How to Insulate Under a Sunroom?

We received a question from Wade in Virginia. And he has a sunroom that does not have a crawl space around it, and he’s wanting to know the best way to insulate it.

What to know before starting

Wade sent in a picture of his sunroom, from the photo we where able to see  that there is some plywood that they put underneath in order to block wind and critters from getting up into the sunroom. From the way he described it sounds like there is some  batt insulation, probably some fiberglass insulation between the plywood and the subfloor of the sunroom. So how would you insulate something like that? Well, you’ve got to be really careful. Since Wade is in Virginia, and the average humidity in January is between 61% and 68%. He has to account for that and make sure the wood is dry.

Install a Dehumidifier

What you need to do is if you’re going to insulate underneath an area that is open to the outside humidity, you might want to think about blocking that area off and drying the wood before you put any kind of insulation. It’s going to be a bit of a pain to do that, but you can staple up some plastic around the entire space to create a containment area, put in a dehumidifier after you remove that plywood and bat insulation that’s already up in there. Use wood moisture meter, check the wood moisture level, get it below 9-12%, somewhere around in there. Then you can take a good foam board, we would use our termite resistant foam board, cut it to fit between the floor joists, And you shove that up in there, and then you can spray foam the exposed wood if you wanted to and lock that foam board into place; or you could just spray foam the edges of the foam board to lock it into place and make sure you’re air sealing that foam board up against the subfloor.

Protect your work from the elements

Then after all of that I would still take some pressure treated plywood and cover all over that back up, and then air seal that plywood with a good caulk, then paint the plywood if it’s a paint-able plywood, which it should be. That way you’re protecting that plywood from moisture and water exposure. You’ve got the plywood caulked, you’ve got the insulation air sealed, and you’ve also got that thermal barrier underneath the sunroom. Sounds like a pretty big project, but make sure you just dry out all that wood before you add any insulation up in there.

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