Under slab insulation

Weezbo

nearby rivers and lakes or ponds on or near your property would indicate that you would want to have someone look at the proposal to insulate and how much ,where and what quantity of r value...Mark Eaterton is doing some studies on a similar note as we speak.

bob_55

Under slab insulation

Does anyone have an opinion on the best insulation for under slab in a radiant system? I have heard Insul tarp is good but some people are saying go with rigid board?

Thanks

Unknown

If you only need a basic thermal break, insultarp will do that. Your perimeter and edges need real (read: rigid) insulation, and the middle may as well if you have clay, ledge or potentially wet ground. Generally 2" thickness is typical.

Insultarp and rigid are NOT equal. Not even close.

Constantin

Like Rob said,

it depends in large part on the local soil conditions. Those of us who are lucky enough to live in a temperate area that is sandy and that drains well, only a thin thermal break is needed. Even Insultarp may work under those circumstances.

Our house isn't that fortunate, as we're surrounded by clay soil 9' deep, it gets very cold, and it's also quite wet around here for much of the heating season. We went with 2" of XPS throughout (on the outside of foundation walls below the surface and below the slab as well), though my preference would have been to use a bit more around the slab edges. However, the looks that my insulation and concrete contractor gave me when I suggested greater thicknesses shelved that idea.