Can I install XPS foam board over plywood sheathing, when using concrete board siding?
m2ew
| Posted in Energy Efficiency and Durability on
I was also going to fill the interior stud walls with Fiberglass bats backed with 6 mill vapor barrier. I’m located in southern ontario Canada; just outside of Toronto.
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Mike,
Yes, you can. Here are two articles to help you:
How to Install Rigid Foam Sheathing.
Calculating the Minimum Thickness of Rigid Foam Sheathing.
By the way, don't install any interior polyethylene if you choose to install exterior rigid foam.
Hi Martin,
I guess I have further questions then. Part of the reno is installing a washroom (powder room; no shower or tub) which would infact 2 of the 4 walls would be on the above mentioned exterior. In this case would it be better to forgo the exterior rigid foam and do interior insulation / vapor barrier?
Also the entire space is over an exterior slab / edition that is 50 years + years old. Below it I have put 2" foam and spray foamed the gaps in joist spaces. I was going to continue the 2" foam to the ground (inside slab) and encapsulate the whole thing with vapor barrier over it; from the joist spaces all the way down, including over the dirt / gravel floor. Will that method work if I do go the exterior route?
Mike,
Your wall assembly will not be able to dry to the interior due to the poly and will only be able to dry to the exterior very slowly due to the xps.
You will want to either omit the poly and replace with vapour barrier paint, replace the xps with a more vapour open roxul board product, or use enough xps on the exterior that you don't need any interior vapour barrier.
Also, you should use a vapour open wrb between the sheathing and the exterior insulation and strapping on on top of the exterior insulation to create a drainage/ventilation gap behind your siding
Ok I get it now. With the crawlspace should I even bother with a vapor barrier on the concrete walls? I've seen encapsulation drawing that have ground poly vinyl sheets going up 6" up the slab wall, mostly to prevent moisture coming up from the ground
Mike,
I'm a bit confused by your description of the foundation - maybe a sketch would help me.
You have a crawl space with poured concrete walls and you have put foam board in the rim joist area around the perimeter and you plan on continuing the foam board down the walls to the dirt floor? If that is the case, you do not need poly on the walls, you just need to seal the joints (sheathing tape is typically used). Then you just need to seal the poly sheet that is on the floor to the foam board on the walls - using sheathing tape or acoustical sealant to complete the air/vapour barrier. Depending on the accessibility of your crawls space, the inspector may also want a fire barrier over the exposed foam board.
Hi Trevor,
Here is the drawing I put together. There is no accessibility to the crawl space. Essentially its a poured slab off the exterior foundation
I have revised the polyvinyl barrier to just be on the floor. The heating will be infloor too.
Cheers,
Mike
I am curious as to how you are going to get R20 insulation in 2x4 wall.
That's a good question. I meant to revise that to R14 at 3.5 inches think
Mike,
Q. "Would it be better to forgo the exterior rigid foam and do interior insulation / vapor barrier?"
A. No. Exterior foam -- the more the better -- is vastly preferable to fiberglass batts plus interior polyethylene.
Q. "The entire space is over an exterior slab / addition that is 50 years + years old. Below it I have put 2" foam and spray foamed the gaps in joist spaces. I was going to continue the 2" foam to the ground (inside slab) and encapsulate the whole thing with vapor barrier over it; from the joist spaces all the way down, including over the dirt / gravel floor. Will that method work if I do go the exterior route?"
A. It's unclear whether you have installed spray foam at the rim joist area or between the floor joists. You wrote elsewhere, "There is no accessibility to the crawl space." If there really is no accessibility to the crawl space, it hardly matters what our advice is -- because if you can't get in there, how are you going to fix anything?
Hi Martin,
I have used 2" rigid foam below. I have access now as joists are exposed at the moment.
Mike,
If you have a continuous horizontal layer of rigid foam attached to the underside of your floor joists, then you have chosen to locate your home's thermal barrier and air barrier at that location. I don't recommend that you insulate your crawl space walls -- especially since you tell us that you have no access.
It sounds like there is no way to enter your crawl space without removing some of the existing rigid foam insulation installed under your floor joists.
Hi Martin,
I do have access at the moment though as there is no subfloor; I can even make access if I need to.
The insulation right now is in the joist cavities only. There is nothing on the walls. We are putting in floor heating in so I thought insulating the slab down to the gravel / dirt floor would be best.
Looking at fig 23 (on pg 20 of http://www.buildingscience.com/documents/guides-and-manuals/gm-read-this-before-you-design-build-renovate) I see exterior foam where the siding is, and then interior for the slab. Are you saying I must remove all the rigid insulation in the interior that is parallel to the exterior foam board?
Mike,
It's your choice: you can insulate the floor (the crawl space ceiling), or you can insulate your crawl space walls. But there is not need to do both.
The wall is definitely easier.
I have attached a jpg with the section of rigid foam in question.
Mike,
Your illustration includes words in red that read, "Remove this [rigid] foam [at the rim joist] in favor of unfaced cavity insualtion.."
No! Don't do it! At least -- if by those words you mean that you want to take out the rigid foam and install fiberglass batts. That is definitely a step backwards.
If you install air-permeable insulation at this location, warm interior air will migrate to the cold rim joists, leading to moisture accumulation, mold, and rot.
Hi Martin,
Ok so if I leave it all as I have drawn (without removing the section with red arrow) then there won't be an issue with the interior floor heat and the drying of the exterior wood that is below the sub floor?
Mike,
Your earlier reference to heating was unclear. Are you saying that you intend to install in-floor hydronic heating (PEX tubing loops) under the OSB or plywood subfloor?
If that is your plan, then the insulation belongs directly under your PEX loops (in the crawl space ceiling), not on your crawl space walls.
Hi Martin,
Its actually going ontop of the subfloor as per ditra's handbook (pg. 12)
http://www.schluter.com/media/DitraHandbook.pdf
Mike,
My answer is unchanged: Insulate the crawl space ceiling, not the crawl space walls.
Hi Martin,
Thanks for the clarification.
I guess my next question is then we are actually putting in a bathroom in part of the space. Is piping insulation up to where the rigid foam sufficient to prevent freezing? We could always run heating wire if its necessary on the pipes. If its possible, for the sake of budget we were only going to place heated floor in the washroom portion (1/3 of the floor area). The rest would be heated from the rads on the main floor.
On another note; according to http://www.certainteed.com/resources/30211292.pdf I should insulate the slab walls because there is pipes. Thoughts?
Hi Martin,
I'm reposting because I had a issue with the account getting temporarily turned off / website error.
Thanks for the clarification.
I guess my next question is then we are actually putting in a bathroom in part of the space. Is piping insulation up to where the rigid foam sufficient to prevent freezing? We could always run heating wire if its necessary on the pipes. If its possible, for the sake of budget we were only going to place heated floor in the washroom portion (1/3 of the floor area). The rest would be heated from the rads on the main floor.
On another note; according to http://www.certainteed.com/resources/30211292.pdf I should insulate the slab walls because there is pipes. Thoughts? I'm only re-asking as it would be difficult to enter / exit the crawlspace; unless I put in an access door on the floor.
Mike,
You're right: if you have plumbing pipes in the crawl space as well as in-floor hydronic tubing, it's a good idea to insulate both the crawl space walls and the crawl space ceiling. But if access is difficult -- if there is no way to get down to the crawl space after you finish your plumbing work -- I think it's a little crazy to put plumbing pipes down there.
If there is ever a future problem -- a frozen pipe or a drip in your plumbing lines -- no one will notice, and repairs will be a nightmare.
Thanks Martin. I think we'll look at putting an access door in just so that we can get down there if there is an issue.
Cheers
Hi Martin,
Just to confirm, I have read several articles on GBA about insulating the ceiling in the crawl space. Should I go full xps or use fiberglass between the joists and wrap the interior walls / joist header space with xps?
My thinking with the bats is that it would act like the thermal barrier / order I have on the walls with fiberglass on the interior and xps on the exterior.
If all xps, should it be flush to the crawl space ceiling; between the joists.
Mike,
Don't install any fiberglass in a crawl space. Use either rigid foam or spray foam.
More information here: Building an Unvented Crawl Space.
A couple of issue: IIRC based on the national building code, Ontario code requires a class-I interior vapor retarder (like 6 mil poly) unless the exterior foam guarantees that the sheathing stays above 5C at the outside heating design temperature, which takes a HELUVA lot more foam than is required in similar US climates under IRC guidelines, but that's between you and your code enforcers.
With a sealed insulated crawlspace that includes at least R5 insulated floors & R10 walls there's very little mold/rot risk with R11-R13 batts on the underside of a radiant floor- but there might be if you took it up to R30. The heat loss from the radiant floor to the crawlspace would be very minimal. The primary humidity/mold risk would occur during the summer when outdoor air infiltrates into the cool crawlspace, but a small room-dehumidifier could easily handle the latent loads, and may not even be necessary. Monitor it with a cheap battery operated temp & humidity meter- if it stays above 15C and below 60% RH in summer (and your nose isn't picking up any musty smells) it's going to be just fine without mechanical dehumidification.
If the house is air conditioned in summer, running a small fan on an interval timer to displace the crawlspace air with dry conditioned space air may ultimately use less electricity than a room dehumidifier since the AC is more efficient than a dehumidier, and puts the latent-heat of the moisture OUTSIDE where you want it in summer. (This approach doesn't work unless the house is air conditioned though.)