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retrofit basement slab edge insulation?

mcmodern | Posted in Energy Efficiency and Durability on

Greetings!

Long time lurker, first time poster. Thanks for all the great info. Searched the archives, but cant seem to find an answer.

zip 12572, NY Hudson Valley Zone 5A

House built in 1961. on top of a hill, very good drainage, dry soil, no standing water after heavy rain, & 3ft eaves overhang walls

26′ x 50′ Monolithic Basement slab, guesstimate 4-6″ thick. poured between the block/basement walls. [R20 basement walls internal insulation planned]

slab is on gravel /stone crush fill, assume no vapor barrier or underslab insulation.

about 10yrs after construction, rear hillside was completely excavated level, and an inground pool installed. so the basement is now at ground level on one side– ie no longer a basement. and will have mostly windows

not sufficient ceiling height for on slab insulation and tests show a decent polished concrete floor is possible.

so my question is, please.

to reduce heat loss through the exposed slab/floor, is it worth cutting a trench around the outside of the basement slab, down to grade [approx 4-6″] and removing concrete, for edge insulation?

If so how much insulation?

I have seen varying estimates of edge loss for an uninsualted slab, from 25-80% of total slab loss.

Any advice comments much appreciated.

Thanks!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    R10 vertical slab-edge insulation down to 2' depth is IRC 2012 code min for zone 5.

    Rather than cutting into the slab to trench around it installing 2.5" of EPS on the exposed exterior of the concrete down to 2-3 ' below grade and finishing it with a purpose made fiber-reinforced cementicious coating (such as Quickkrete Foam Coating) that is still sufficiently vapor permeable for outward drying works. You'll have to install some Z-flashing tucked behind the WRB at the foundation sill to direct bulk water out to the exterior side of the EPS works. Use only unfaced EPS or it'll be too vapor retardent, and could accumulate & drive moisture up in to the foundation sill. (Low, but not no risk, given your excellent drainage.)

  2. mcmodern | | #2

    Thanks very much for the replies/info.

    "With the perimeter thermal break, the heat loss will be worse than with a slab on grade with wood-framed walls, because the concrete walls act like heat sink fins. "

    I'm not sure I understand this. Do you mean "without"? the perimeter thermal break?

    Can you please explain how insulating the top 2-3' of the wall exterior will mitigate loss from heat transferring from the slab edge, to the still uninsulated first 4-5' of the exterior of the basement wall, and to the soil ?

    I had considered digging out and insulating externally completely, however, it isn't possible to access the walls completely, externally. [There is a large slab on grade entry porch and a large granite chimney obstructing this] and given the slab is uninsulated it seemed there would be little advantage, but much more work than internal insulation [excepting slab edge loss]. interior space is not an issue.

    The walls are 10" hollow concrete block. Is it worth retro filling these with perlite? as the sill is 6", there is access to the hollow cores from above

    any best guesstimates on slab edge loss please?

    thanks again for the advice

  3. charlie_sullivan | | #3

    Good question, as that's not easy to model accurately, much less to do a back-of-the envelope accurately. My hunch is that your proposed insulation would help significantly. Otherwise heat will conduct through the floor to the exterior wall. With [edit: should have been without] the perimeter thermal break, the heat loss will be worse than with a slab on grade with wood-framed walls, because the concrete walls act like heat sink fins. Hollow concrete block ("cinder block" or CMU) walls are not as bad as poured concrete in that regard, but will still conduct heat up from the slab.

    An alternative would be to insulate the outside of the concrete wall, and continue that down to the footer or lower. That would also protect the footer from frost damage. It seems you escaped that problem by having good drainage, but it still might be prudent to avoid having the footer below freezing temperatures even if you expect no water there.

  4. charlie_sullivan | | #4

    Oops--you are right--I meant "without". Original post edited.

    If I understand right, you've got one foundation wall exposed on the exterior, and others with ~8' of soil, and your question in the most recent post is about the ones with 8' of soil--how 2-3' deep insulation would help. But I think Dana and I were recommending the 2-3' deep for the side with the exposed foundation wall so it's basically slab on grade.

    For the walls where there is soil against them, what are the options that you want to compare? You are nixing exterior insulation and comparing internal wall insulation vs. internal wall insulation plus a thermal break at the bottom? In that case I don't think it would help that much because at 8' deep, the wall is going to at the same temperature as the soil, and so it's not so different from the uninsulated slab.

    I'm not sure about filling the blocks. On the one hand you've got the interior insulation which is going to be much better, but on the other hand the vertical heat flow there could undermine my idea that the deep reaches of the wall would be at soil temperature--they might get cooled closer to exterior temperature. I'd don't know how much that would cost but if it's not bad, it seems like there is no downside.

  5. Expert Member
    Dana Dorsett | | #5

    "..please explain how insulating the top 2-3' of the wall exterior will mitigate loss from heat transferring from the slab edge, to the still uninsulated first 4-5' of the exterior of the basement wall, and to the soil ?"

    The entire exposed concrete both above grade, and down to 2-3' below grade would be insulated, not JUST the below grade section. The soil has both thermal mass and R-value (varies with soil type, & moisture content). Ideally you'd insulate all the way down to the footing, but the IRC only requires 2'.

    On the above grade exposed concrete, R10 contiuout foam covered with cemeticious finish would make it a mass-wall assembly. To meet code it would then only need another R3 on the interior side, which could be half-inch polyiso, for at total of R13 continuous insulation, with more than 50% of the R on the exterior side of the thermal mass per TABLE N1102.1.1 :

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

    If the R10 then continues another 2' below grade, it's effectively the same as insulating the slab edge to 2' below grade ( also per TABLE N1102.1.1 ).

    Adding perlite to the open cores does improve the thermal performance, but if you're going with the exterior foam concept it may not be worth it, depending on what the stuff costs. Filling the cores with non-expanding injection foam would do more than perlite, and would make it much more air-tight, but that's not super-cheap either. Bumping the foam-R by another R3 would offer more performance than either injection foam or perlite in the cores.

  6. mcmodern | | #6

    thanks again for the replies/info

    Ive attached a sketch.

    "But I think Dana and I were recommending the 2-3' deep for the side with the exposed foundation wall so it's basically slab on grade." thanks-- this is doable

    on the walls 8' inground, I can insulate externally, down to 2'+ below grade, just not the entire perimeter due to the obstacles [equal to about 20 lf of wall in about 100 lf.

    So if I now understand correctly, insulating the top exposed part of the wall and at least 2' below grade, has the same effect as cutting the slab edge, to put in R10 edge insulation?

    soil temps 50ish, interior temp 70ish, so I still dont understand why energy will not travel from the slab edge to the soil because of insulation 4' above it.

    obviously energy will travel down thru the uninsulated slab as well, and into the soil, but the lack of info on uninsulated slab loss down vs edge loss makes it hard to determine the value of a retrofit thermal break at the edge

    thanks

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