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Final design: Roof, walls, and floor

Dave B | Posted in Green Building Techniques on

So getting final details of a new build in Zone 6 (Osaca, ON) Hopefully pretty good house , that will be Net-Zero.
Any comments or suggestions before I finalize this? Thanks !!!

Roof setup:
Standing seam metal roof
Blueskin self-adhered membrane
5/8 plywood
2” air gap
attic truss with raised heal
24” minimum blown in cellulose
1/2” drywall
latex paint

Wall setup Outside to Inside:
Vinyl Board and Batten Siding
1×4 furring strips mounted horizontal 12” OC
2 layers of 2” Silverboard EPS taped seams
Blueskin VP100 ,maybe something else?
1/2” OSB possibly advantech
2×6 frame walls 16oc. 2 Stud corners.
Roxul R22 although R24 is available for $.80 more sq’
1/2 drywall
latex paint.

Walls outside to inside
dimple mat
poured concrete
2 layers of 2” Silverboard EPS taped seams
2×4 frame walls filled with R14 roxul
non-paper faced drywall
latex paint.

Floor: 4” concrete with pex tubing for in floor heating
6mil poly
2’” of Silverboard UC R10 taped
4” gravel

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  1. GBA Editor
    Martin Holladay | | #1

    Only one problem: You don't have enough insulation under your hydronically heated slab. Two inches of EPS isn't enough, especially in Ontario. Double the thickness of the sub-slab foam, and you are good to go.

    The quoted R-value for 2-inch thick EPS (R-10) seemed high to me -- but I guess it's graphite-infused EPS, so R-10 is possible.

    -- Martin Holladay

  2. Dave B | | #2

    Thanks for the reply Martin.
    so 4" of silverboard UC EPS under the slab, or would it be better to go with 2" of something else XPS? It doesn't mention graphite infused?
    IS the 2 layers of 2" silverboard EPS on the exterior walls not going to give me R20 ?
    Honestly I wanted to do Roxul on the exterior but the price difference is too great... To bad, because they make it just down the road.


  3. GBA Editor
    Martin Holladay | | #3

    It appears that Silverboard makes two types of EPS. Some, but not all, of their EPS is graphite-infused. As far as I can tell, the graphite product is referred to with the designation "SBGSX." For more information, see Silverboard Graphite.

    It is possible the graphite-infused product has an R-value as high as R-5 per inch.

    If you are using a type of EPS that isn't graphite-infused, a more reasonable estimate for the R-value would be R-4 per inch.

    It's possible that the manufacturer is trying to claim some R-value advantage from the foil facing. If so, that claim would be illegitimate. You will only gain an R-value boost from the foil facing if the foil faces an air space -- and that is clearly impossible when the insulation is installed under a slab.

    -- Martin Holladay

  4. Steve Knapp CZ 3A Georgia | | #4


    You might want to search online for reclaimed foam. It is typically about one-third the cost of new product.

  5. Dave B | | #5

    Thanks Martin
    So I didn't realize that they over estimate the R-value, that sucks!! Should not be legal.
    So walls are R16 not R20 for the 4" of EPS. I though EPS was the way to go for cold climates.
    Not sure if the price difference to graphite will be worth it, I`ll have to price.
    Maybe Roxul will be a better choice in the end!!

    Steve I will look into used although I think it is less common up here Thanks

  6. GBA Editor
    Martin Holladay | | #6

    I don't have enough time today to delve deeply into the documents posted online by the manufacturer. I want to make it clear that I am not accusing the manufacturer of false R-value claims. If I were you, however, I would verify these details before making a purchase.

    -- Martin Holladay

  7. Dave B | | #7

    I`ve looked up used, some places do have Polyiso foil faced and kraft faced. Although it all looks like it sits outside uncovered. Will this degrade the foam anything to watch for in used material?


  8. Dave B | | #8

    That's fine martin I didn't except for you to do that.

    No I don't think you are accusing anyone of false claims, I do understand that some manufactures over estimate there claims and may have different testing methods. I think it happens in all sectors.

    For anyone interested in the physical properties and testing, here is what is listed on thier site.

    It shows if I'm right for 1" R6" at 0F and R5 at 75F for the SB-35XS ASTM standard testing...

  9. Expert Member
    Michael Maines | | #9

    I was curious about the Amvic Silverboard, as we used it for the sub-slab insulation on Fine Homebuilding's ProHome, so I contacted their VP-technical services. They used the standard ASTM C-518 testing protocol, at a mean temperature of 75°F, and found their 2-lb and 2.5-lb EPS to be R-4.90/in., ±2%. The testing protocol does not take into account the reflectivity of the surface films. It is an impressive value for EPS.

  10. GBA Editor
    Martin Holladay | | #10

    Thanks for tracking down that information. Although the claimed R-value seems high, I have no reason to doubt it.

    -- Martin Holladay

  11. Dave B | | #11

    Yes thank you Michael, I did have concerns.
    My decision is now between used polyisocyanurate which I can get in 3"x 4' x 8' at about $.62 sq' unfaced (there may be some foil faced) or new silverboard 2" x 4 x 8 $.70 sq'
    both options I would double up, so either 6" or 4" exterior foam.
    I know there is a decrease in R in cold climates(i'm zone 6) for the poly, so I'm still trying to decide which makes more sense!!? or some combination of both??

  12. Expert Member
    Dana Dorsett | | #12

    DaveB: What's the dilemma here?

    At 3" used polyiso is both cheaper and higher performance than 2" SilveRBoard, even with derating the polyiso & uprating the EPS for temperature. The longer pancake head timber screws makes the cost closer to parity, but it's till going to be ~50% or more R going with 6" of used polyiso than 4" of SilveRBoard.

    Going with the reclaimed goods also has a lower footprint for the polymer use than with virgin-stock EPS, especially the higher density stuff, if that's where you're going.

    On the outside of an R20-something fluff-filled 2x6 framed wall only the outer inch or so of polyiso would under perform the outer inch of EPS, and then only on the coldest hours of the coldest days. Much of the season even that outer inch will outperform the EPS, and replacing the outer 3" of polyiso with 2" of SilveRBoard would be somewhat lower overall performance. On it's best day the 4" of SilveRBoard might hit R23 (not really sure it'll even hit R22), and on the worst hours of the worst day 6" of polyiso will perform north of R25 in that stackup.

  13. Dave B | | #13

    Thanks Dana
    Now I have to see how much stock they have and if they get it in regularly or its time for me to find storage.

  14. Dave B | | #14

    So struck out with the used stuff, it seems we just don't have the used quantities that I see down in the US. Guys seem to have small quantities only, not enough for me to plan around.

    If anyone knows of used rigid foam in Ontario, let me know. Other wise I guess I'm buying new.

  15. Dave B | | #15

    So I have found some used 3" polyiso paper faced, question I have is if I do 6" of exterior polyiso and drop down to 2 X 4 wall with R14 Roxul . Do you think this is better than the 4" Silverboard with 2x6 walls filled with Roxul R22 both seem to be around R40 ??
    or I could do 6" Poyiso and 2x6 walls with R22 Roxul, but this seems over kill for my area (zone 6).
    Costs will obliviously be more for the last setup.
    Trying for a pretty good -Net-Zero Home .

    Thanks for any input.

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