Airtightness and Spray Foam Insulation
Spray foam makes houses tighter, right? Maybe. Usually. Having blower-door tested hundreds of homes with every insulation combination you can imagine, I’ve developed some opinions.
Spray foam insulations have a reputation for making homes more airtight than fiberglass or cellulose. If you look at a sample of the various insulation products, it’s easy to see why. The foam itself is airtight, while most other insulation products aren’t. This contributes to a tight house, but it isn’t the whole story.
My number one favorite place for spray foam is in floors over unconditioned spaces. Particular favorites are spaces whose air I don’t want in the house (garages, vented crawlspaces), or if the floor is cantilevered. In addition to the obvious air sealing benefits, pests commonly enter homes from these spaces. Pests. Stink bugs, spiders, mice, snakes, squirrels, raccoons, that sort of thing. My problem with using batts in floors is that gravity isn’t working in your favor. Any of the these pests might decide to make a nice nest between the batt and the floor, and their weight might pull the batt down. Or the plumber might pull it down to work on a pipe. No one ever puts it back up.
Complicated roofs and attics are another first-place favorite. Here’s a short quiz you can take to determine whether Amy would really prefer that you spray foam the roof of your home. Put a check mark next to anything that you have in your home:
- HVAC equipment and ductwork in the attic
- Desire to store things in the attic
- Cape-style home or bonus room truss for the top floor
- Vaulted ceilings on the top floor
- Attic knee-walls (walls between conditioned space and the attic)
- Many can lights or other penetrations in the top floor ceiling
- Wood paneling is used for the interior ceiling finish (in lieu of drywall)
If you checked any of the above, you should consider using spray foam on the roof of your home. If you checked several, you really, really should consider it.
I have three goals in attics: keeping the insulation intact for the life of the home, keeping equipment out of the hot attic, and air sealing between the house and attic. Knee walls and can lights are hard to air seal perfectly. You can try, but you’re not going to get the house as tight as spray foam at the roof will. The number one risk to attic insulation is the homeowner packing it down or brushing it out of the way to install decking and store stuff that they should probably just get rid of. What’s the R-value of Christmas decorations?
As long as the foam truck is coming to your house to insulate your floors and complicated attics, I’d like to have them foam just a few more areas. The rim and band joist are nearly impossible to cover evenly and completely with anything other than spray foam insulation. I would also spray any other locations that would require an interior air barrier per code. Foam is considered its own air barrier (if it’s thick enough or sprayed onto a rigid surface), so you can spray dead areas under stairs, behind tubs, fireplaces (mind the clearance), window seats, duct chases, double walls, dropped soffits, and other funky places where your insulation wouldn’t be touching sheathing outside and drywall inside. If the foam truck isn’t coming you can certainly do some other sort of air barrier. But while it’s here, it’s just so easy to foam it.
That leaves us with a few places where you can save money and not use foam. If you have a simple flat ceiling over an attic, go ahead and use R-50 blown insulation of some kind. I’d like you to use a raised heel truss and to vent the attic well, using vent covers that are strong enough to keep squirrels from getting into your attic for the next hundred years. Take my word for it – the aftermath of squirrels partying in your attic is pretty gross. Air seal every single penetration in the attic ceiling, caulk your drywall to the top plates even on interior walls, and insulate your attic hatch. Then put the attic hatch in and never go up there again. Or if you do, take a broom along to fix the insulation that you mess up.
I’d rather see blown insulation than batts up there. The attic probably has a truss with a 2×4 or 2×6 bottom chord. Your R-38 batts are going to be taller than that, so every 24 inches, they’re going to be sticking up above the bottom chord leaving a 1.5 inch gap. Also, they probably won’t get cut/split around light fixtures and wiring perfectly, and after the electrician pulls them out to fix your bath fan he won’t put them back where they belong. Blown insulation fills in around all this stuff and it’s cheap and easy to install at an above-code R-value. It’s also easy to move around with a broom.
What about walls? I’m not a strong advocate for foam here. Most walls are pretty easy to air seal, and at most of the places where walls leak foam won’t help you anyway. Caulking your bottom plates, using tight windows, and weather-stripping doors will all help. Properly installing zip sheathing, a high-quality house wrap or a spray on exterior sealant will all help. The foam doesn’t seem to help as much on airtightness in walls. I can think of only one exception: homes with interior paneling instead of drywall seem to test leakier, and foam would likely be helpful there.
If you need to save money on your insulation job, using a blown product (fibergblass BIBs or cellulose) in the walls can give you a comparable airtightness and R-value to foam. In fact, I prefer this cost-cutting approach over reducing the thickness of the foam. If you build a 2×6 wall and then only get R-15 with 4″ of spray foam, you’re no further ahead thermally than you’d be with a 2×4 wall. For all the valid criticisms out there of what the R-value doesn’t tell us, it’s still pretty relevant to conduction heat loss.
If you really, really need to save money, you can even go with batts in the wall and make sure they get installed to a real, proper grade 1. I feel like that advice should come with a “don’t try this at home” warning though. If you’re going to work with batts, make sure you have a third party involved who really knows what to look for. A bad install can really degrade the performance. To be fair, keep in mind that any insulation can be installed badly and perform badly. Batts just happen to be especially prone to this problem, in my experience.
Yes, you really can get equivalent airtightness with batts in the walls. I once had a client who gave us a great comparison group of nearly identical homes. All of them had spray foam at the roof, but they had a mix of fiberglass insulation and spray foam insulation in the walls. The blower door test results were remarkably similar in the group of homes. Some of the homes with fiberglass walls actually tested slightly tighter than some of the homes with spray foam in the walls. I should probably sit down someday and see if I have enough data to do an actual statistical analysis, but I seriously doubt that there would be a statistically significant difference.
My goal for all of my clients is to get an airtight enclosure with insulation installed evenly, neatly and permanently over the entire exterior envelope of the home. You can accomplish this with any type of insulation, but some types make the job easier and increase your probability of success in different applications. I don’t care so much which one people use, as long as the one they choose gets done well. Personally, I like using the easy method.
copyright 2013 Amy Musser.
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