Final word on wall construction…?
I am building a new home in New Brunswick, Canada. We are all framed up (booming trusses today) and have been installing 2″ XPS rigid foam to the exterior and taping the seams as the walls went up.
Can someone give me clarity on these 3 issues:
1. Do I need to install tyvek over the 2″ sealed foam?
2. I read that I should NOT install poly vapour barrier on interior due to the 2″ exterior foam acting as a VB, thus allowing the walls to dry to the inside, BUT, do I need to install anything in place of the poly?
3. Can my deck ledger boards be installed over the 2″ XPS? If not, can you explain how it should be installed/sealed, etc?
Thank you in advance.
Kind Regards,
David
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
David,
Q. Do I need to install tyvek over the 2" sealed foam?
A. It's up to you. You certainly need a WRB. If you don't use housewrap as your WRB, that means that you must detail the foam as a WRB. While this last option is challenging, some people are doing it. You must integrate all of your window flashings with your WRB, no matter which route you take.
Q. I read that I should NOT install poly vapour barrier on interior due to the 2" exterior foam acting as a VB, thus allowing the walls to dry to the inside, BUT, do I need to install anything in place of the poly?
A. Vapor-retarding paint, kraft paper facing, or MemBrain all work fine.
Q. Can my deck ledger boards be installed over the 2" XPS? If not, can you explain how it should be installed/sealed, etc?
A. The best approach is an independent deck on independent footings with no structural connection to the house.
1. Do I need to install tyvek over the 2" sealed foam?
No, you can use Typar (which is a better product) or any of the multitude of non-perforated WRBs.
2. I read that I should NOT install poly vapour barrier on interior due to the 2" exterior foam acting as a VB, thus allowing the walls to dry to the inside, BUT, do I need to install anything in place of the poly?
It depends on your code requirements. In the States, where we have vapor and not vapour, the code requirement is for a minimum 1 perm vapor retarder (not vapor barrier), and Martin's suggestions will meet this standard.
3. Can my deck ledger boards be installed over the 2" XPS? If not, can you explain how it should be installed/sealed, etc?
An free-floating deck is fine as long as you can guarantee that it will not settle (or heave) differentially from the house. Otherwise, XPS has sufficient compressive strength to bolt your deck ledgers through (as long as they're tied to solid wood framing such as the band joist), or you can accept a minor thermal bridge and build out 2" of wood nailer (such as a 2x4 over ½" CDX) to secure the deck ledger to. In either case, I would insulate the band joist on the inside to minimize thermal losses through the heavy steel fasteners.
Thanks guys.
Regarding question 2, what purpose does the kraft paper serve, air barrier? Would tyvek faced in be a good alternative?
Robert, hope you're not making fun of how us Canadians spell, eh! :-)
The Kraft paper facing on fiberglass batts is a class II vapor retarder, but is often discontinuous at the studs and makes it more difficult to make sure the batts fully fill the stud cavities. I never recommend Kraft-faced batts. It does not comprise an air barrier because of the discontinuity and the likelihood of hidden punctures by fasteners later placed in the drywall.
Please don't use "tyvek" like "kleenex". They're both brand names, not generic descriptors. The generic is "weather-resistant barrier" (WRB) or "housewrap". There are better housewraps than Tyvek.
If the purpose of an inner membrane is to create an air barrier, then polymeric housewrap would work, as long as all seams were taped and all edges caulked. If the purpose is to limit outward vapor diffusion, then highly-permeable housewrap on the interior would be counterproductive.
I bet you don't like to go ski-doo'ing either?
JK, good point.
I am a bit of a novice. Can you please spell it out for me. What should my wall construction look like if it is to dry to the inside as this article instructs:
https://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/can-exterior-foam-insulation-cause-mold-moisture-problems
-vinyl siding
-2" XPS taped tight to act as WRB
-OSB
-2x6 wall with R20 fiberglass in cavities
-housewrap sealed/taped tight
-gyprock (not sheetrock :-)
Does that look right?
All help is much appreciated. Thanks again.
No, I don't like snowmobiling. But that' s another story.
I can't recommend the "proper" method for building an exterior-insulated wall since (as noted in the blog you linked to) I believe it creates more problems than it solves.
What is the code requirement for an interior "vapour" retarder in your part of Canada?
Here in NB it is widely popular to insulate exterior with XPS, but all builders are still installing 6mil poly on interior. I assume it is a new understanding that this can cause problems and they just don't know better yet, maybe it is still code, I will call my inspector tomorrow to find out
At this point, I have already committed to the exterior XPS as it is already installed, so now i just want to get it as right as I can from here on.
Thanks.
David,
Your proposal to install interior housewrap is mystifying to me.
1. Housewrap is not a vapor retarder.
2. If you intend the housewrap to be an air barrier, you should consider easier alternatives. Your rigid exterior foam sheathing can be an air barrier if properly detailed; so can your interior gypsum wallboard, if you follow the Airtight Drywall Approach.
Thanks Martin.
I thought that I shouldn't have any vapor barrier or retarder on the interior, that's why I thought housewrap would work.
I guess what I really need is for someone like yourself to tell me what they would do in my position.
Also, when you refer to detailing the exterior foam, does that just mean taping all the joints? That is what we've done.
David,
Here's some basic reading to get you started:
Vapor Retarders and Vapor Barriers
Forget Vapor Diffusion — Stop the Air Leaks!
One Air Barrier or Two?
Vapor Profiles Help Predict Whether a Wall Can Dry
Q. When you refer to detailing the exterior foam, does that just mean taping all the joints?
A. No, it means a lot more than that. Every penetration, including windows, doors, plumbing penetrations, wiring penetrations, and handrails, needs to be properly flashed. (Understanding the best way to flash penetrations is a separate -- but very important -- field of knowledge.) Once these penetrations are flashed, the flashings need to be integrated with the WRB. This can be done with an overlap (a method that depends on gravity) or with a high-performance tape (a more risky strategy that depends on chemistry).
Vertical seams between rigid foam panels need to be taped, as you point out. Horizontal seams can either be taped or flashed with Z-flashing.
The main advantage of using housewrap as a WRB (instead of rigid foam) is the fact that housewrap allows easy lapping of horizontal seams, whereas rigid foam does not.
David,
Although your proposed wall will probably work without a vapor retarder, the assembly becomes riskier as interior humidity levels increase. Moreover, you need to meet local code requirements, which probably include a vapor-retarder requirement.
In the northern areas of the U.S., it is usual for walls to include a vapor retarder. As I explained in a previous post, good options include vapor-retarding paint, kraft facing, or MemBrain.
just spoke to my local inspector and he confirmed 6mil poly is minimum code. he was determined that the XPS will not be enough of a VB to stop the wall from drying to the outside.
David,
You should determine the actual language in the code, either by asking your local building official or obtaining a copy of the code book.
If the code calls for "a vapor retarder with a permeance no higher than 1 perm," or language to that effect, you may be able to convince your code official that products other than polyethylene meet your local code requirement.
VERY interesting article from the manufacturer, what are your thoughts?
http://commercial.owenscorning.com/assets/0/144/172/174/e45fe07d-5cc9-4e4b-866a-5e35d75090ec.pdf
"A common interior vapor retarding material such as 6 mil poly has a rating of 0.06 perm.
Using the perm comparison alone, FOAMULAR® XPS (higher perm rating) lets water vapor out, much faster than 6 mil poly (lowest perm rating), lets water vapor in. With that imbalance of inside(lower) versus outside (higher) perm ratings, the possibility of trapped moisture, or a “double vapor barrier, is reduced."
David,
The thicker the XPS foam, the lower the perm rating. If you want the wall to dry to the exterior, you're stuck with thin foam -- making the sheathing subject to condensation. Thick foam stops the condensation risk, but lowers the drying potential.
The possible sources of moisture inside the wall are not merely diffusion through the 6 mil poly; they also include the occasional penetration of wind-driven rain, as well as moisture piggybacking on exfiltrating interior air. All moisture sources must be considered when designing a wall with adequate drying potential. That's why the Owens Corning statement is potentially misleading.
David,
Owens Corning is selling a product, not educating the public.
Their argument for double vapor barriers considers on vapor diffusion, which we know to be the least important method of vapor intrusion into a building assembly (when relative humidity is within normal limits). The primary mechanisms of wetting are ignored, including convection and bulk leakage.
To say that a 2" XPS layer (perm 0.55) will allow drying by vapor diffusion is like saying that R-38 insulation allows heat to escape (sure, but not much). In a cold climate, most drying is to the outside, so it's sensible to use a relatively vapor-permeable (perm ≥5) exterior, and it's imperative when there is an impermeable interior layer like 6 mil poly.
The argument that exterior insulation keeps the sheathing above the dew point most of the time is also an argument that it keeps the sheathing at temperatures more conducive to mold and decay organism growth most of the time. Such vapor-closed assemblies work only if they don't start out wet (all new construction is moist) and never leak. This is, of course, an impossible hope. All buildings start out wet and get wet.
Such reliance on plastic membranes, whether vapor barrier or foam "outsulation", also ignores issues of moisture buffering, liquid diffusion, hygric redistribution, capillarity, and the reduced heat flux that is necessary for drying to occur.
Double vapor barriers are always a bad idea, whether the layers are true vapor barriers (class I retarders) or merely code-compliant vapor retarders (class II). By Building Science (Joe Lstiburek) standards, a material isn't considered "vapor-permeable" until it reaches a perm of 10. I'd settle for 5 perm as a breatheable layer.
Regarding the deck ledger connection detail, I would consult your building inspector, or build a free standing deck as suggested. In areas of the US governed by IRC, we cannot space the ledger more than 1" away from the face of the structural sheathing (which XPS is not) and this must be accomplished with washers, not compression of any material between the ledger and the sheathing/rim joist (IRC '09 R502.2.2.1 and table R502.2.2.1 (see note b and h). So, in the US we cannot "sandwich" the foam between the ledger and the rim joist. Exacerbating the problem, if your foam is over wood structural sheathing for your wall bracing, The thickness of this sheathing counts towards your 1", so if you have 1/2" ply, only 1/2" of foam is permitted, and must be washered. Good luck! I hope Canada is easier to navigate than the US on this issue.
There is an error in my above answer; the line 1" away from the face of your structural sheathing is wrong. It is 1" away from your structural framing (band joist or properly solidly-blocked end joist).