Replies

1. Expert Member
   Akos | Aug 10, 2020 02:06pm | #1

   Dan,

   You don't want poly directly over the concrete, this can create mold issues. A vapor barrier coating on the concrete is a better option.

   The insulation bellow the subfloor is mostly for comfort, more is always better. Unless you have height issues, I would go for thicker.

   Not sure where this steel stud for basements fad started from but it is silly. You loose about 1/2 the R value of the insulation between them, they are harder to work with, harder to run utilities and harder to hang shelving. Stick to wood studs. A properly detailed basement should never encounter conditions that would cause the wood studs to rot.

1. 
   danlikesbikes | Aug 10, 2020 02:19pm | #3

   Hi Akos - i have previously read that if i don't have poly under the slab (i don;t - confirmed when slab was cut for plumbing) - i should be putting some overtop. You say the poly can create mold issues - on what side?

   From Martin Halloday - https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab

   Do you need polyethylene above the slab?
   Ideally, your basement slab was installed over a layer of 6 mil polyethylene. If you’re not sure whether your slab has poly underneath, you can always drill a hole through your slab in an inconspicuous area of your basement. If the diameter of the hole is at least 3/4 inch, you should be able to tell (with your finger, a probe, or a flashlight) whether there is any poly under the slab.

   If there’s no poly under your slab, or you’re worried that there may not be any poly under your slab, it’s probably a good idea to install a layer of 6-mil poly between the top of your slab and the new rigid foam.

   The purpose of the polyethylene is to limit vapor diffusion from the damp slab to the new OSB or plywood subfloor. Of course, the rigid foam that you are about to install is already a vapor retarder — perhaps enough of a vapor retarder to put any worries about vapor diffusion to rest. But poly is cheap, and an extra layer of poly in this location does no harm. So you might as well install the poly.

1. Expert Member
   Akos | Aug 10, 2020 06:00pm | #5

   You can look at Fig 2 here:
   https://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems

2. 
   PLIERS | Aug 11, 2020 12:42am | #6

   Akos,

   What kind of vapor barrier coating would you use? Only one I heard of people using around me is drylok not sure if there is something else I should be using. So height is an issue for me, and floor has uneven spots. So I’m thinking a skim coat of self leveler, then vapor barrier coat, and lvp right over floor. I know lvp isn’t green but not sure what else would be good in an area like a basement without building up a subfloor.

2. Expert Member
   Dana Dorsett | Aug 10, 2020 02:16pm | #2

   Can we assume there will be interior latex painted gyprock on the interior side of the assembly?

   The R14 studwall section would only perform at about R7 if steel studs, but R11-ish with wood studs. With R10 of EPS there is no risk of moisture accumulation in wooden studs from interior moisture drives in your location, and would be fine to use if there is no history of flooding.

   What will be the finish flooring above the subfloor? The difference in overall energy use and comfort between R2 vs. R3 under the subfloor is pretty small, and R2 is reasonably moisture safe as long as the finish floor isn't carpeting. Carpeting with underlayments would lower the average temperature of the subfloor, raising the moisture content of the wood, especially when the dew point of the humid summertime air is above the subfloor temp. R3 gives a bit of safety margin over R2, but if the basement is mechanically dehumidified or only ventilated with dry air-conditioned air it's fine to leave it at R2.

1. 
   danlikesbikes | Aug 10, 2020 02:36pm | #4

   Hi Dana - yes the interior will be 1/2" regular paper faced drywall painted with "dulux diamond acrylic latex interior paint".

   For my floors - tiles in the bathroom. Rest of basement will be some form of "wood look" product. My wife will likely want some decorative rugs over the wood so i will go with the R3 EPS if that is safer.

   For wood type flooring do you have any opinions on vinyl plank vs laminate vs engineered hardwood? I believe i had previously read the vinyl plank could possibly lead to a double vapour barrier situation (vapour barrier on top of slab, eps, osb, vinyl plank also acts as vapour barrier).

2. 
   danlikesbikes | Aug 11, 2020 01:14pm | #7

   Hi Dana - do you also have any opinion on my subfloor?

   I do not have vapour barrier under slab (my construction drawings filed with the city say i should have it but when i cut slab to move plumbing i confirmed i do not).

   Would you place a vapour barrier on top of slab?

   Thinking
   - 6 mil poly
   - 3/4" EPS
   - 5/8" osb

   Another user commented that sealing the concrete would be better but my preference would be 6 mil poly as it will be much faster install.

1. Expert Member
   Dana Dorsett | Aug 11, 2020 01:25pm | #8

   6 mil poly under or above the EPS is fine. In some ways it's better to put it above the EPS, between the EPS & OSB. That's because unless the concrete is perfectly flat and the poly is between the slab & EPS there will be voids where water can pool between the poly & EPS, taking forever to dry due to the vapor retardency of the foam & OSB. If the poly is between the EPS & OSB the size/volume of any voids will be smaller, ergo less potential for localized spots with chronic high moisture conditions that can affect the subfloor & finish floor.

1. 
   danlikesbikes | Aug 11, 2020 03:08pm | #9

   For finished floor i'm guessing i cannot use something like vinyl plank since the low perm rating would result in a double vapour barrier effectively sandwiching my OSB?