basement flooring
ken_jensen
| Posted in General Questions on
What flooring is best on basement slabs regarding moisture issues?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
> What flooring is best on basement slabs regarding moisture issues?
If you have a moisture issue in the basement, you need to fix that first. No flooring is meant to be installed where you have liquid water on the substrate.
The only thing that will really work with moisture seeping up through the concrete is to just polish the concrete. If the water is coming through the walls and only pooling on the floor, then you can use epoxy on the floor. If the water is seeping up through the concrete and you use epoxy, the epoxy will lift and fail over time.
You may want to look into the dricore system that is made for damp basement floors, but remember that if you have major water ingress issues, you're going to need to do more than just a fancy floor -- you need to correct whatever issue is bringing in bulk water first.
Bill
I really don't like Dricore. It's a bandaid. I guess there are situations where it works but there's no guarantee in any given set of circumstances. But what I really don't like is their marketing, which I find deceptive and leads people away from best practices.
"DRICORE® Subfloor is designed with a high-density plastic membrane containing a patented air gap pattern to ensure that moisture emitted from concrete floors can naturally evaporate." While I doubt that the air gap actually allows meaningful evaporation, if it did it would be exactly what you don't want to happen. You don't want moisture in the concrete coming into the living space, you want it to stay in the concrete.
What kind of moisture issue? Moisture gets into basements a lot of different ways. Rainwater can run into the basement from above. Groundwater can seep in from below. Moisture in the soil can wick through masonry. Moisture in the air can condense when it comes in contact with a cold basement wall.
Each one of those has a different solution. You need to figure out which one -- or ones -- is happening in your basement before we can help you.
A new 4" slab / waterproof membrane / 2" spray poly foam / 3" crushed stone drain layer with perforated drain pipes and sump pump / filter fabric / existing grade or compacted fill - should keep the slab dry from underneath and also not as cold as the ground temperature.
I'm considering waterproof LVP in the bathroom and 5/16" or 7/16" waterproof carpet cushion with cut pile carpeting in the bedrooms.
Anyone have experience with those finish materials on a basement floor?
Sounds like a well-constructed basement. LVP will work fine. I've never known carpet to work out in the basement. If it gets wet at all the there's just no drying. You have to carefully monitor humidity to avoid condensation.
Don't use spray foam under slab, use rigid foam board. XPS is the best material for this particular application. EPS can be used too, but is not ideal. Polyiso is not suitable here.
Bill
Zephyr7
Care to share your opinions about the suitability of types of foam under a concrete slab?
Polyiso is a no-no, period. Polyiso will take on water, so it can't be used below ground. Note that "below ground" here basically means "buried", you can use it just fine below grade, somewhere like the INSIDE of a basement wall. You just can't put it anywhere where it can soak up water from the ground.
EPS is OK, but has been shown to take on water over time (there was a study in Alaska showing this in comparison to XPS under roadways). This means EPS is technically usable, but not ideal.
XPS is better for below-slab insulation, since it is less prone to take on water, so you're more liekly for it to maintain it's R value over time -- even when derated due to blowing agent diffusion (basically it will stabilize somewhere around what EPS would be in air).
Yes, XPS is a "less green" material, but in this particular case it really is the better choice. You always have to consider the full system here, and not just the product itself. XPS is what I would use HERE, but I would avoid using it in most other locations in a typical home. I'd use polyiso, for example, if you want exterior rigid foam on the exterior of your walls. Use XPS only where it's buried, so sub-slab, and any exterior foundation walls insulation.
I would think like this:
XPS where it is IN CONTACT WITH the ground, meaning buried locations.
EPS where it MIGHT GET WET, but won't be buried in the ground.
Polyiso everywhere else, unless you want to cut costs, then look at EPS.
Bill
>Polyiso everywhere else, unless you want to cut costs, then look at EPS.
Per unit of R-value I find polyiso ever so slightly cheaper than EPS. Of course there's some debate about the long-term R-value of polyiso and whether the foil facing really contributes as much as claimed. So I'd say it's a toss-up.
Of course the cheapest is reclaimed polyiso.
Polyiso is often cheaper per unit R value, but not per unit thickness. It depends on what you're trying to do -- hit a particular R value, or fill an available space. As an example, I'm in the middle of a little mini-renovation fixing a T+G ceiling in my house that had no air barrier behind it. I did an anti mold treatment today, and I also found that a wall I was going to reinsulate in that attic as 2x4s flat, not the usual stud way. I have some leftover 1.5" XPS that will fit the space between those "studs". I'd rather use polyiso, but I don't have any in the right thickness.
R value of polyiso does drop a bit over time, but not as much as XPS. I think the aged R values stated on polyiso are probably pretty honest these days too, since the industry got their butts kicked decades ago for being waaay overly optimistic.
The foil facer doesn't contribute to the rated R value. The foil facer does help to slow the diffusion of the blowing agents out of the material though, which is a plus. The foil facer can also get you a freebie (my favorite kind :-) radiant barrier too, which makes it a good product for building vent chutes in attic assemblies.
+1 for reclaimed polyiso when you can get it. Don't shy away from kraft or fiberglass faced polyiso, either -- it can be used in lots of places just like the foil faced stuff.
Bill
Thanks for the great info on various types of insulation! I should have been more specific on the spray-foam that I'm considering for under the new slab.
Closed cell sprayed polyurethane is what I hope will work for this floor assembly since it will be also used on the existing concrete walls behind furring studs. The structural engineer says it should be strong enough to support the concrete slab and other loads, but I don't know about specifying the foam's optimal density for this condition. Any advice on this? Thanks to all!
I've never heard of spray foam being used on a slab like this. There is certainly no good reason to use spray foam here over rigid foam board. I would NOT recommend using spray foam under the slab, I would use sheets of XPS. XPS is rated for this application, and it's flat and even -- something you'll never get with spray foam. Any variation in the thickness of spray foam will result in variations in the thickness of your slab, which is something you don't want.
My advice is DO NOT use spray foam under the slab. Use sheets of XPS rigid foam. This is how nearly everyone installs sub slab insulation.
Bill
Bill,
I agree. I can't think of any advantage of using spray foam, and a lot of downsides.