Spray foam insulation or pink batts??

5 years ago

Hi - we are renovating a drafty 60+ year old cottage in Quebec (cold winters, hot summers). We have had a new, radiant heated slab put in and are looking at wall insulation options. It's gutted inside, we have 2 x 6 studs and 6 inch wide sill. there are places where we can see daylight through the walls - and are thinking spray foam insulation will get into all of the weird nooks and crannies we've discovered in the walls. We've received a quote for closed cell spray foam 2.5 inch (R15) or 3 inch (R18). From reading I see that a much higher R value is recommended - how do we get the R value up to 24 or more? Do we have to put pink batts on top of the spray foam to increase the R value?

Also, if we get spray foam of 3 inches in a 2 x 6 cavity, there's still empty space between the insulation and the gyprock, is this ok? Last question - does the cold radiate from outside via the 2 x 6, which hasnt been completely covered with spray foam as we would only have 3 inches of foam? Thanks for any advice -

Comments (4)

Bruce in Northern Virginia
5 years ago
last modified: 5 years ago
If you have a 6" stud cavity, its very common to not completely fill the stud cavity with spray foam. However, the closed cell foam will completely seal and insulate the wall, so it will still be very effective.. With the stud bay open they can get good adhesion on the sides so that it really seals the wall. There is some transmission of cold/heat through the studs, but its not as significant. My opinion is that keeping out cold air is more important than the depth of the insulation. You need to also ensure that your windows are installed and sealed to the wall properly as part of the project.

If it was my house, I would spray the wall insulation first, and then have the electrician come in and run the wiring and install the boxes in the remaining space in the stud bay. I don't really like to have all the electrical buried in foam insulation where its not easy to access.

Bruce
strategery
5 years ago
Closed cell spray foam is approx 6.5 R per inch, and eliminates need for vapor barrier. It is excellent. Yes! You can definitely use spray foam and then add batts on top.

If you literally can see daylight, then if you have the siding off consider using an air-sealing tape on the exterior over all gaps, with suitable primer.
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Open cell foam does stop most of the airflow, and it costs a bit less than closed cell foam; however, it is spongy and can retain moisture. There are at least two ways that moisture can get inside the wall. Water vapor can migrate into the wall, which a properly-installed vapor barrier should prevent. The second way moisture can be present in the wall is via condensation. The warm air from inside the home will meet the cold air from the outside somewhere within the wall cavity. Where the two temperatures meet, moisture is created. When one uses only open cell foam, the condensation will form within the foam, but won't be able to dry out because the airflow has been cut off, potentially leaving wet insulation which could lead to unhealthy mold. Building science offers that, if a home is heated during the winter months, the best way to prevent condensation from forming in a wall is to be sure that 50% of the R-value in the wall is closed cell foam. The remainder of the cavity can be filled with open cell foam, fiberglass, or any other insulation. Because the cells of closed cell foam are solid, they can't hold moisture. By following the 50% rule in external walls, the hot and cold temperatures will be forced to meet within the closed cell foam, condensation will be neutralized, and you'll have a wall that is air-tight and moisture-free, making the use of closed cell foam as the first line of defense against Mother Nature well worth the investment. Linda - Foam It Green http://www.sprayfoamkit.com/videos-a-how-tos/insulating-a-2x6-wall
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Thanks for the replys. Forgot to add that the firm that wants to do closed cell + fiberglass said it was a code requirement to cover the foam with what he called a code approved ignition barrier. I thought that any foam rated E-84 was OK by itself. Comments? One question I need to ask is why not just spray 4 inches of closed cell and be done it with. That should provide at least R-25. Compared to the inefective R-11 that was in there it would be a big improvement. One reason this may not be practical is it may require two passes as I'm not sure you can apply closed cell at a 4 inch depth in one pass due to excess heat generation and risk of self ignition. Two passes would add to the cost and most likely push it well above the open cell. As for converting the crawl space into a mini basement, I think the jury is still out on that one. If not done correctly I think the consequences are worse than leaving it the way it is now. I also think the cost would be much higher since you still have to insulate the foundation walls and completely seal the volume from the ground.
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Try www.buildingscience.com/ for information about vapor barriers. Another good source for local building advice might be your state land grant university. I do not know anything about your floor finish. I guess you will cringe when I mention pre-finished flooring, but keep reading ;-) The spaces between the boards lets moisture through. With finished in place floors (poly finishes) you have a good vapor barrier. I guess that you understand the "sandwich" prohibition now. If you have a vented crawl space, off gassing will primarily be through the crawl. Ignition is a good question. I am afraid that I do not know much about fire and foam. I know that it burns and that crawl spaces raise fire insurance so it is a good thing to look into. Any kind of covering over the underside of your floor is going to increase the risk undiscovered or delayed discovery of rodent and insect damage. That includes batts, foam or boxing-in with rigid insulation. I do not know if there is any hard information about the relative risks of each method. The rigid insulation alternative is not one that has been addressed in this thread yet. It would be less expensive than foam, if DIY, but it is a lot of work. It has to be taped very well. You could take a hybrid approach and cover selected areas with foam board -- under the most troublesome plumbing areas for example. When you get done with that, you foam the rest. Low density foam pulls out pretty easily. I do not know about high density. You would just have to spray some more in after you make repairs. The problem would be finding the stuff that you need to fix. I would considerer tagging stuff before foaming if it will be covered by foam. You could use durable plastic tags in some color code. If you have a leak, water will run right through low-density foam. I do not think that is so with high-density foam. One important feature of spray foam is the air-sealing capacity. Batts will not do that. That is the problem with it where I live in the humid Gulf South. Air gets around batts even when freshly-installed. It is even worse when it sags. Humid air hits the cool floor, water condenses and does not dry well. Floor rot happens. Even with the best batt installation, the joists become the coolest thing under the house. Condensation happens there and then rot happens. These things worked great before AC existed, but not anymore.
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rwiegand
5 years ago
If you have the siding off consider wrapping the outside with 1-4" of rigid foam then filling the stud bays with blown in cellulose. This is a very cost effective approach to get both good air sealing and high r value, you can get to the r-40 range. Thermal bridging across the framing is very significant-- 20% or so of the wall will have only negligible r value with a straight spray foam approach, though the air sealing is excellent.

If you take a "flash and batt" approach the spray foam needs to be sufficiently thick that you don't create a condensing surface inside the wall (similarly with the rigid foam plus cellulose). There are tables that show the required values for different climates. Right offhand I'm not sure R15 is going to be enough for Quebec
dhamcof
Original Author
5 years ago
Thanks very much for the comments - Rwiegand: we have found a firm (with excellent referrals) that uses a 'better/different' brand of foam which has an R7 value per inch, so with 3.5 or 4 inches, I think we'll be ok. we keep reminding ourselves that this is a cottage, not our principal residence, if we need to wear an extra sweater in February that's fine. Bruce: yes, the electrician has said that he prefers to do his thing after the foam has been applied, so wiring etc are not forever buried in foam. thanks again -

Spray foam insulation or pink batts??

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Bruce in Northern Virginia

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Spray foam insulation or pink batts??

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Bruce in Northern Virginia

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