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Community and Q&A

Using Fanfold Insulation Bundles

drewintoledo | Posted in Energy Efficiency and Durability on

In north Ohio where frost line is just over 36″ down.  I am in the process of insulating my basement walls where I’ve already used a layer of 1″ EPS over a layer of 2″ EPS for a total of R12 around the house basement.
however, I have been gifted about 12 bundles of fanfold from a friend.  As I understand, this is EPS insulation and I’m not installing vinyl so I shouldn’t need it on the sheathing.
Where is it more useful to place this around the house?  It seems that it would make sense to keep it folded, near grade vertically along the basement walls instead of burying it deep below the soil as to “break” the frost line from the basement walls.
Another thought would be to place it around the garage.  I didn’t plan to insulate the garage but now I may.   If I place it around the garage, does it make sense to place it horizontally around the walls either on the exterior or interior side?
Just looking for suggestions for the best way to place this free gold.   WWEND?  (What would energy nerds do?)  🙂

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  1. drewintoledo | | #1

    I did not receive any replies, most likely because I was too vague with my question.
    I've decided to lay the EPS flat around the garage walls. But, can anyone tell me if it makes more sense to lay it around the exterior wall in attempt to slow the cold ground from reaching the wall, or to place it around the inside under the concrete garage floor in attempt to slow heat loss from the garage to the environment?

  2. Expert Member
    BILL WICHERS | | #2

    That fanfold stuff is usually XPS, not EPS, but that's not a big deal here. It's probably the 1/4"(ish) thick stuff rated R1, so it's not going to do much BUT it's hard to argue with free! :-)

    I'd probably put it around the exterior of the foundation wall, which is probably where you'll get the most benefit. I don't think it's thick enough to do much for the slab, and insulating the perimeter wall helps the slab a little too by keeping the ground under it ever so slightly warmer than it would otherwise be. Note also that the fanfold stuff is not rated for compressive strength as far as I know, so you could possibly have issues with an inspection if they want to see a PSI rating for material used as subslab insulation. Using the material on the wall eliminates the need for any compression strength rating.


  3. gusfhb | | #3

    I think insulating the perimeter is more advantageous than insulating under the slab without insulating the perimeter. While R1 is not much, it is doubling the insulation value of what is there[there is not such thing really as R0] and if you are not actually heating the space it will help manage the temp swings, and keep the slab more consistent.

  4. drewintoledo | | #4

    Thanks guys! Exterior it is.

  5. drewintoledo | | #5

    My neighbor is a laborer in the commercial construction business. He told me in commercial applications they will place insulation on the interior side of the wall perpendicular to the wall. I asked him why and he replied "that's just what we're told to do". He was clear that he was not referring to full under-slab insulation

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