Exterior foundation insulation: polyiso or extruded polystyrene

I dont thonk poly isocyanurate absorbes water, I used about 70 sheets on a house years ago, Manufacturere sites will have spec sheets that im sure include the info you need or their tech support. Polyisocyanurate board also has foil covering which should help as a radiant barrier. I used polyisocyanurate everywhere and under the basement concrete.

Styrofoam is carried by most every building supply house, if that is what you really want. Styrofoam is the trademark name for Dow Chemical's extruded polystyrene board. Is that what you are looking for?

For exterior and under slab, you don't look for R value per inch, but look for R value per dollar. If you can buy 3" or 4" of a lesser R value for the same price as poly iso, it is a better deal and the thickness is still going to work. You are not limited in space.

What is probably the best deal is expanded polystyrene board, often called bead board and often mistakenly called Styrofoam.

Where are you located? I know a half dozen places in New England that make it and maybe can help you find a supplier in another area.

CMU walls are covered with tarp so there is no direct contact. For underslab my understanding is polyethelene vapor barrier is under insulation, so concrete slab is poured over insulation sheets.

Styrofoam is good, but I wouldn't use the bead board version because it absorbs water a lot worse than the type that's molded in one step.

"larry moe 'n curly" wrote

There is no bead version.

I live in Central NJ. In my area both Lowes and HD carry Super-TUFF-R and none carries Styrofoam. Lowes has its own brand of extruded polystyrene which costs the same for 2" 8' X 4' baord as Super-TUFF-R but it has lower R value. I checked a couple of building suppliers, they sell foam insulation for much more then either HD or Lowes. So if extruded polystyrene is preffered over polyiso for exterior foundation can I use the Lowes brand in place of Styrofoam?

"ls02" wrote

Sure you can, but you can probably find expanded bead foam for less and can go thicker for better insulation.

Give these guys a call. If they can't help you then can tell you who is close to you.

Last I checked R value per dollar put polyisocyanurate and polystyrene equal, the polyisocyanurate has foil both sides which has to account for some radiant advantage.

And some has fiberglass felt on both sides instead of foil. Generally the thicker stuff used primarilly on built-up flat roofing.

There is, to people who aren't troublemakers. ;)

I'm not referring to Styrofoam=99=A9=AE but generic styrofoam, AKA expanded styrene foam. which is made in both 1-piece foam and bead board versions. Bead board, made from tiny expanded styrene foam balls pressed and heated together, absorbs water much more easily than 1- piece styrofoam does.

"lesbian bob" wrote

Sorry to tell you this, but there is only one version of Styrofoam. There is no generic version. I've been in the industry for 40 years. Dow has a staff of lawyers looking for mis-use of their trade name for extruded polystyrene board. As you can imagine, they are very busy.

Polyiso insulation board is made with a variety of facing materials. Some are specifically made for use inside concrete forms( Rmax Concrete Core is one such product.) Check with commercial building supply houses.

I rather suspect he's got the project finished by now , after all he started it in 2010 by the time stamp on the message you responded to .