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Thoughts on this Wall Assembly

Krist0ff | Posted in Energy Efficiency and Durability on

Hello All,

New home construction in Minnesota. Builder is providing the below insulation for the Walls. Can you please let me know if there is a better way to insulate or is this good enough?  Thank you

WALLS:

         5 ½” fiberglass insulation (R-21) house walls

5 ½” fiberglass insulation (R-19) garage walls

         4 mill vapor & sealed air barrier

         3″ Closed cell spray foam at rim joist area

         Tyvek air infiltration barrier on exterior

3” closed cell foam insulation on interior of foundation walls

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Looks like they'll be using high density (highER density, anyway) fiberglass batts on the house. That's a good thing. I'd check what the upcharge would be to use the same on the garage, if it's not much I'd do it.

    What is the "4 mill vapor and sealed air barrier"? Polyethylene sheet on the interior? It would be better to use a smart vapor retarder (Membrain, Intello, etc.) if that's what they're planning. Smart vapor retarders are a little safer.

    Closed cell spray foam in the rim joist is a good way to air seal, nothing wrong there. Make suer they are still planning on using a sill gasket though, and make sure there is going to be a capillary break between the top of the foundation wall and the framing (which a sill seal gasket can act as, although I like to see something beefier like some HDPE or a strip of EPDM roofing membrane).

    Closed cell spray foam on the foundation walls works fine too, although I'd probably spec rigid foam here like Polyiso, assuming it's on the interior. If the insulation were going on the exterior, then I'd use XPS.

    I would ask about putting on some exterior continous insulation, which usually means rigid foam (Polyiso, EPS, etc.). This helps a lot with thermal bridging and is a big benefit in terms of boosting overall R value of the walls.

    What are they planning for the roof/attic? I would normally want to see a vented attic, an air sealing step for the attic floor (sealant and canned foam), and then loose fill cellulose to at least code minimum (R49), although it's easy to go up to R60 when blowing in loose fill so you should shoot for better than code minimum, especially in the extreme northern climate zones like Minnesota is in.

    Bill

  2. Krist0ff | | #2

    Bill, Thank you. I'll find out about the foundation walls. We plan to have radiant heat in the basement. Windows deciding between Marvin essentials vs Alpen - depending on the price difference. Quick question - for exterior continuous insulation, it goes above the foundation walls as well?

    For ceiling, Blown fiberglass – R60 in house, R30 in garage
    4 mill vapor & sealed air barrier
    6” fiberglass at truss heel
    SkuttleTight attic access door

  3. Andrew C | | #3

    RE radiant heat in basement - will that be in-floor heating? If that's the case, you'll need to install more than the normal amount of sub-slab insulation...you are planning sub-slab insulation and vapor barrier, right?

  4. Krist0ff | | #4

    Andrew - This is all new to me and am going by what builder provided. Yes, it is hydronic infloor heating in the basement. Basement floor is 4" concrete. I see a reference to 2" polystyrene R-10 insulation and 6 mil vapor barrier under basement floor.

  5. Walter Ahlgrim | | #5

    What is in the basement that will make it worth the luxury of a boiler to heat the slab?

    Generally basement rooms are only rarely used spare bedroom work out space and storage.

    Electric boilers are expansive to operate, gas boiler not high on the list of green things, Heat pump boilers are almost as rare as hen’s teeth and expensive to buy.
    Will you have Hydronic in floor heating upstairs?

    “6” fiberglass at truss heel” this sounds bad to my ear only R20 where the trusses set on the walls seems like a recipe for ice dams.

    Seems to me with Hydronic slab needs R30 under it and R10 if you drop the Hydronic slab.

    Tell us about the insulation on the basement walls.

    R21 walls are marginal at best consider adding 2 inches of exterior foam getting you R35.

    R60 ceiling is perfect.

    Understand insulation is useless if the warm air in your house is allowed to escape because they did a poor job of air sealing your home.

    Will your home be blower door tested? Did they guarantee you an air tightness number from your test say 3ACH50?

    Walta

  6. Expert Member
    Zephyr7 | | #6

    "Polystyrene" insulation could be either EPS or XPS, both are made of polystyrene, they're just made using different processes. XPS is really best for underslab use.

    I don't really see a problem with your attic insulation plans, although I'd consider blown cellulose instead of blown fiberglass.

    Yes, continous insulation is typically installed above the slab on the regular walls of the home -- all of the exterior walls. You install continous insulation over the exterior sheathing, so that insulation is on the EXTERIOR of the home. Insulation installed that way helps to keep the sheathing warmer, and helps to limit thermal bridiging of the studs which is a big help. I personally like to go for 2" of polyiso (R13) or more, although going much above 2" can complicate some exterior trim details. Note that you cannot use polyiso underground, so if you want to insulate the outside of your foundation walls, you'd need to use something else. I would use XPS for any underground applications.

    Bill

  7. Krist0ff | | #7

    HI Bill & Walta,

    Spoke to my contractor today. He said we could do 2" of polysio on the exterior but would need a plywood to cover the foam as well? Typically is that how it is done? Can you please guide me here? Thanks

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