What are the pros and cons of using expanding foam insulation?

What are the advantages and disadvantages of using expanding foam insulation? In the project I’m working on now I’ve been installing 4″ and 6″ batt insulation (depending on the studs, some are 4, some are 6 inches). It’s proving to be a real pain to cut the batts into the appropriate size for all the spaces. Seems like using expanding foam would make all this easier but what if I later need to get into the wall to fix the wiring? And are there any risks with things catching on fire when foam is used?

Answer from Green Energy Efficient Homes

One of the great things about expanding foam insulation is that it provides a very effective vapor barrier. While the R value of expanding foam insulation is typically around R-3.5 to R-4 per inch – not much better than batt insulation – I’ve seen claims backed up by government research that foam insulation such as icynene actually provides far better insulation than a comparable thickness of batt. My own builder sprayed 2″ of foam insulation on the underside of the flat roof he installed in our addition, and on the underside of the floor as well, and then installed batt insulation under that. The benefit, he said, is that it provides a much better vapor barrier than installing polyethylene plastic sheeting over the batt insulation, because poly vapor barrier is frequently compromised by drywall screws or other actions that pierce it, both during construction and any time the homeowner drives a screw or nail through the wall.

As you mention, cutting batt insulation to fit in small spaces can be a bit awkward. Not only that, but people have a tendency to overstuff when they pack fiberglass or rockwool insulation into small spaces, which actually decreases the insulation value. Expanding foam insulation is great for these applications, for example if you have small gaps between the stud framing and a window frame or door frame. Be careful, though about injecting foam into a wall cavity that already has batt or blown fiber insulation, because the expansion of the foam may actually compress the batt or blown fiber insulation, which reduces its insulation value.

Another great thing about expanding spray foam – Icynene, for example, which has an R value around R-4, or sprayed polyurethane, which can get as high as R-6.5 – is that you can add it to older homes that have no insulation at all, without even resurfacing your walls, because it can be injected from small holes cut into the walls, and then the holes are simply patched over and repainted.

That was one of the first home efficiency upgrades we did in our 1920’s brick house. There was just a 2″ gap between the bricks and the plaster and lathe, not enough to inject fiberglass spray-in insulation, and I really didn’t want to tear off all the plaster and lathe, reinsulate with batt insulation, and then redrywall. (For one thing the house is fairly narrow and I didn’t want to lose another 4″ of width, especially on the stairwells.) Another problem with the house in its original state was all the air gaps between the bricks, since the brick facing was in need of a thorough tuck pointing job. We hired a local company – Jerry’s Insulating – to inject Icynene insulation into the walls of our ground floor – in the space between every pair of studs – and it really helped cut down on our heating bills and improved our comfort. That was probably the best $1700 I spent on heating efficiency in a long time.

You can certainly do better than the Icynene we bought in 1998 – there are higher R value expanding foam insulation products available. And you should also read up on factors such as the VOC emissions of the particular foam you’re considering. All other things being equal, I recommend going with the highest R-value per inch you can find. You can now buy expanding foam insulation guns, and the foam itself, in large enough quantities for a do-it-yourself application in walls or sprayed directly onto the inside of new exterior sheathing – you no longer need to bring in experts. (If you do it yourself, get adequate protection – gloves, old clothes, mask and glasses – you don’t want that stuff getting on your skin or in your eyes!) I wouldn’t recommend a do-it-yourself if the surface area being sprayed is greater than about 8×20 feet, but for areas that size or smaller you may find it hard to even find a contractor interested in doing the job.

For attic insulation, I recommend blown cellulose, and you probably don’t want a vapor barrier between your living space and an attic, so that’s another reason to avoid expanding foam insulation in attics.

One disadvantage of expanding foam insulation you should be aware of is its tendency to expand more than you might have anticipated. If you cut a hole into a wall and inject more foam into it than you should have, the foam will generally work its way out the hole and it’s pretty easy to cut off the excess once it has cured. But there are two problems. First, sometimes the foam inside the wall cures in places, and the expansion can cause the wall to bulge, which can cause cracking of plaster or popping of drywall screws and bursting of drywall seals. Second, any cured foam that gets onto the living area surfaces of your home is very hard to get off cleanly and can leave unsightly stains.

Another disadvantage of the kind of expanding foam insulation you buy in a small can is that it has a tendency to cure inside the injection tube after first use, so it’s hard to use a bit on one project, then save the cannister and use the leftovers later. You’re supposed to be able to hold the can upright with the injection straw removed, push the nozzle to clear it out, and let the material cure in the injection straw, then snip off the end before the next application, but I find just about every time I try this that the foam inside the injection straw cures and completely seals off the straw. It would be great if they sold extra straws with each cannister (but maybe that would decrease the number of cannisters they sell!). I recommend going with a better quality product such as the Great Stuff PRO pictured above at left, since a single cannister can be used over the course of several days or weeks. Of course, if you’re just filling a few gaps for a one-shot deal, the cheaper cans will do fine.

Removing already cured spray foam insulation from an insulated area is a bit messy, but the only risk I think you’ll run into is that you could cut a live wire you didn’t know about while trying to remove it. I recommend cutting the main circuit breaker to your house before trying to cut out sections of cured foam insulation in wall cavities, unless you’re absolutely certain there are no live wires in the area you’re working in. The foam itself does not increase the risk of fire hazard.

Green Energy Efficient Homes articles cited

3 replies
  1. Ern
    Ern says:

    Great stuff’s propellant is highly explosive, i found out when my hot water tank burner came on! I was outside and the now ex wife was doing dishes so, it ignighted in the basement and blew me away from the outside wall. Use with spark flame source OFF please. I still use the great stuff, just with all pilot lights completely off and then move the indoor air out, doors windows open .i don’t think the propellant has a smell but it may be heavier than air …

  2. tlhonmey
    tlhonmey says:

    For the home user who wants to avoid trouble with the foam curing inside the application straw, just buy one of the cans of cleaning solvent they use with the professional gun-type applicators. Apply what you want, then detach the straw and clean out the tube and valve and the can will have only slightly less than its normal shelf life remaining.

    A can of cleaner costs about as much as a can of foam, but can be used for cleanup many times before it’s empty, so if you’re using small bits of foam here and there it’s well worth it.


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