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2x4 wall buildups with polyiso/air barriers/vapor retarders

Hi all,
I am building a 16x40 / 2 storey residential addition in climate zone 7 (Columbia Valley, BC Canada)
My wall section that I am currently specing is as such:
1/2" GWB
2x4 framing, c/w roxul R14 batt insulation
1/2" plywood sheathing
1-1/2" polyiso
3/4" rain strapping
Wood siding.

My question is mostly concerned with the right polyiso product to use and how this will affect whether or not we have to install an interior vapor retarder, and if so which class - 1, 2 or 3?
A lot of this will depend I imagine on the type of polyiso and their respective perm ratings ie foil back (atlas energy shield), fiberglass (Atlas rboard) or basic unfaced, cardboard backed.
Also whether or not we then tape and caulk all seams of the polyiso to make it air tight.
If the energy shield or R-board polysio is taped and detailed properly it can also be used as the WRB and Air barrier, otherwise I would need to employ a housewrap of some description between the polyiso and the rain screen.

My choice would be to use either the energy shield or Rboard, taped, flashed and detailed to be the effective air and water barrier and eliminate both the exterior house wrap and interior vapor retarder, although class 3 could be used.

The local inspector and code still prescribe a VR of some sort but I think we could remove it and we would lessen the risk of "double bagging" the wall assembly which would not allow it to breathe.

I know this is long winded but I hope it makes sense to someone out there who can lend me their advice on the best way to proceed. I am almost done framing and will soon need to decide on the polyiso in order to start getting my windows in.

Asked by jeremy harris
Posted Sep 3, 2014 5:12 PM ET
Edited Sep 5, 2014 1:11 PM ET


28 Answers

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That's an insufficient amount of exterior-R for R14 cavity fill in a zone-7 type climate, to be able to skip interior side vapor retarders, especially when you properly derate the polyiso for it's performance at mid-winter temperatures. You would need at the very least R10 performance from the polyiso in winter and at 1.5" you would be looking at something like R7-R8 whenever the the outdoor temps are below -5C (which is probably warmer than your wintertime temperature average.)

At 1.5" even with a paper facer most polyiso would be only about 1.5 perms. While that isn't nothing, but not a super drying capacity.

Putting Certainteed MemBrain or Intello Plus on the interior side would make it work though. In winter when the RH in the house is under 35% the MemBrain would be running less than 1-perm, slowing the rate of moisture migration into the cavity. But in the spring when the house is warming up and the sheathing is releasing it's moisture, raising the air in the cavity above 50% RH the MemBrain goes well above 5 perms, and your interior wall paint becomes the limiting factor on the drying rate. This behavior makes these materials something of a "moisture diode", with a much faster drying rate in spring/summer than the moisture accumulation rate in the winter.

Answered by Dana Dorsett
Posted Sep 3, 2014 5:42 PM ET


While the BC building code does require a vapour barrier, it allows you to use poly, membranes, gypsum board, sheet and panel-type materials or foamed plastic insulation as long as it meets a certain permeance criteria. (, (9.25.5) so you have more options than your inspector is letting on.

Answered by Malcolm Taylor
Posted Sep 3, 2014 8:13 PM ET


If you can convince your building inspector that you don't need an interior vapor retarder, that's the best way to go. Walls with exterior rigid foam are designed to dry to the interior. For more information on ensuring that your foam is thick enough to keep you out of trouble, and for information on the theory behind this type of wall, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

If your building inspector insists that you need some type of interior vapor retarder, choose a smart retarder like MemBrain -- not polyethylene.

Concerning the need to tape the polyiso: all walls need to have an air barrier, but you don't have to depend on taped polyiso as your air barrier. Many builders tape their sheathing -- in your case, plywood -- and use that layer as the air barrier.

Answered by Martin Holladay
Posted Sep 4, 2014 6:24 AM ET


Hey all thanks for the response.
Based on the above and reading the link that MArtin posted, I may have to look at upping my insulation to 2". Not really a big issue, just a little added cost.
I like the idea of taping the sheathing AND the polyiso so I get both an effective air barrier and WRB / drainage plane.
I will skip the interior poly, and will research more on the appropriate VR that will be accepted by my local inspector.
I was suprised to read that closed cell foam should not be used for the interior cavity with the exterior rigid foam?

Answered by jeremy harris
Posted Sep 4, 2014 2:33 PM ET


It's a good idea for the sheathing to be able to dry in at least one direction -- so if you are installing exterior rigid foam, you don't want to block drying to the interior.

Answered by Martin Holladay
Posted Sep 4, 2014 2:40 PM ET


fair enough
i just read thru the article again and tried to reconcile your comments above with the alternative flash and batt method?
Am i missing something here?

Answered by jeremy harris
Posted Sep 4, 2014 2:59 PM ET


The typical flash-and-batt installation does not include any exterior rigid foam. That means that the plywood or OSB sheathing can dry to the exterior.

Answered by Martin Holladay
Posted Sep 4, 2014 3:17 PM ET


If going to 2" of foam in a zone 7 climate with R14 cavity fill you'll be better off with the exterior 1" being EPS, and the inner 1" being polyso. This is due to the severe performance penalty that polyiso has when the average temperature through it is below freezing.

With polyiso the performance peaks when the temperature through the polyiso layer is about +15C, but when the average temp then drops back to it's rated-R at an average temp of +10C. When the average temp through the layer is 0C, its R-value is less than half it's rated value. It's a strange and non-linear curve, but in a zone-7 climate don't expect the outer inch to perform better than R3 on average during the winter months.

But EPS outperforms it's rated R as it's average temperature falls- when the average temp through an EPS layer is -5C it's actual performance is about 12% better than labeled. The crossover temperature at which EPS begins to outperform polyiso is about +5C, so swapping the outer layer for EPS would keep the remaining iso warm enough to still have some performance.

So while the rated R of 2" of polyiso is about R12.5, compared to about R10.5 for the 1" EPS + 1" polyiso stackup, during the months that matter the 2" of polyiso will perform at less than R10, whereas the EPS + polyiso stackup will still be running about R10. (2" of just EPS, no polyiso would run about R9 for a winter average.)

In fact, in a zone 7 climate you're probably better off with 1.5" of EPS + 1" of polyiso, extending the performance range of the polyiso. At a 22C mid foam temp (the temperature at which this stuff gets tested for labeling) a stackup of 1.5"EPS + 1" polyiso stackup performs at about R12.5, same rated R as 2" of polyiso. But during cold weather it will outperform 2" of polyiso by quite a bit.




Note in the concluding paragraph of the article where it states:

"Use a hybrid insulation approach – install cold temperature-tolerant insulation over top of the polyisocyanurate insulation to increase the mean temperature of the polyisocyanurate."

That is the most-appropriate approach for insulating sheathing applications on a batt insulated studwall in a zone-7 climate. Simply de-rating to an R5/inch estimate would only be appropriate if the polyiso were the ONLY insulating layer (as is often the case in commercial applications.) By giving it a blanket of EPS to keep it warm the iso layer will hit the low to mid-5s/inch performance during mid-winter, at the same time the EPS is performing a the mid to high 4s/inch (rather than it's rated R4.2/inch.)

Answered by Dana Dorsett
Posted Sep 4, 2014 4:40 PM ET


Sorry I was assuming that they still were factoring in a layer of exterior foam when talking about the flash and batt - my mistake?
Have you had any experience with Atlas sheathing products?
I was looking at two of their products - energy shield and Rboard.
Based on the above I would now assume that the lower perm rating for my exterior foam the better?
This would be the energy shield (foam backed) with a .3 perm rating as opposed to the rboard with a 1.05 perm rating.
Is my thinking correct or is this perm rating negligible?

Answered by jeremy harris
Posted Sep 4, 2014 4:42 PM ET


Correction - energy shield is foil backed, rboard is fibreglass

Answered by jeremy harris
Posted Sep 4, 2014 4:42 PM ET


If you install rigid foam that is thick enough to keep the sheathing above the dew point in winter, and if the wall assembly is designed to dry to the interior, then the perm rating of the rigid foam layer is irrelevant.

A low perm rating means that the material is relatively impermeable to water vapor. A high perm rating means that the material is relatively permeable to water vapor.

Answered by Martin Holladay
Posted Sep 4, 2014 5:01 PM ET


Thanks again for your input.
I was just curious if you would consider the perm rating of the r-board (1.05) sufficient enough to allow the wall assembly to dry to the exterior? I would still be specing 2" polyiso (r 12), and taping and sealing all joints to create my air and wrb. But it would give me the option to then spray foam the interior cavity to and use a typical vr - ie 6 mil polyethelene.

Answered by jeremy harris
Posted Sep 5, 2014 9:14 AM ET


The traditional definition of a vapor barrier is a material with a permeance of 1 perm or less -- which for all intents and purposes is the material you are describing. So, no -- you won't get any meaningful drying through a 1 perm material.

It's fine to install exterior rigid foam, as long as it is thick enough to keep the wall sheathing above the dew point in winter. But this approach requires that you specify vapor-permeable materials on the interior side of the wall sheathing. (That means no closed-cell spray foam between the studs, and no interior polyethylene).

Answered by Martin Holladay
Posted Sep 5, 2014 9:36 AM ET
Edited Sep 5, 2014 9:38 AM ET.


Thanks Martin, got it.
And based on all the literature you specify from the IRC there is enough scientific backup to take to my local building to show that I can design my assembly as such.
Do you still find that there is resistance at the local/municipal level to the removal of a class 1 VR on the interior when using exterior foam?

Answered by jeremy harris
Posted Sep 5, 2014 10:52 AM ET


Q. "Do you still find that there is resistance at the local/municipal level to the removal of a class 1 VR on the interior when using exterior foam?"

A. The level of resistance to building science varies hugely from one municipality to another. The biggest remaining problems occur in Canada and Minnesota. While it isn't hard to marshal evidence that supports the concepts summarized in this Q&A thread, the marshaling of evidence provides no guarantee that the ears and brains at your local building office will be receptive. Some are, and some aren't.

Answered by Martin Holladay
Posted Sep 5, 2014 11:02 AM ET


"Do you still find that there is resistance at the local/municipal level to the removal of a class 1 VR on the interior when using exterior foam?"

Where the vapour barrier can be placed, using exterior foam, the permeance of all construction materials that can be used as vapour barriers - and answers to all the questions you are asking are deal with directly and in detail both in the code reference I provided and in the code appendix table (1)
It really is worth a look rather than just speculating.

Answered by Malcolm Taylor
Posted Sep 5, 2014 11:19 AM ET


A more nuanced definition of a vapor barrier is 0.1 US perms or less (aka "Class-I vapor retarder", with 0.1-1 perm being considered only "semi-impermeable" (aka "Class-II vapor retarder.") The ubiquitous 6-mil polyethylene runs about 0.05 perms or a bit less. Foil facers on foam board are similarly low-permeance.

You DO get meaningful drying through 1-perm layers, but that's not to say the assembly can dry very quickly through only 1 perm. Two heavy layers of interior latex paint on wallboard runs about 3 perms- not super tight. It takes 3-5 weeks of drying through 1-perm foam for every week that it would take to dry through standard latex paints, which is NOT a disasterously slow drying rate. But you get effectively zero drying through Class-I vapor retarders, so 6 mil poly on the interior side of an assembly with exterior foam rates less than 1-perm has some risk.

For protecting the sheathing from interior side moisture drives in winter a Class-II vapor retarder between 0.5-1 perm on the interior side is usually sufficient for limiting the level of moisture accumulation in the sheathing, provided it's air tight, but you would want at LEAST a half-perm of drying capacity toward the exterior if you drop below 0.5 perms on the interior. Smart vapor retarders are great, since they are below 1 perm when they need to be in winter, but well above 1 perm when the moisture levels inside the wall are too high.

The 2" of exterior polyiso does NOT deliver R12 performance during the cold season in a zone 7 climate, and won't even perform at R10 in mid-winter, which makes it risky. A stackup of 1" of polyiso + 1" of EPS would be (barely) enough though, since EPS performance increases with low temperature, whereas polyiso performance takes a nose-dive. Allow yourself the time to digest my prior post (including the linked diagrams & articles) regarding the derating of polyiso for cold temperatures. The derated performance of 2" polyiso is not adequate dew point protection for you stackup in your climate, whereas 1" polyiso +1" EPS does make the grade.

If polyiso performance stayed at R12 during the winter it would be enough, but the real performance hit is too severe. Even in much warmer climates than yours (say, North Carolina or Tennessee) you'd have to derate it to about R10 for dew point control in an exterior sheathing application, R11 if the polyiso is the only insulating layer (as it often is on commercial building roofing, where the derating standards are usually applied.)

Answered by Dana Dorsett
Posted Sep 5, 2014 11:29 AM ET


"Where the vapour barrier can be placed, using exterior foam, the permeance of all construction materials that can be used as vapour barriers - and answers to all the questions you are asking are deal with directly and in detail both in the code reference I provided and in the code appendix table (1)
It really is worth a look rather than just speculating."

Yes I have looked but what I find as a journeyman carpenter and builder with recent code changes and differing opinions that it is hard to get definitive answers. The code book is also sometimes confusing and contradictory, so I wanted to clear some issues up with the panel of well informed people that are on this site.
Also the officials enforcing the code often have to be directed to alternative views and evidence before being willing to discuss code interpretations.

Answered by jeremy harris
Posted Sep 5, 2014 12:52 PM ET
Edited Sep 5, 2014 1:16 PM ET.


I realised the LTTR of the polyiso, as we use it alot on roofs around here, usually two layers of 4" on our vaulted timberframe ceilings but never on the reduction due to cold weather. I will read your links further and think again. The 1" poly/1" EPS sounds interesting and also allows an overlap of materials which is always nice to have. I am guessing the poly would go against the sheathing, EPS outside?

Answered by jeremy harris
Posted Sep 5, 2014 1:06 PM ET


Yes we have had issues even getting straight answers from building envelope engineers and professionals, let alone building code officials who are sometimes just trying to avoid conflict by following the code and limiting their liabiltiy.
There still seems to be a lot of differing opinions out there.
I really appreciate the input on this site, thanks for all your prompt and enlightening responses.
Much to think about

Answered by jeremy harris
Posted Sep 5, 2014 1:16 PM ET


God knows I agree with you. Much of the code seems like it was written by people with a fair understanding of the issues but no idea how to describe what they are trying to get across.
However I do find the section on vapour barriers, especially the appendix, really quite lucid.

The section of A- on Sheathing reads:

"Like cladding, sheathing materials have different permeances and different degrees of susceptibility to moisture deterioration.

Low permeance sheathing... ( and the table below lists the ones, including plywood and some rigid insulation, under 60 ng/pa.s.m2)… may serve as the vapour barrier if it can be shown that the temperature of the inner surface of the sheathing will not fall below that at which saturation will occur. This may be the case where insulating sheathing is used"

So they are inviting you to use the tables Martin provides to calculate the thickness of the exterior insulation necessary and eliminate the inner vapour barrier.

I wish I could reproduce or link to the entire section. I think posters here would be surprised at how sound and progressive the approach is, and how many options it allows.

Answered by Malcolm Taylor
Posted Sep 5, 2014 1:52 PM ET
Edited Sep 5, 2014 1:53 PM ET.


In many cases the problem is not with the code; the problem is with a poorly informed and rigid code official. Of course, most (but not all) code officials can eventually be convinced if a relevant code paragraph can be shown to them.

Answered by Martin Holladay
Posted Sep 5, 2014 1:58 PM ET


Thanks for clarifying that, I will investigate further and get more backup to take to my local inspector.
Lucidity is nice.
Unfortunately the code often just reads like legalese and sometimes makes one boggle eyed!

Answered by jeremy harris
Posted Sep 5, 2014 1:58 PM ET


"I am guessing the poly would go against the sheathing, EPS outside?"

Assuming that "...poly..." means polyisocyanurate (and not polyethylene) yes- the EPS goes on the outside, the polyiso goes next to the sheathing. EPS gains performance above its rated-R by being colder, whereas the polyiso loses performance at low/very-low temp, and the EPS keeps it warm enough to avoid COMPLETELY falling off the performance cliff. At the coldest hours of the coldest days the EPS might be performing at about R5, whereas the polyiso would be running about R4, but over the mid-winter average you'll be getting something like R4.5 out of the EPS, and R5.5 out of the polyiso, whereas with 2" of polyiso you'd be looking at R9-ish for a winter average in that climate. The winter average is what matters from a moisture control point of view. (Making the EPS 1.5" with 1" of polyiso would be even better, it gives you some margin.)

The shoulder season average of the 1" +1" dual-foam stackup will be about R10.5-11 whereas the polyiso would be hitting around R12, but during the shoulder seasons the sheathing is already warm enough.

Answered by Dana Dorsett
Posted Sep 5, 2014 3:30 PM ET


Yes I did mean Polyiso and based that assumption on what your reply clarified. Thanks for getting back to me with your detailed take on things.
I do have the option of upping my polyiso to a 3" cardboard faced for a good price as we use that alot on our roof systems. It will obviously expand my jambs etc but not real issues design wise. It is not able to be detailed to be an air or WR barrier, but I will still tape all seams and make as airtight as possible and will now employ typar or eq. housewrap along with 3/4" strapping/rain screen on top of this, followed by wood siding.
Based on all the comments above I should be able to satisfy the critieria for keeping the sheathing above the dewpoint for my climate zone.
And I can give the building dept the backup reqd to ensure that we can eliminate the interior VR.
I will look into the smart VR as I have never been exposed to that and it might be worth considering, subject to pricing of course!!

Answered by jeremy harris
Posted Sep 5, 2014 3:57 PM ET


I have a lot of sympathy for inspectors. I spend an inordinate amount of time trying to keep on top of our Part 9, which covers most small residential projects, and still have large gaps in my knowledge. Part 9 represents a quite a small part of the increasingly complex code. Inspectors are supposed to understand the whole mess, plus pertinent zoning bylaws, and in rural area water and septic may fall to them too. Small wonder they retreat to what they have always seen done and avoid falling into the role of building scientists.

Answered by Malcolm Taylor
Posted Sep 5, 2014 4:01 PM ET


Now onto the next loaded question - innie or outie windows??

Answered by jeremy harris
Posted Sep 5, 2014 4:01 PM ET


Ditto as a builder.
Sometimes on larger projects we employ code consultants to make sure we are in compliance, as there are constant evolutions in products or buidling science and a lack thereof for the code to keep up.
ALways good to learn more and I appreciate the depth of knowledge flowing around on this site and use it to stay abreast of different protocols, procedures and product.

Answered by jeremy harris
Posted Sep 5, 2014 5:52 PM ET

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