not need a WRB of any sort with these types of products and taped joints on the sheathing layer will achieve my airtightness that I am looking for.
The SilveRboard is about $1.00 CAD/sqft for their claim of R22 (3 layers of 1.5″ @ R5/inch). I have been quoted two other products which are more expensive.
I am wondering if anybody has any advice on which route to take with an assembly like this? Should I be using a WRB over my sheathing or is it redundant in this application? Is there a simpler/better/higher performance option that I am missing which would cost similar?
I appreciate anyone’s time taken to read this and reply! Thanks 🙂
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Replies
Reflective facing only adds R value where it is facing an air cavity. So de-rate it slightly.
> We are wanting to avoid stud cavity insulation
I'm curious - why? It's low cost and effective.
Consider Drainwrap between OSB and foam - for drainage, not air tightness.
Don't worry about low perms - your proposed "all exterior foam" wall doesn't need to breath to the exterior.
With no cavity wall insulation it looks like you are going for a perfect wall house. Matt Risinger has a video on one he did in Texas --- https://www.youtube.com/watch?v=d8i-93ABo3I
Hi Jon,
From what I understand there is a safe ratio of interior to exterior insulation? Or does this not apply when eliminating the vapor barrier entirely from the interior side of the assembly? There is a house a colleague of mine is building very similar with EPS exterior ~R20 and then an R16 inside. Would fiberglass batt be recommended in the cavities?
Would drainwrap be a better choice than something like conventional Tyvek or Novawrap? I have a hard time getting products in my town. Drainwrap would be a special order item and its cost seems quite a bit higher than Housewrap or Novawrap.
Tim,
I have watched and am a fan of Risingers videos. I would prefer not to spend a few thousand on a peel/stick WRB though.
There isn't a single "safe" ratio. There is a range of ratios that, depending on climate, interior humidity and interior to exterior perm ratio, will work without the sheathing accumulating too much moisture. In Zone 6, 3" of vapor permeable (no facer) EPS over a batt filled 2x6 wall and interior side Class II vapor barrier paint will work well with a conservative margin.
Hey everyone,
Thanks for getting back to me, sorry to dig up an old thread. I am running in to some other concerns as we further progress on this build. We have decided on R14 RockWool in the cavities as well as 4.5" of EPS on the exterior. I'll try to provide some background as simply as I can. I have also attached two drawings for reference.
Foundation is 6" ICF on a 28" footing (4' crawlspace)
Typical framed box I-Joist w/ OSB Rim (LVL Rim for deck connections)
Originally my plan here was to extend the wall insulation with a below grade rated EPS over the box joist and connect it to the ICF foundation. I have also considered spray foam as an alternative. The deck connections on either end of the house would be built using a method I have seen on these forums before with stand out blocks and through bolts https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2018/08/08100708/Deck%20ledger%20attachment%20detail%20-%20CCHRC_2-700x511.jpg. I have also considered Maine Deck Brackets but don't know how our building officials would feel about that..
My main question: Is there any issue with going from exterior insulation to using spray foam in specific areas? This leads me in to my second concern; the roof.
The roof is a 10/12 which changes to a 4/12 over the decks. The pink highlighted areas on the engineered drawing is where the roof changes pitch over the exterior wall and where we will be framing in the truss ends with plywood so that they can be spray foamed. As it is now - the entire underside of the roof deck will be foamed with 6" closed cell foam (leaving the areas where this would be useless, obviously). This will continue down the plywood ends above the exterior walls. Exterior insulation will be used on the gable ends of the house with a 1" closed cell foam sealing the gables on the inside. The attic space will have air circulation via the furnace/air handler/hrv.
Is this assembly going to give us any grief? Other than the cost and being specific about where the foam goes..