Basement subfloor – different philosophies on sealing perimeter
Hi,
Im planning a basement renovation which has had carpet installed directly on the slab – house is about 30 years old. The walls are finished with drywall, so im planning on finishing my floor to my existing walls.
I’ve been reading about my options for subfloors, and have read many great threads and articles on this site.
Unfortunately, due to several issues (ceiling height, door height – its a walkout, bottom step etc..) I am going as thin as possible. So R2.5-3.2.. 1/2 rigid foam + ply/osb.
Ok so here is the question: why the different philosophies on whether or not to leave a gap for air flow around the perimeter? This site is full of people recommending the rigid foam, osb/ply, tapcon method. and with this method most people seem to recommend using expanding foam around the edges. This sounds good and it is the option I am leaning towards.
But when I read about some of these “all in one solution” products like dricore R+, barricade, amdry etc. they all say you have to leave a 1/4 gap around the edges and their manuals usually use strong language saying you “MUST leave a 1/4 gap”.
One answer would be that they are accounting for potential expansion. This makes sense except for the fact that these products recommend using tapcons for most installations (for carpet they want tapcons around the edges of the room, for tile floors they want tapcons throughout) So in these cases it doesn’t seem that expansion is a concern. But yet, they still insist on the 1/4″ gap.
Im obviously missing something here.
I am considering the ply/osb rigid foam, tapcon, sealed gap method. But depending on cost, I am also considering a Dricore installation and just sealing around the edges.
Any insight would be appreciated.
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Replies
User 68etc,
I think you may be reading too much into the method of attachment. Both plywood subfloors and roof sheathing should be gapped to allow for expansion, and both are heavily nailed (and in the case of subfloor glued). Hardwood flooring is stapled or nailed (and now often glued) but should always be laid with gaps at the perimeter.
Why not leave a gap? Stopping any flooring system shy of the wall doesn't preclude filling the gap with caulking or foam, as the GBA posters you allude to suggest.
User,
First, I hope you can take the time to update your user profile to change your screen name. Here is a link to an article that tells you how: Advice to GBA readers with goofy screen names.
Second, Malcolm's answer was a good one. The manufacturers of the products you refer to are worried about expansion due to moisture absorption. The risk of this type of expansion vary, depending on the dampness of the slab. A small gap (usually covered by the baseboard) is standard. It's OK to use canned spray foam in the gap.
Bryce,
1. If you cover a basement slab with a vapor-impermeable layer, and you leave only a 1/2-inch wide crack at the perimeter of the room and you cover the crack with baseboard, you can't expect any significant upward drying to happen. The crack isn't wide enough to make a difference, and the baseboard covers it anyway. The "leave a crack for upward drying" theory is nuts.
2. A concrete slab isn't damaged if it stays damp. It can stay damp for 200 years without damage. It doesn't need to dry out.
3. Inviting soil moisture into your home is not a good idea.
DRIcore talks about airflow/drying under their panels, but that's asking for condensation/mold under the panels when there is insulation involved.
Well sealed foam + ply/osb makes sense, although if you don't have plastic under your slab, then I'd use a fully adhered moisture barrier. And in either case, don't use impermeable flooring - ie, make sure drying rate exceeds wetting rate so you don't get mold or plywood rot.
Concrete has thermal expansion issues, wood has moisture expansion. A 1/4" gap is about right.
Don't expect spray foam to seal a gap that changes in width. A good movement rated sealant might, gaskets are best.
With a caulked or foamed gap, it seems you're destroying a central premise of the subfloor: the ability for humid underfloor air to dry out eventually. I'm doing some dricore but lucky in that I have one edge that must be sealed with acoustical caulk, but another that opens to an air space.
AmDry says: "Once the (flooring) has been installed install the baseboard
around the perimeter of the wall, this will cover the ½” air space and continue to allow
the AmDry system to perform properly. " So be they crazy or not, the AmDry company seems to feel the drying effect is significant.
@Jon note that DriCore seems to be discontinuing their un-insulated panels.
Both DriCore and AmDry require the floor to be cleaned, AmDry says this is because trapped dirt can mold. I'm pretty sure both would deny a claim if you gasket-ed the edges contrary to their instructions. Note that DiCore just changed the gap requirement from 1/2" to 1". I can't say they are right, just pointing out that they are very specific on these points.
For my personal residence I used a delta brand dimple mat against the concrete, then taped rigid foam. No prefab system, no warranty. And that's over a slab from the 1930's with zero chance of a vapor barrier below.
I suspect DriCore and AmTry, Barricade and Tyroc actually have the building science wrong. Their products don't work by drying to the edge. They work by a capillary break to protect the subfloor. The venting structures allow a local moisture excess in the concrete to equalize out more readily. And perhaps, maybe, the products allow occasional tiny amounts of liquid seepage to equalize out.
I'd love to see a real building science person analyze how the products actually perform.