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Community and Q&A

Insulating basement wall

cgeorg07 | Posted in General Questions on

Hello all, looking for some input on insulating my basement. Home is new construction in New Jersey, and builder installed a drain channel? around the perimeter of the basement – 1/2 thick dimples off the wall and goes up the wall 2-3 inches. What are my best options for insulating? Township requires R15 total insulated value.

1. Install dimple membrane on entire wall surface, then 2″ Foamboard over membrane, then and regular batt insulation between stud wall
2. Frame the basement 6+ inches off the exterior walls and use regular batt insulation
3. Foambard flush to wall and stop at top of drain channel
4. ??

Ruling out spray foam as the cost will likely be out of budget to do 1,200 sq ft.

Your guidance is much appreciated!


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  1. Expert Member
    BILL WICHERS | | #1

    I would run dimple mat all the way up the wall and then put rigid foam (polyiso) directly over the dimple mat. The dimple mat will preserve the function of the drain to carry away any water that comes through that wall.

    If your basement is completely dry and you’re confident it will stay that way you could omit the dimple mat, but I wouldn’t recommend it.


  2. Expert Member
    NICK KEENAN | | #2

    If there is someone near you who sells used polyiso that is usually the cheapest route. A thickness of 2.5" would give you the R15 you need. Check with your town whether they would require a fire barrier over it.

  3. Expert Member
    NICK KEENAN | | #3

    Used insulation is commonly found on Craigslist in the Materials section.

  4. cgeorg07 | | #4

    Thank you all for your input! It's been dry for 2 years now, but better to be safe than sorry

  5. Expert Member
    AKOS TOTH | | #5

    An easier option than dimple mat is to use grooved foam board. This is stock item at the local box store.

    Another option is to tape up a layer of crinkly house wrap on the concreate.

    If you are gluing the foam up, the glue itself will space the foam off an 1/8" or so and create a gap. Just don't put any long horizontal beads that can hold water.

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