I am in Climate Zone 6 (on the Atlantic coast of Nova Scotia). Currently, my wall assembly will be as follows from the inside out:
- drywall
- poly vapour barrier
- 2x6 16in OC stud frame with R24 fiberglass batts in the cavity (still thinking about changing that to Roxul or even spray foam if the cost difference is reasonable)
- OSB sheathing
- Tyvek house wrap
- 1" XPS with taped seams
- vinyl siding

My builder was going to put the XPS foam directly over the sheathing and then was going to wrap the house over the top of the foam. I just saw a video from Dupont that this is how they recommend the installation goes.
However, after talking to my pre-construction energy advisor, he recommended that they wrap the house, install XPS and skip the taping of the foam seams to enable some outward drying of any potential trapped moisture. Also, every house I've seen around here gets wrapped first (on the sheathing) and then whatever exterior insulation is thrown over top of it. That doesn't mean it's right, but makes sense to me to protect the sheathing with a wrap first since any stapling and nailing that happens would penetrate that wrap if it is installed over the foam and may allow direct water access to the sheathing and be trapped there.

So, I ended up asking the builder to build the exterior detail as described above with the wrap directly on the sheathing, then boards around the window sealed, then the rigid foam and then siding. I think they may even put another wrap on top of the foam anyway.

Does this make sense or did I give them the wrong instruction?
They already started it this way on a portion of one wall and will be continuing on Tuesday.

Thank you!

P.S. After reading the excellent advice here and on GBA, I know that the best way to reduce the risk of moisture in the wall cavity in my climate is to add R12 exterior rigid insulation, but I cannot afford to add that cost to the build at this point. :( So, I'm sticking with R5.

Consider drywall then taped XPS (or EPS) on the inside, cellulose fill and then taped plywood.

Thank jonr.
I'm not sure that local city inspectors would pass that assembly even if building science supports it. They typically look for an interior vapour barrier.

In my current setup, do you think the wrap should go on the sheathing or over the XPS?

when we've used exterior foam, we install flashing and building paper (if used) outside of the foam. There should be no reason to get water behind the foam, and the since foam joints needs to be taped, paper under the foam is unnecessary.

The vapor permeance of the housewrap is very high relative to the foam. Which side of the foam to install it matters, not from a vapor permeance point of view, but on how you are going to mount & flash the windows. If the window glass is going to be roughly co-planar with the exterior siding, the housewrap goes on the exterior side of the foam, lapped properly with the flashing. If the glass is going to be roughly co-planar with the structural sheathing, use a crinkle type housewrap, and put it between the foam & plywood., with the window flashing properly lapped & taped to the housewrap.

Either way you install it, tape the foam, and reinforce it with a paint-over the tape with duct mastic at the edges to ensure a long term adhesion. Skipping the taping of the seams does next to nothing for drying capacity, but does create a measurable heat leak, and is thus a bad idea. If you want more drying capacity than 1" XPS (which runs about 1 perm) use 1" rigid rock wool, which at about 30 perms- as permeable as housewrap, or 1" unfaced Type-II (1.5lb per cubic foot nominal density EPS ,which is a bit under about 3 perms, comparable permeance to a coat of exterior latex paint. EPS is usually cheaper than XPS at any R value too. You can even go with 1.5" of EPS (R6) and have substantially higher permeance than 1" XPS.

Thank you Bob and Dana.
Yes, the windows will be pulled out of the sheathing so the rigid foam board come up to the flashing. They were essentially going to do this install:
http://www.dupont.com/products-and-services/construction-materials/building-envelope-systems/videos/install-homewrap-over-rigid-foam-board-and-window.html

I will call them tomorrow and ask them to install it over as they intended to do.

They already put a section of the wrap on the bottom directly on the sheathing since the siding contractor did not know that he was supposed to install the XPS too. They said they will leave the wrap that was already installed on the sheathing there and proceed with the rest as usual.

If the cost is not dramatically different, what shall we stuff into the wall cavity? I guess we'll have the interior poly, so worrying about spray foam not having any drying capacity in the cavity is likely pointless, correct?
In such case, would it be spray foam, then Roxul, then fiberglass in that order of thermal resistance preference?

The net difference in thermal performance of the cavity-fill is remarkably small, due to the high thermal transfer rates through the framing itself. With 16" on-center framing the framing comprises ~25% of the face area of the wall, but it has about 4x the heat transfer per unit area as closed cell foam, about 3x the heat transfer of Roxul. So the framing fraction is already at least half of the total heat transfer through that layer of wall.

The most important aspect of the cavity performance is to make it air-tight, and a complete fill with no compressions or voids. Caulk the framing to the sheathing (a power caulking gun and a case of acoustic sealant caulk makes quick work of it). Blown fiberglass at 1.8lbs per cubic foot density (Optima, Spider, L77) has about the same R-value as Roxul, but a guaranteed tight fit. Blown cellulose is slightly less, but at 3.5lbs/cubic foot (a standard "dense packing" density) resists convection better than 1.8lb fiberglass or standard Roxul. Cellulose at 5.5" thickness also has a modest thermal mass benefit, which narrows the performance difference between 1.8lb fiberglass and cellulose. Cellulose is also hygrosopic, which protects the structural wood by sharing the wintertime moisture load that would occur from minor air leakage through your interior vapor barrier.

Open cell foam is more air-tight than fiber insulation, with roughly the thermal performance of cellulose, but none of the protective aspects. It would eliminate the need to caulk the framing to the sheathing, but that's not a huge cost savings. It's also easy to screw up the mixture/temperature of open cell foam, but hard to remove a botched foam job. That said, there are experienced pros who can hit it right every time, and it goes in quickly.

When caulking, be sure to lay a bead on the seam between doubled-up framing such as top-plates, etc, and where the bottom plate of the studwall meets the subfloor, which are all bigger air leaks than is obvious.

While it's possible to do a good job with batts, not everybody does- make sure they tuck the corners & edges then lightly tug it out slightly proud of the stud edge for a complete fill. Simply stuffing them in until their flat to the stud edges goes faster, but it inevitably leaves voids where the studs meet the sheathing, a performance difference big enough to show up in thermal imaging. The other common faults are batts trimmed slightly too short with a void at the top or bottom or batts too long that buckle, with a compression somewhere in the middle.


If you can find it and the inspectors don't object, it's worth using Certainteed MemBrain instead of 6 mil poly for the vapor barrier. MemBrain meets Canadian code definition as a vapor barrier in an ASTM E96 dry cup test, but is higher than code permeance in a wet-cup test. What this really means in practical terms is that as long as the assembly is relatively dry it behaves as a vapor barrier, but should the cavity take on much moisture (from any source) it becomes more vapor open, which speeds drying by about an order of magnitude. Intello Plus is another such "smart" vapor retarder, but even harder to find (except online), but is tested to a European spec, not ASTM E96, which could make it a harder sell to an inspector in NS. By using a smart vapor retarder instead of 6 mil poly a great deal of moisture resilience is built into the assembly- think of it as cheap insurance.

http://www.certainteed.com/resources/30-26-074.pdf

Posted By Dana1 on 17 Feb 2015 02:15 PM
The net difference in thermal performance of the cavity-fill is remarkably small, due to the high thermal transfer rates through the framing itself. With 16" on-center framing the framing comprises ~25% of the face area of the wall, but it has about 4x the heat transfer per unit area as closed cell foam, about 3x the heat transfer of Roxul. So the framing fraction is already at least half of the total heat transfer through that layer of wall.

The most important aspect of the cavity performance is to make it air-tight, and a complete fill with no compressions or voids. Caulk the framing to the sheathing (a power caulking gun and a case of acoustic sealant caulk makes quick work of it). Blown fiberglass at 1.8lbs per cubic foot density (Optima, Spider, L77) has about the same R-value as Roxul, but a guaranteed tight fit. Blown cellulose is slightly less, but at 3.5lbs/cubic foot (a standard "dense packing" density) resists convection better than 1.8lb fiberglass or standard Roxul. Cellulose at 5.5" thickness also has a modest thermal mass benefit, which narrows the performance difference between 1.8lb fiberglass and cellulose. Cellulose is also hygrosopic, which protects the structural wood by sharing the wintertime moisture load that would occur from minor air leakage through your interior vapor barrier.

Open cell foam is more air-tight than fiber insulation, with roughly the thermal performance of cellulose, but none of the protective aspects. It would eliminate the need to caulk the framing to the sheathing, but that's not a huge cost savings. It's also easy to screw up the mixture/temperature of open cell foam, but hard to remove a botched foam job. That said, there are experienced pros who can hit it right every time, and it goes in quickly.

When caulking, be sure to lay a bead on the seam between doubled-up framing such as top-plates, etc, and where the bottom plate of the studwall meets the subfloor, which are all bigger air leaks than is obvious.

While it's possible to do a good job with batts, not everybody does- make sure they tuck the corners & edges then lightly tug it out slightly proud of the stud edge for a complete fill. Simply stuffing them in until their flat to the stud edges goes faster, but it inevitably leaves voids where the studs meet the sheathing, a performance difference big enough to show up in thermal imaging. The other common faults are batts trimmed slightly too short with a void at the top or bottom or batts too long that buckle, with a compression somewhere in the middle.


If you can find it and the inspectors don't object, it's worth using Certainteed MemBrain instead of 6 mil poly for the vapor barrier. MemBrain meets Canadian code definition as a vapor barrier in an ASTM E96 dry cup test, but is higher than code permeance in a wet-cup test. What this really means in practical terms is that as long as the assembly is relatively dry it behaves as a vapor barrier, but should the cavity take on much moisture (from any source) it becomes more vapor open, which speeds drying by about an order of magnitude. Intello Plus is another such "smart" vapor retarder, but even harder to find (except online), but is tested to a European spec, not ASTM E96, which could make it a harder sell to an inspector in NS. By using a smart vapor retarder instead of 6 mil poly a great deal of moisture resilience is built into the assembly- think of it as cheap insurance.

http://www.certainteed.com/resources/30-26-074.pdf

Thank you very much Dana!

It sounds I will be picking up a few bottles of an acoustic sealant and going over the whole house from the inside and lay a bead along all studs and plates where they meet the sheathing or floor.
Under the bottom plate, we have a vertical strip of rigid foam for thermal break of the concrete slab from the foundation wall. Should I lay a bead over the top of that rigid foam where it meets the bottom plate? Will that cause any grief to the floor tile installers? I believe they will be using a Ditra membrane, so a small bead should be fine around the perimeter, right?
Should I also caulk any horizontal gap between sheathing boards from the inside?

Since I've never done this, do you have any sealant recommendations? I found these:
http://www.rona.ca/en/sealant---aco...0330133--1
http://www.homedepot.ca/product/pl-...5ml/972916
http://www.kent.ca/kbs/en/product.j...kuId=40520

Also, the high tomorrow will be -5C, but Sunday is +5C but a ton of rain. Am I OK applying the sealant in these conditions?

I'm hoping to get Roxul for the cavity insulation and I will certainly look for that CertainTeed MemBrain. Sounds like exactly what I need to allow some drying to the inside if needed.

Thank you very much!!

Acoustic sealant is even more susceptible to being squeezed out when wood expands. Which leaves you with a leak when the wood shrinks backs. There is a reason that they make gaskets, tapes and highly flexible silicon sealants and that something like Tremco acoustic sealant is marketed only for acoustic use on drywall and for sealing lap joints on poly sheets.

I second that. You want to use high-grade sealants/caulks that will expand and contract with the wood. Wood moves, you can can't stop that. If the caulk/sealant doesn't move with it, then it will fail to seal very quickly.

Do you have any suggestions for specific caulking products I can buy tomorrow at my local stores (Home Depot, Rona, Kent)?

Acoustic caulk remains flexible forever and can squeeze out and suck back in forever. I am not sure there is a better sealant then acoustic caulk.
http://www.tremcosealants.com/fileshare/pds/AcousticalSealantDS-English.pdf

Any drywall supply store will carry it and out west all Rona and Home Depot stores carry it in at least the small tubes and most carry the large tubes.

You might want to read the comments here http://forums.jlconline.com/forums/showthread.php?45054-acoustic-caulk-with-poly-sheeting on the collateral effects of this most loved product before you start ;-)

Posted By eljay on 20 Feb 2015 09:31 PM
Do you have any suggestions for specific caulking products I can buy tomorrow at my local stores (Home Depot, Rona, Kent)?

Not all caulks and sealants are the same. Some are good, some perform poorly, while others are top of the line. I would try PROSOCO.

Consider an ASTM C-920, Class 50 or 100/50 rated sealant. Use a good tape to seal a film to a solid surface.

Thank you for all the input.
Didn't do anything today and I'm not sure we'll have time to make much progress tomorrow. The electrical will be roughed in this week, so we will do this in the evening. I guess the electrical crew may even appreciate not having this stuff all over the walls and cavities. Of course, it will be more difficult for us to caulk everything  after around the wires and outlet boxes etc.

I still haven't found a suitable product and after talking to a couple of construction guys at my hardware store, they look at me like I have two heads, especially, when I mention the smart vapour retarder. Most have never heard about it. I just hope that the inspectors are fine with it and that the builder can get it and install it. Perhaps, we should wait until we speak with the builder onsite to ensure they can and will install the MemBrain product. I guess it would be pretty bad to caulk everything with a sealant and then learn they cannot get the MemBrain and we'd be stuck with a sealed wall with a standard poly vapour barrier. We can already see that they put the standard poly sheets behind some of the interior walls where they meet the exterior stud wall, but it's only around those joints.

Here's a picture from our house. Did I mark the correct areas where we would need to apply the sealant?



The builder will be sealing the interior vapour barrier to the studs with an acoustical sealant. Is doing all the extra sealing of the studs to the OSB sheathing as discussed above and overkill since the interior will be air-tight with their vapour barrier application anyway? Or is the idea to keep air from entering the cavity from the outside and preventing condensation in the cavity?

p.s. I will confirm, but they will be caulking/sealing all around the windows and doors. The joists cavities at the exterior walls will be insulated with fiberglass and then squares of rigid foam will be spray foamed in place for an air-tight seal.

Posted By Lbear on 21 Feb 2015 11:12 AM

Posted By eljay on 20 Feb 2015 09:31 PM
Do you have any suggestions for specific caulking products I can buy tomorrow at my local stores (Home Depot, Rona, Kent)?

Not all caulks and sealants are the same. Some are good, some perform poorly, while others are top of the line. I would try PROSOCO.

Are you referring to the Prosoco R-Guard Joint & Seam Filler?
Not sure where I would find that stuff around here. :(

Posted By eljay on 21 Feb 2015 10:57 PM

Posted By Lbear on 21 Feb 2015 11:12 AM

Posted By eljay on 20 Feb 2015 09:31 PM
Do you have any suggestions for specific caulking products I can buy tomorrow at my local stores (Home Depot, Rona, Kent)?

Not all caulks and sealants are the same. Some are good, some perform poorly, while others are top of the line. I would try PROSOCO.

Are you referring to the Prosoco R-Guard Joint & Seam Filler?
Not sure where I would find that stuff around here. :(

Yes, PROSOCO is top grade stuff. Used on commercial high projects and residential. You won't find it at the big box stores. You will have to call PROSOCO and have them tell you where the rep is located in your area.

Thank you.

Can you comment on my picture above? Do I really need to seal all of those places I marked with green when the builder will be sealing around electrical outlets and any wall protrusions and will seal the vapour barrier to the studs?

Yea that's where you seal, where you put the green lines. Definitely second Dana's comments on the Membrain or Intelloplus. Don't use poly! Especially with only 1" rigid foam on the outside. There's a rule which indicates how much inner vs outer insulation you need to prevent dewpoint moisture. In my zone I put 1.5in XPS on outside and R19 batts in wall to prevent dewpoint condensation with Membrain to allow it to breathe to interior should any moisture condense (and it will eventually). I sealed with foam and caulk around the stud bays as well. Took me several weeks to seal. Good luck. Not for the faint of heart! Now that it is done, I am happy to have an awesome house. I'm finishing up the finals like cabinets and trim now. Occupancy soon. No contractor would have put in the time and patience as I have on my own home. It's worth it!

If you can find it and the inspectors don't object, it's worth using Certainteed MemBrain instead of 6 mil poly for the vapor barrier. MemBrain meets Canadian code definition as a vapor barrier in an ASTM E96 dry cup test, but is higher than code permeance in a wet-cup test. What this really means in practical terms is that as long as the assembly is relatively dry it behaves as a vapor barrier, but should the cavity take on much moisture (from any source) it becomes more vapor open, which speeds drying by about an order of magnitude. Intello Plus is another such "smart" vapor retarder, but even harder to find (except online), but is tested to a European spec, not ASTM E96, which could make it a harder sell to an inspector in NS. By using a smart vapor retarder instead of 6 mil poly a great deal of moisture resilience is built into the assembly- think of it as cheap insurance.

http://www.certainteed.com/resources/30-26-074.pdf