Hot Topics: Sealing Off Crawlspace for Temperature and Moisture Control

An unfinished basement crawlspace.

Here on we enjoy providing a place where home improvement novices and experts can come together to share ideas and advice. Inside our Forums, users can browse threads to see what exchanges are taking place on a topic of interest or start their own dialogue by posting something for the community to take part in. With over 250,000 members and counting, this resource is quite active so each week we highlight conversations that may just help you with that next DIY project.

Original Post: Sealing off 'crawlspace for temp and moisture purposes?

newbie0101 Member

Quick background: Prior owners expanded the back of the house out about four feet (and about 30' long) which created the 'crawl space.' It's actually more like a long additional basement space than a crawl space (concrete floor, five or six feet high), and it has a cement floor and cinder block walls (the rest of the basement is poured concrete walls).

When we moved in six years ago, we had attic insulation put in and the company convinced me to spray foam the rim joist of the entire basement, including the crawl space. They did not cover the four vents in the cinder blocks (that even when closed, don't seal well).

A year or so later, I noticed that the temps got down to around 40° in the winter (not bad, but got me a bit worried as there are one or two pipes there), and humidity got high in the summer (80% or so). If I leave the door open between the basement and crawl space, the temps stay above 40° and humidity stays below 60% in the summer. We don't heat the rest of the basement and it stays around 60°. I don't want to have to leave the door open forever, though.

Separately, we also have a cold kitchen floor above the crawl space and a few years ago a contractor convinced me to seal the vents in the crawl space and insulate the ceiling. He cut rectangles out of foam block insulation and put those over the vents, and used regular rolls of insulation in the ceiling. This helped the kitchen floor a little, but the temp in the crawl space has actually gotten lower in the winter (approaching freezing) and humidity is still high in summer.

I'm now looking for a permanent approach to fixing both the cold in the winter and the humidity in the summer (and don't care as much about the kitchen floor above). I almost don't care what it costs (within reason). I just need it to be done with.

mossman Member

You have two options. A ventilated crawlspace, or an unventilated crawlspace. With a ventilated crawlspace, you'll want to insulate pipes and between the floor joists. With an unventilated crawlspace, you'll want to insulate the foundation walls and can leave the floor joists and pipes uninsulated. If you're having issues with humidity, then I would go with the ventilated option.

How is the drainage around the house? Make sure all your gutters are clear, downspouts are well away from the foundation, and the grade slopes away from the foundation. Do you have a french drain/sump system installed?

newbie0101 Member

Will opening the vents be enough to keep humidity down in the summer?

There is no insulation on any of the walls within the crawlspace, but I believe there is insulation behind the wood paneling that goes around the basement itself, including the wall that abuts the crawl space.

mossman Member

It should, but we need to determine if the main source of humidity is coming from inside or outside, which is why I asked about drainage. Sealing the slab may help amongst other things. If you have high humidity, closing the vents is going to make it worse because you just took away any sort of air circulation.

If the space is always going to be unfinished, then I'd go with the ventilated option. Ideally, you'd want a drain tile around the basement perimeter that drains into a sump, a waterproof membrane on the outside of the foundation wall, seal the slab, insulate the pipes, insulate the floor joists, and open the vents to get some good circulation down there. Install an exterior door to the basement if you don't have one already. The drain tile and waterproofing membrane would be a little pricey, so it depends on your budget. You could start by opening up the vents, sealing the slab, insulating the floor and pipes, and ensuring your drainage outside is in order, and then seeing what difference that makes.

newbie0101 Member

Drainage is good for the most part. Gutters are clear. Downspouts generally point far away, though two of them go right down into the ground into some sort of 5" diameter white tubing. The two downspouts closest to the crawl space are traditional ones and I have extensions running them a few feet away from the house.

That being said, one thing I have noticed is that after an extremely heavy rain (well over an inch) there is sometimes some moisture on part of the floor in the crawl space. I can't tell where it's coming in. Also, the deck at the back of my house runs along almost 100% of the exterior wall of the crawl space. Not sure if that might affect the way water falls near the crawl space. Finally, the lawn does slope away from the foundation (particularly in the back of the house which is where the crawl space is), though I think it's pretty flat under the deck, but I believe may slope away a little. No french drains or sump pump as, knock on wood, the main part of basement is very dry.

To read the rest of the thread, look here: