Basement Insulation

[Netspirit]

Hello Dana,

Many thanks for educating the entire Internet (Google links to your posts on all searches related to basement insulation - this is how I ended up here).

I have a concrete slab, insulated on the perimeter (only) with what looks like 1'' or 2'' XPS. It has a poly vapor barrier all the way underneath it. The slab appears dry. The basement is force-heated & air-conditioned just like the rest of the house. There are no plans to install radiant heating. The climate is marine (Seattle). There can be no carpet in the basement (the future occupant is suffering from severe allergies). In the bathroom, tile is the usual answer. In the bedroom and living (media) room, the choice is harder: engineered hardwood vs. laminate vs. LVT vs. cork vs. bamboo. For the media room, hardwood, laminate and bamboo echo a lot (bad for acoustics), so vinyl and cork are perhaps the only options I am considering.

Based on your posts, would you recommend laying 1'' XPS on top of the slab, covered with plywood, covered with some vapor-permeable finished floor?

If that is (at least partially) correct, there are a few related questions:

1. If there is a poly vapor barrier under the slab already (not sure how it is holding up), would another layer of poly above the slab help, hurt, or make no difference? If water is "trapped in the slab" between the two vapor barriers, is there any potential for mold growth on the slab? Is 1'' XPS with no poly, being a vapor retarder, sufficient to protect the wooden subfloor above it from vapor?

2. What is your opinion on DMX-1, Platon, Delta MS, Subflor, Tyroc, Dricore, Barricade, Amtry, ThermalDry, SuperSeal and other products with dimpled membranes at the bottom? Does that airgap do anything better than a simple 6 mil poly? Do you subscribe to the theory that having the airgap somehow equalizes vapor (hydrostatic) pressure below and above the slab, and therefore stops further vapor diffusion?

3. Is 6 mil poly a good enough vapor barrier? Does upgrading to 10 mil poly, as some people recommend, have any positive effect?

4. Plywood subfloor over XPS - what grade / kind of plywood is both strong enough and suitable for flatness-sensitive finished floors (like Loose Lay vinyl planks)? Which is a better installation method: Tapcons or T&G/floating? If Tapcons, how many (here are Advantech instructions - http://www.huberwood.com/assets/user/library/AdvanTech_to_a_Concrete_Slab_v3.pdf - do they apply to plywood, or is that an overkill)? Do Tapcons need to be dipped in some adhesive like Loctite PL 300 in order to air-seal the holes punctured in the vapor retarder?

5. Loose lay luxury vinyl planks are vapor barriers themselves, but after the installation they will have tiny gaps between them. Would such finished floor be considered vapor-permeable ("breathing to the interior"), or vinyl a bad, bad idea?

6. Leveling / flattening - where does it make the most sense? The slab is in a decent shape, but has a few minor highs and lows. Does it make sense to grind / self-level the slab first, or leveling / flattening the wooden subfloor is the way to go?

Thanks a lot in advance!

 

[Dana]

Hello Dana,

Many thanks for educating the entire Internet (Google links to your posts on all searches related to basement insulation - this is how I ended up here).

I have a concrete slab, insulated on the perimeter (only) with what looks like 1'' or 2'' XPS. It has a poly vapor barrier all the way underneath it. The slab appears dry. The basement is force-heated & air-conditioned just like the rest of the house. There are no plans to install radiant heating. The climate is marine (Seattle). There can be no carpet in the basement (the future occupant is suffering from severe allergies). In the bathroom, tile is the usual answer. In the bedroom and living (media) room, the choice is harder: engineered hardwood vs. laminate vs. LVT vs. cork vs. bamboo. For the media room, hardwood, laminate and bamboo echo a lot (bad for acoustics), so vinyl and cork are perhaps the only options I am considering.

Based on your posts, would you recommend laying 1'' XPS on top of the slab, covered with plywood, covered with some vapor-permeable finished floor?

If that is (at least partially) correct, there are a few related questions:

1. If there is a poly vapor barrier under the slab already (not sure how it is holding up), would another layer of poly above the slab help, hurt, or make no difference? If water is "trapped in the slab" between the two vapor barriers, is there any potential for mold growth on the slab? Is 1'' XPS with no poly, being a vapor retarder, sufficient to protect the wooden subfloor above it from vapor?

2. What is your opinion on DMX-1, Platon, Delta MS, Subflor, Tyroc, Dricore, Barricade, Amtry, ThermalDry, SuperSeal and other products with dimpled membranes at the bottom? Does that airgap do anything better than a simple 6 mil poly? Do you subscribe to the theory that having the airgap somehow equalizes vapor (hydrostatic) pressure below and above the slab, and therefore stops further vapor diffusion?

3. Is 6 mil poly a good enough vapor barrier? Does upgrading to 10 mil poly, as some people recommend, have any positive effect?

4. Plywood subfloor over XPS - what grade / kind of plywood is both strong enough and suitable for flatness-sensitive finished floors (like Loose Lay vinyl planks)? Which is a better installation method: Tapcons or T&G/floating? If Tapcons, how many (here are Advantech instructions - http://www.huberwood.com/assets/user/library/AdvanTech_to_a_Concrete_Slab_v3.pdf - do they apply to plywood, or is that an overkill)? Do Tapcons need to be dipped in some adhesive like Loctite PL 300 in order to air-seal the holes punctured in the vapor retarder?

5. Loose lay luxury vinyl planks are vapor barriers themselves, but after the installation they will have tiny gaps between them. Would such finished floor be considered vapor-permeable ("breathing to the interior"), or vinyl a bad, bad idea?

6. Leveling / flattening - where does it make the most sense? The slab is in a decent shape, but has a few minor highs and lows. Does it make sense to grind / self-level the slab first, or leveling / flattening the wooden subfloor is the way to go?

Thanks a lot in advance!

In order:

1: Concrete is not adversely affected by moisture, and adding a second vapor barrier above the slab only helps. If installing foam above a slab known to have a vapor barrier under the slab, putting the second vapor barrier between the slab & foam is slightly better than putting it above the foam, since the foam might otherwise take on moisture over time. (Fully saturated EPS or XPS has somewhat lower thermal performance.)

2: The air gap in the dimpled membrane products is a capillary break against the migration of liquid water, not a vapor barrier (though the materials they are made of vapor barrier materials. ) Unless you have known moisture problems like visible seepage or occasional minor flooding it's a bit overkill. Doesn't hurt, but it isn't necessary.

3: 6 mil poly is fine as a vapor barrier. 10 mil good are a bit more rugged and less likely to tear, but the differences in vapor diffusion quantities are not large enough to warrant going with anything thicker than 6 mils. (Don't use 4-mil stuff though.)

4: Any grade of plywood is strong enough even at 1/2" since it is fully supported everywhere (unlike subfloors over joists) The issue is the depth required for the fasteners for the finish floor. It's common to use a double layer of 1/2' CDX with the bottom layer glued to the foam with foam-board construction adhesive, with the edges of the second layer overlapping the first by a foot or so, nailed & glue to the first layer, rather than a single layer of 3/4". With a glued double-layered approach you can float the subfloor rather than putting it down with TapCons. If the slab isn't perfectly flat or level and there are some "waves", you can TapCon the sections that need it 16-24" o.c. to keep it from flexing. With a single layer of subfloor each sheet needs to be screwed to the slab to prevent "potato chipping" curl lifting the corners from normal seasonal moisture shifts, whether half-inch or 3/4".

There is no need to dip the screws in sealant for the vapor barrier to function as vapor barrier. Vapor diffusion is a vapor pressure times cross sectional area type of thing, and the annular ring of "leak" around the screws are quite small, and sealed with vapor-retardent foam insulation.

5: Vinyl flooring over a wood subfloor in a basement is a bad idea. The seams don't provide sufficient drying area for the subfloor- it's a moisture trap.

6: Leveling the slab is by far the better choice, where practical.

 

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[Netspirit]

Amazing. Thanks a lot Dana. It all makes sense.
If you published a book or created your own YouTube channel, I would definitely buy or subscribe. )