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Basement insulation (hybrid) – guidance needed

jaymack1979 | Posted in Energy Efficiency and Durability on

I am in the process of installing basement insulation in a new construction. I am looking at cost effective methods. I have 145′ of below grade basement wall (9.5′ tall). I have had the builder install a small amount at this point in one of the “immediately finished areas” approximately 30′ x 9.5′.

Here is what I have so far (PLEASE COMMENT ON THIS)

ABOVE GRADE LIVING AREA 2×4 walls(scheduled insulation):
3.5″ open cell in walls
R49 fiberglass blow in ceiling

ABOVE GRADE GARAGE 2X4 WALLS (scheduled insulation):
R13 fg batts in exterior walls
R19 fiberglass blow in ceiling

BASEMENT BAND (2′ floor truss)
1.5″ closed cell foam

1″ XPS glued to wall
2×4 stud cavity “pressing” XPS to wall.

2×4 studs are held 1″ away from wall
have 1.5″ of closed cell spray insulation scheduled for this area

no exterior insulation in basement
*no exterior insulated sheathing on house
****KY climate zone 4

—- I plan to insulate the rest of the basement myself

1. would it be wise to add unfaced fg batts to the cavities?
2. what is the best method to hold the unfaced insulation in the wall cavity (I have never done this before)?
3. is there a more cost effective method for insulating my basement than this hybrid approach?
4. is 1.5″ of closed cell sufficient for the 2′ band below the floor and above the basement?
5. any comments on above grade insulation strategy?

Thanks once again.

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  1. GBA Editor
    Martin Holladay | | #1

    This article will explain what you need to know about insulating basement walls: How to Insulate a Basement Wall.

    It sounds like you probably need a higher R-value to meet minimum code requirements, but it's hard to say because you didn't tell us your climate zone.

  2. jaymack1979 | | #2

    " **no exterior insulation in basement
    ***no exterior insulated sheathing on house
    ****KY climate zone 4
    ---- I plan to insulate the rest of the basement myself "

  3. jaymack1979 | | #3

    Martin - great article. I have read that before and may have been where I got the idea of adding fiberglass to the interior of the XPS. I would love to be able to use all foam for my basement... but it seems that my budget keeps getting blown everywhere else before I get to that point. My hopes were that the 1" of plastic faced "blue" and/or "pink" board would provide enough protection to keep water from condensing on the interior walls... thus allowing me to add unfaced FG batts (to lower costs). I have read tons of literature and opinions on basement insulating strategies. I am a novice at home building and hope this is the LAST house I build... but am still amazed at the number of recommendations for poly sheeting in basements and other bits of info that just don't make sense scientifically.

  4. GBA Editor
    Martin Holladay | | #4

    In your climate zone, you need at least R-10 basement wall insulation if you are using foam insulation, or R-13 if you are using fluffy insulation. I don't recommend the use of fluffy insulation.

    The best approach would be to install 2 inches of polyiso insulation or 2.5 inches of EPS insulation. But 1 inch of XPS plus fluffy insulation between the studs will also work.

  5. charlie_sullivan | | #5

    Given that budget is a concern, and, I assume, being green is a concern (since you posted here), I'll second Martin's recommendation for EPS over XPS or spray foam. Cheaper, especially compared to XPS, and avoids the major global warming impact of the gas used to form the bubbles in XPS and spray foam.

  6. Dana1 | | #6

    "ABOVE GRADE LIVING AREA 2x4 walls(scheduled insulation):
    3.5" open cell in walls
    R49 fiberglass blow in ceiling"

    The wall insulation is way out of proportion (on the low end) relative to what you're doing with the rest of the house. IRC2012 code min for above grade walls in zone 4 is R20 (2x6) or a continuous R5 over R13 2x4 framing.

    The best you'll get out of 3.5" of open cell is still shy of R15, and it's thermally bridged for ~25% of the area by R4 framing. Even if you changed the open cell specification to closed cell to get R20 at center cavity, it would still underperform R20 in 2x6 framing, since the framing fraction in 2x6 framing is R6.5 not R4, and you can get the same structural capacity at 24" o.c. spacing for a lower framing fraction.

    It's finacially rational to go higher than code min too, if you plan to live there for more than a decade (even more so if you have expensive heating fuels such as propane, resistance electricity, or heating oil.) A good starting point for the financial rationality discussion can be found in Table 2 on p.10 of this document:

    Note, those are "whole assembly" performanc values, with the thermal bridging of the framing averaged in.: A code-min R13 + R5 c.i. wall comes in at about R15 "whole-wall", not R18, due to the R4-ish thermal bridging, and similar to the R14-R15 you would get out of R20/2x6 construction. For Zone 4 the Building Science Corp analysts are are suggesting R25 walls may still be cost effective over the long term, which would be a 2x6/R20 wall, with R10 insulating sheathing.

    If the siding is already up it's too late to add continuous insulating sheathing on the exterior, but it's not too late to put it on the interior. After the o.c. foam goes in, but before the gypsum goes in you can add a couple inches of polyiso and nearly DOUBLE the performance of the above grade walls, or install just an inch to at least hit IRC 2012 code-min performance. If the siding isn't up yet you still have other options.

  7. jaymack1979 | | #7

    regarding basement -- I can purchase 3" EPS for $1.16 per unit R ($7/1.5" sheet). This will be against a 4month old poured basement wall. Should I not have any concerns with using EPS in the basement directly against concrete?

    EPS definitely is the most cost effective solution and very green. My intentions would be to layer as follows:

    **foam R10+ (prospectively 3"EPS)
    ***2x4 stud wall
    ****covering would be unfaced drywall, "greenboard" gypsum, or plywood/barnwood

    Any comments on this sequence?

  8. Dana1 | | #8

    Assuming a "sheet" is 4' x 8' (32 square feet) and ~R6 @ 1.5", that's (R6 x 32'=) 192 R-ft^2. At $7.00/sheet that's 3.6 cents per R-ft^2, which is pretty cheap. (Virgin-stock EPS tends to run about 10 cents/ R-ft^2, reclaimed or factory seconds anywhere from 2-5 cents/ R-ft^2).

    EPS is perfectly fine against concrete (freshly poured or century-cured). EPS is the most common material used in insulated concrete forms.

    It's still worth dropping in "contractor roll" or "soundproofing" R11s or R13s into the 2x4 studwall (either kraft faced or unfaced, but not foil) even if you have R10 EPS between the studwall & concrete. The BSC recommendations in Table 2 assume a cost considerably more than the 3.6 cents/ R-ft^2 you're looking at. By insulating the studwall you roughly double the performance to about R20 whole-wall (higher than BSCs starting point of R15), but still have zero condensation risk inside the studwall in a Zone 4 location.

    Installing 1.5" of EPS between the bottom plate of the studwall and the slab as a capillary & thermal break against the slab is also worthwhile, unless you already installed more than R6 under the slab and a continuous vapor barrier between the slab foam and the concrete.

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