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Community and Q&A

Appropriate insulation system for building on piers

Hotpond | Posted in Energy Efficiency and Durability on

I live in Mass (Zone 5) and am renovating as a DYI an existing wing on piers that is framed with 2×10 joists. I’ve been trying to develop a floor system with accommodations for energy efficiency and code, ant and rodent infestations, and weatherproofing. After looking at a lot of the guidance on this site, and links to other articles, I’ve come up with the following (top to bottom in structure):
-Click flooring with plastic moisture barrier underneath
-subfloor
-Rockwool comfortboard (ants don’t like it) against the subfloor with sprayfoam seal along edges
-fiberglass batts between joists
-Rockwool comfortboard across bottom of joists
-hardware cloth (rodent proof)
-1-way vapor barrier/screen to keep weather out but allow moisture to exit the system
-exterior plywood

I’m looking for any specific items that readers would take issue with, as well as input on an appropriate vapor/barrier or screen. I would prefer to use the Rockwool rather than the foam because ants like it less, but of course the rockwool is rather permeable…..
Thank you!

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    You can look at a simpler option.

    If you are looking to reduce the thermal bridging of the 2x10 you can cross strap it with 2x3 or 2x4 on edge either on the top or the bottom. You can then pack the cavity with a cellulose with the blower form the box store. Borate treated celluose is toxic to the guts of most insects.

    Your assembly (assuming 16 OC and 1" comfortboard) ends up around R39. The above with 2x3 gets you the same R value, going to 2x4 bumps it up to R43.

    It would be much cheaper assembly as you can use the cheapest insulation and saves one extra layer of plywood. Most important part is make sure you air seal the plywood on the outside. Air leaks in the floor means cold floors.

    P.S. You can' t lay click directly over comfortboard, you need an extra layer of plywood, otherwise the click joint will come apart if you walk on it.

  2. Expert Member
    Dana Dorsett | | #2

    >"-Rockwool comfortboard (ants don’t like it) against the subfloor with sprayfoam seal along edges"

    Comfortboard is not an air barrier- there isn't much point to foaming the edges- just make it a friction-fit (for ant control).

    Air seal the subfloor as the primary air barrier, and air seal the plywood bottom sheathing for redundancy.

    >"-1-way vapor barrier/screen to keep weather out but allow moisture to exit the system"

    The plywood sheathing on the bottom is a smart vapor retarder, and there's no weather happening inside the cavity between the bottom plywood and the subfloor. The subfloor is also a smart vapor retarder. Omit the "...-1-way vapor barrier/screen..." layer. There is no such product, and even one did exist, it wouldn't be doing anything useful at that layer.

    There also isn't any weather to speak of happening in the crawlspace under the plywood sheathing, except perhaps during hurricanes.

    Using cellulose with all-borate fire retardents instead of fiberglass for the cavity fill goes a long way toward ant-proofing the place, at least for wood boring ants. Borates kill the gut flora wood boring insects need for digesting the wood.

    Rigid rock wool is expensive- replacing the cavity fill with rock wool batts instead of fiberglass would likely come in cheaper than cut'n'cobbled rock wool between the joists tight to the subfloor, with the rest being fiberglass.

  3. Jon_R | | #3

    Don't put a vapor barrier on the exterior side of a zone 5 floor. +1 on air barrier(s). Preferably on all sides of air permeable insulation.

  4. Hotpond | | #4

    Ok thanks to all three of you! To clarify on the vapor barrier, because this floor is on piers (not technically a crawl space), it is essentially open to exterior atmospheric conditions such as fog, which I was worried I needed to account for. Jon R. I'm not sure I understood your comment.
    Any....so to combine the advice from all of you it seems like revised version would be?:
    -Click flooring with plastic vapor barrier underneath
    -subfloor
    -dense pack cellulose with strapping below the joists for the air gap
    -hardware cloth (rodent proof)
    -exterior plywood

    1. Expert Member
      Dana Dorsett | | #6

      >"To clarify on the vapor barrier, because this floor is on piers (not technically a crawl space), it is essentially open to exterior atmospheric conditions such as fog, which I was worried I needed to account for. "

      Fog will not condense on surfaces warmer than the outdoor air. Do get 75F fog in summer, when the subfloor is 72F?

      Even if you did, the vapor retardency of half-inch plywood is more than adequate for managing short-term super-tropical conditions such as that. Zone 5 does not include the Amazon rain forest.

      >"-dense pack cellulose with strapping below the joists for the air gap"

      You don't need or want an air gap between the insulation and the bottom sheathing. Installing 2x strapping below the joists delivers about R5 -R5.5 of cellulose between the bottom sheathing and stud edges, which is a decent enough thermal break. But that thermal break depends on the 1.5" being filled with cellulose, not air.

      1. Expert Member
        NICK KEENAN | | #8

        "Do get 75F fog in summer, when the subfloor is 72F?"

        OP doesn't say what part of Massachusetts but on the South Coast, Cape or Islands the answer would be yes.

  5. Expert Member
    AKOS TOTH | | #5

    During the times when there is high exterior humidity, inside your house the humidity should always be less. This means, if you want your floor to dry, you don't want a warm side vapor barrier. Celluose can absorb a lot of water without issues, minor interior water spills are not a problem as long as you have a drying path.

    I would check your local code, ours for exposed flooring calls for house wrap (acting as air barrier) underneath and no vapour barrier on the top. The house wrap as the long term air barrier is questionable, I would rather trust plywood with taped seams though.

    The rest is good. Make sure your hardware cloth does not interfere with air sealing.

  6. user-6893127 | | #7

    I know this is an old thread but it was addressing some of my questions. I’ve finally got a little piece of land to build on, but it’s kind of a swampy area and I’ll need to build on piers. So, In the end was the verdict something like this? (top to bottom):
    Subfloor
    Blown cellulose
    Exterior plywood

    I read the article where the guy used zip wall underneath, but in my area of Georgia right now they’re selling zip at about $68/sheet.
    Could I just use some cheaper exterior ply? Should I put tyvek under it? Tape the seams?
    Much appreciated

    1. Expert Member
      MALCOLM TAYLOR | | #9

      User ...127,

      I would go with Ako's suggestion in post #1 and strap the underside of the joists to reduce thermal bridging. Another alternative is to use either a continuous layer or strips of of EPS foam above the exterior plywood.

    2. Expert Member
      NICK KEENAN | | #10

      Generally what you want to do with an insulated assembly is put a vapor barrier on one side and keep the other side vapor open so the assembly can dry to that side. The general rule is that vapor moves from humid to dry and from warm to cold so the vapor barrier should go on the warm, humid side. In a climate where you heat more than air condition the inside is the warm, humid side most of the time and the barrier should go on the inside. In a climate where you air condition more than heat the outside is the warm, humid side most of the time and the barrier should go on the outside. I don't know where you are in Georgia but I suspect it is cooling dominated.

      You really want to prevent air movement in your insulation, so air sealing both sides if you can is a good idea. You also need to seal against rodents and other pests. Half inch plywood with tight joints and the seams taped is a good air barrier and pest barrier. Tongue and groove is even better but it's tricky to put up from below. It is not a vapor barrier. You could paint it with a vapor barrier primer before putting it up, use a layer of faced insulation along the bottom.

  7. user-6893127 | | #11

    Thank you all. DC, you’re right, it’sa cooling dominated climate but we do have about 4 months of running heat.
    So to recap:
    Subfloor
    Blown cellulose
    Strapping to reduce thermal bridging
    1/2 exterior ply, with seams taped
    Vapor barrier (either foil-faced foam or a painted on Vapor barrier [would tyvek work]).

    1. Expert Member
      NICK KEENAN | | #12

      Tyvek is not a vapor barrier.

      Many common materials that are waterproof actually let vapor through. You need to find the permeability rating for what you're using. Most paint is not a vapor barrier but they make vapor barrier primer. In this location primer alone is probably sufficient. You could even prime on the interior which would allow the plywood to dry to the exterior if it got wet.

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