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Zone 4 basement remodel: how much insulation is enough?

BrianM72 | Posted in General Questions on

I’d like to insulate the basement on my central Virginia home to save on energy costs, and so I can more comfortably use it as a home office this coming winter — when I expect I will again be working from home, just like I’ve done since mid-March of this year.  It’s a walk-out with a shady yard; one wall below grade, two are half below grade, and one (southern) is above grade; all walls are cinderblock.  It is currently unconditioned, though I’m considering opening a register in one of the ducts. I’ll probably put interlocking foam mats on the concrete floor for now. I’m also insulating the rim joists myself (foam board + great stuff + rock wool barrier). My budget is limited ($5K tops). I’m also a half time single parent working from home with little childcare this summer (thank you COVID) so my time for DIY is at a premium. 

I’m considering Insofast, with additional concrete fasteners. This would be R10 continuous which meets code for below grade mass wall in an unconditioned basement. The other option is 2″ XPS foam board, frame, batt + drywall. Significantly more work, and slightly more expensive.

My question is, would this be sufficient in practice to be reasonably comfortable?  Mostly looking for perspective from folks who’ve done something like this in this zone.

Thanks in advance for any advice, comments, or experiences!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi user-7700882.

    I imagine that with good work at the rim, code-minimum R-10 is enough insulation on the walls. Insofast is an EPS product, I believe, and EPS is much more environmentally-friendly than XPS. I would be curious to see a cost comparison on Insofast vs regular EPS or polyiso plus stud walls. Framing lumber is not expensive and the walls give you a place to run electrical and studs to fasten shelves and such to which may be helpful in a home office. Also, if it is comfort you are after, a layer of insulation on the floor may be worthwhile too. Here are a couple of articles that may be helpful:

    How to Insulate a Basement Wall
    Stay Dry, No Mold Basement

  2. Expert Member
    Akos | | #2

    I'm in colder climate than you. Here the common basement insulation is batts in 2x4 walls (works out to around R11 whole assembly), which is plenty enough for comfort.

    Usually the bigger issues with comfort is air leaks, but it sounds like you are already on that.

    For a quick budget finish/insualtion, your best bet might be one of the rated insulation products (Thermax) without any studs or wall cover. This comes in white and you can tape the seams with white tape for a reasonable finish. You can also strap out the wall down the road if you want to finish with drywall.

  3. BrianM72 | | #3

    Akos, for my project -- 77 linear feet of wall, 7' high -- the price comes in comparable. I estimate:
    traditional: $650 for lumber and nails + $550 for 2" XPS = $1,200 or so + tools
    inSoFast: $1,500 (14 cases)
    plus drywall, adhesive, and vertical fireblock materials for both. Traditional you can then relatively cheaply get to a higher R value by putting fiberglass or roxul batts in. This is for the 2.5" (EXi) inSoFast that has built in electrical raceways and supports installation of electrical boxes (admittedly not in an ideal fashion: you cut out a section of the panel).

    The appeal to me is mainly convenience -- my time is limited, I've never framed a wall nor do I likely have all the tools I'd want for that. Secondarily, it's appealing to use EPS instead of XPS for environmental reasons. I've been unable to find EPS panels for sale, the big box stores appear to have all gone to XPS and polyIso.

    I like your suggestion of thermax and will most likely do that for the other half of my basement, a lower ceiling, more cellar-like space with the mechanicals, washer & dryer. The only concern I have with this is if the thermax facing would act as too much of a vapor barrier and encourage condensation to build up on the cinderblock walls.

  4. Expert Member
    Akos | | #4

    Generally for any of these projects, the hard part is the final finish. The insofast panels do save on framing, but you still have to apply drywall and mud and tape. That last step is what usually takes time. I can see why you would choose this product for DIY, it does seem to have some nice features that will make your life easier.

    Condensation comes from air leaks reaching cold masonry surfaces, if you detail the foam well and air seal the seams and top and bottom, no air can back there. The fact that there is a vapor barrier on the foam does not matter.

  5. BrianM72 | | #5

    Thanks Akos. To seal and detail the thermax, is foil tape sufficient? or would you use great stuff or the like around the whole perimeter?

    1. Expert Member
      Akos | | #6

      If you are looking to finish that space down the road, you should include fire blocking above the foam layer.

      For example, if you go with 1.75" of foam, you can put a 2x3 on the ceiling. This would leave .75" of space on the inside for 1x4 strapping for drywall down the road.

      For air sealing, the foil tape across the seams is enough. At the top of the wall, you can tape against the 2x3 or if uneven, go with canned foam (canned foam will be quicker). The rim joist air sealing should seal to the top of this 2x3 as well.

      A bead of canned foam doesn't hurt along the bottom as well.

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