Insulation under slab

Like others said, don’t use Polyiso. Use only XPS or EPS. EPS is cheaper and some like it better because it dries if it gets wet. XPS tends to retain the moisture and loses its insulating properties faster over time. Whatever you choose, it needs to have high compressive strength to hold the weight. I found these 2 articles helpful: http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-insulate-foundation-0 http://www.greenbuildingadvisor.com/blogs/dept/musings/polyethylene-under-concrete-slabs

mopeslope

in Insulation 9 years and 7 months ago

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workisfun | 9 years and 7 months ago

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We are pouring a new slab on our ground level. We were strongly advised to lay a vapor barrier first and then use 2″ Polyiso, r-value 13\. (We are also laying tubing for radiant). Now, I’m being told that we can’t or shouldn’t use the Polyiso, but rather EPS. If there is a vapor barrier, will the Polyiso defrade at all? At what rate? I can’t stack enough EPS to achieve the same r-value of 13, that the 2″ Polyiso is giving me. PLEASE ADVISE!! Thanks.

ellenlourie | 9 years and 7 months ago

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You need something like this: http://www.foamular.com/foam/applications/concrete-slab.aspx with certain compression strength. Polyiso is for walls and roof. If you do radiant, then do also perimeter insulation of the slab. Get advice from good professional.

Augustiner | 9 years and 7 months ago

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EPS can carry more weight than Polyiso I guess. just go with 2″ EPS or concrete floor rated panels. It is a good idea to insulate the cellar floor if you want to use the cellar for a recreational room. But if you have an R value of 11 or 13 on your cellar floor is really not of any importance. Unless you are the only homeowner in Brooklyn who plans to do all of the following – insulate and waterproof cellar walls from exterior – take out the end floor beams and properly seal exterior walls in between floor assembly – make a real passive house with an advisor who really knows what he’s doing and contractors that are supervised every inch they move -> and even then the difference betweem 2″ poly or EPS is neglectable

workisfun | 9 years and 7 months ago

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Thanks for the perspectives. It was a trusted architect who recomended the Polyiso. Also, to August’s point, the ground floor is technically a basement, I guess. It is 3 feet in ground on 2 sides, at grade in the back, and attached on the fourth side. Our bedrooms and baths will be there. It is the lower floor of our duplex. Big windows and semi-detached. Just want to make the right choice here, and haven’t gotten a consensus yet. Thanks. I hope more comments roll in to help me make sense of it. I thought Polyiso could take the load of a 4″ slab. Arghhh.

Augustiner | 9 years and 7 months ago

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Is it the garden level or the cellar?

workisfun | 9 years and 7 months ago

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Garden level. We don’t have a cellar.