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Block house. Floor joists have concrete in between. Should I insulate?

user-5776488 | Posted in Energy Efficiency and Durability on

I live in a block house in zone 5. So I want to finish out my basement with 2″ eps followed by 2×4 walls 24″ on center with roxul insulation then drywall. I don’t have your typical rim joists to insulate. My wood floor joists look to be inbedded in concrete. Built in 1947. How should I handle insulating this area? Or should I leave it alone and let it breathe. I’m concerned because I believe cinder blocks normally contain moisture and the exterior I believe has 1/2″ eps or xps and then vinyl siding. I just don’t want to push moisture into my floor joists. Any help would be appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    With vinyl siding the CMU wall has excellent drying capacity to the outdoors as long as there are no facers on the half-inch foam. If it's foil faced EPS it's blocking drying toward the exterior, so find out what's really there- it matters.

    Take a 2-prong wood moisture meter, and measure the moisture content of the joist where it enters the concrete. If it's high now, it'll probably be higher if you put 2" EPS on the interior.

    It's unlikely that it's a real issue, but clearly putting the additional foam on the exterior would be better than putting it on the interior. There will be some flashing details to attend to.

    Is there any sort of capillary break or gasket between the sill at the top of the wall that supports the rafters?

    1. user-5776488 | | #2

      In the attic I believe the wood lays directly onto the concrete blocks. I'll double check when I get home. Is that what you're talking about?

      1. Expert Member
        Dana Dorsett | | #3

        Yes- the plank that rests atop the wall that supports the rafters. Ideally there would be at least some #30 felt or something even more moisture repellent between the plank & CMU.

    2. user-5776488 | | #4

      Its definitely either xps or eps. I've seen both but have no idea what the bulk of it is. It is between pieces of 1x with untaped seams. Eventually I plan to reside with insulated vinyl siding. So something like this would be a more worthwhile investment? But then wouldn't it cause the block to dry inwards?

      1. Expert Member
        Dana Dorsett | | #6

        XPS is usually pink blue or green with no facers. EPS has visible coarse bead structures (like cheap coolers or hot beverage containers.

        Yes insulating on the exterior means it dries toward the interior- so? The CMU gets less rain wetting, and the embedded joist ends stay warmer (= drier) in winter.

        In zone 5 as long as more than half the R-value is on the exterior of a CMU wall R13 would meet code min. Assuming you have about R2 out there now adding 2" of Type II EPS would bring it up to R10.5, and adding 3/4" of unfaced EPS on the interior strapped to the wall with furring through screwed to the CMU onto which the wallboard could be mounted would bring it up to current IRC code. At 3/4" EPS is more than 3 perms (unless it has a facer) about the same as latex paint on wallboard, which is still an adequate drying path, and will not trap moisture in the wall. If you can't find 3/4" EPS without facers, half-inch XPS should be fine too, at about 2-2.5 perms, but not thicker.

        The 11% moisture content isn't of any concern, but the 15% might be. If there has been a lot of rain wetting of the glass block portion that could explain it. As long as it doesn't dwell in the high teens or 20%+ range for many weeks on end it's unlikely to become a problem. If the glass blocks are going to be covered with insulation that would likely fix it all.

        Window flashing and weather resistant barrier details should be thought out in advance when insulating on the exterior.

  2. user-5776488 | | #5

    I measured the moisture content of floor joists ranging from 11-15%. The 15% being the ones near the glass block windows. I dont know if it matters but its been in the 40s and rainy today.

  3. user-5776488 | | #7

    So there isnt a way for me to finish my basement until i tear off all of the siding, add insulation and then reinstall the siding. Ive included a picture of what it looks like.

  4. user-5776488 | | #8

    I have another question. What about the glass block window sills? My glass block windows are outties. Should I put a layer of eps around the window sills too?

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