Home Improvement

Any reason not to spray foam on top of batt insulation?

By October 2, 2019 2 Comments

I’ve got a project in my crawlspace where the framing on an interior stairway sticks down into the crawlspace area. It is always very cold down there, so I went to inspect and saw some seriously subpar insulation with badly installed fiberglass batts, missing batts, etc. As well, a couple of outlet boxes have rather large gaps where the cold air is certainly going straight into the home.

My initial plan was to pull the batts then use a home spray foam kit to lay down a couple inches of foam. Then perhaps later trim and fit batts back into the remaining space.

But I was thinking about it today and wonder if I can’t just spray right over the batts and wood framing. It’s the crawlspace so it doesn’t need to be flush to the framing studs. It would form an air seal on the entire thing, and would likely make those existing batts a lot more efficient. It would also fill the voids.

So, I’m thinking I’m pretty clever at the thought of spraying on top of the existing batts, saving time, and having a better outcome.

Any reason not to do this?

Thanks for input!

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  • TheTimeIsChow says:

    > Any reason not to do this?

    Moisture buildup/mold.

    The long explanation – Air travels from high pressure to low. So cold air (high pressure) is constantly trying to push it’s way into the warm home (low pressure).

    The batts trap this cold air and, depending on r rating, absorb enough to protect it from coming into the home.

    By spray foaming over this, you are now insulating the insulation. Essentially neutralizing it’s effect and making it a ‘part’ of the home (in a way). So warm, moist, air from the home will eventually start to slowly bleed off into this area.

    The warm, moist, air will sit there with no where to go. It won’t be easily circulated. It won’t be able to breath/evaporate off.

    In other words, you’ll be creating a perfect breeding ground for mold.

    The sort explanation – You’re insulating the insulation. The pocket of air will eventually equalize out to the same temp/moisture level inside the home. It won’t be able to breathe. You’ll develop mold.

  • weaponized_sourdough says:

    Spray foam needs to be applied by professionals; that you can turn around and sue. This is because if it doesn’t expand right or is the wrong stuff or reacts with something, it often ends up smelling like FISH. You end up with guests coming in and thinking, “man they eat a lot of fish. They should open a window.” Ever thought that of someones house when you visited? Spray foam house. Not broiled halibut.

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