Above-slab basement insulation
James_Kinder
| Posted in General Questions on
I’m looking to finish my basement. My question is about adding insulation above the concrete slab. From researching articles I’m leaning towards using 1.5″ EPS directly on top of the concrete and 2 perpendicular layers of 1/2″ OSB floating on top of the foam. my question is do I need a poly vapour barrier between the foam and OSB? or between the slab and the foam? I’m located in climate zone 6 (southern Ontario). There is no poly under the slab.
Thank you
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Replies
Hi James -
You certainly need a continuous moisture barrier under the EPS. The question is how much moisture protection do you need? You concrete slab may seem "dry" but to really know the moisture content of your slab you need to drill down 40% of the depth of the slab and test there (see this GBA article: https://www.greenbuildingadvisor.com/article/extending-the-reach-of-a-moisture-meter).
If there is significant vapor making its way up through your slab and you simply put poly down on top, you will likely end up with a ton of condensation under your poly and where will it go?
You may need to consider dimple mat to give a space for this condensed water to accumulate and drain to a sump--of course all a much bigger proposition.
And in terms of EPS - make sure it's the right density, which is identified by "Type" - you should spec Type IX.
Peter
Thank you very much Peter for replying so quickly. I will read through the recommended article and test my floor.
This is something I've been wondering about... whether moisture trapped by poly would accumulate (and cause problems) or if a saturation point would be reached and find equilibrium.
If there are bulk water issues, obviously there would need to be some drainage addressed. But it seems you are suggesting, Peter, that even a wicking type of moisture transfer could cause an accumulation of water worthy of draining?
I guess to some extent, I'm wondering if poly laid flat to the concrete would actually limit the accumulation of water via gradient change (once the thin film of air between the poly and concrete became saturated, it would cease to take on more moisture). Just a theory. I'm guessing it would depend a lot on the situation and whether or not there was any hygrostatic pressure or simply a moisture gradient.