Add 2" or 3" foam to raised ranch?

Discussion in 'Remodel Forum & Blog' started by CMB2018, Feb 12, 2020.

  1. CMB2018

    CMB2018 New Member

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    Gentlemen; I am considering adding foam, either EPS or XPS to 1300 sq. ft. raised ranch , state of Maine, zone 6.

    House is 2x4 studs, fiberglass insulation with a plastic vapor barrier inside. Tar paper on outside of sheathing with hardboard siding.

    Windows and doors will be moved out, some windows replaced. New vinyl siding will go back. Some sort of rain screen under vinyl.

    Lots of roof overhang so moving everything out 2 0r 3" is no problem.

    Is it worth it to go another inch in this zone? Or just stay at 2"?
     
  2. Dana

    Dana In the trades

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    First, don't go with XPS, since at 2" it would be a Class-II vapor retarder (="vapour barrier" under Canada's code definition). With 6 mil poly on the interior that creates a moisture trap, which puts the window flashing details on the "critical" list for making it perfect. In construction there's good/better/best, but perfection is really really hard to achieve.

    Decide ahead of time whether the windows will be mounted innie (glass roughly co-planer with the sheathing) or outie (roughly coplanar with the siding), since that affects how the window flashing is installed, and where the housewrap gets installed. If innie, a crinkle type housewrap (eg Tyvek Drainwrap) is strongly recommended, to allow bulk water that gets between the foam & sheathing to fully drain.

    Furthermore XPS slowly drops to the same performance of EPS of similar density over time, as it loses it's powerful greenhouse gas HFC blowing agents. XPS is by far the least green insulation material in common use today:

    [​IMG]

    The greenest foam-over solution that works would be RECLAIMED roofing EPS, which is usually 1.25lbs per cubic foot density "Type VIII" material. At a mean temperature through the foam layer of 40F (appropriate guesstimate for the wintertime averages in your area) it would run about R8.5 @ 2" , R12.75 @ 3", and about 1.5 perms @ 2", almost dropping to about 1 perm @ =3" (which is about as vapor-open as 1" of XPS), which is reasonably sufficient seasonal drying capacity.

    The next-greenest approach (that's still affordable) that works would be virgin-stock Type-VIII EPS.

    A 2x4/R11 wall runs about R9- R10 "whole assembly" after factoring in the thermal bridging of the framing, and the R-values of the wallboard, sheathing, siding & air films, etc, so even 2" of exterior foam would be nearly doubling the performance of the wall assembly. Before putting up the foam it's worth detailing the sheathing as the primary air barrier using polyurethane caulks &/or sheathing tapes, where appropriate. (With a rollered-on layer of latex primer spanning the seams even housewrap tape works on plywood & OSB).

    Vinyl siding needs no rainscreen- it's inherently back ventilated, but it can't be long-nailed through 3" of foam. Using 1x4 furring through screwed with pancake head timber screws long enough to penetrate the studs by 1.5" is the common approach, but you may want to review this older blog piece before deciding your approach.
     
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  4. CMB2018

    CMB2018 New Member

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  5. CMB2018

    CMB2018 New Member

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    Dana ; Thank you for the quick reply!

    EPS it will be. Most likely new.

    The sheet rock, outside walls, shows thermal bridging. You can see each stud and most every sheet rock screw is black.

    In regards to the windows. I was going to pull each window, extend the bottom of each window rough opening to the thickness of the foam plus the furring. Most of the window weight sits on the sill. Install the foam to the dimensions of the rough opening and picture frame the furring around the rough opening, nailing the window to the furring frame. If there is a better way, I am all for it?
     
  6. Dana

    Dana In the trades

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    It's better to fab-up full depth window bucks out of 3/4" plywood to the correct depth and flash the buck with a fluid-applied flashing such as Prosoco Fast Flash or similar. If there's not enough space in the rough opening to do it with 3/4", it's OK do the sides & top with half inch (or even 3/8" if you absolutely must), as long as the bottom of the buck is 3/4".

    This example of a plywood window buck is on new construction, but is similar to what can be done in retrofits. They use a flashing tape rather than liquid applied, which is fine if you lap it correctly.

    [​IMG]

    [​IMG]

    [​IMG]

    There are a few ways to skin a cat- take a peek at this primer., which included these detail in the gallery.

    [​IMG]

    [​IMG]

    Dudley Box type flanged bucks have some advantages when installing windows in foam-sheathed houses too as outlined in this FHB article.
     
  7. CMB2018

    CMB2018 New Member

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    Thank you, looks good!

    So when the buck is installed, the foam would butt up to the buck and the vertical furring would be flush with the outside edge of the buck, right?
     
  8. Dana

    Dana In the trades

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    Yup!

    With a Dudley Box type buck the foam slips behind the flange, which gets caulked to the foam for the full perimeter:
    [​IMG]

    [​IMG]
     
  9. CMB2018

    CMB2018 New Member

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    Very good.

    Thank you
     
  10. Dana

    Dana In the trades

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    Given that it's a raised ranch, have you worked out the details of insulating the cantilevered overhangs usually found on a raised ranch?
     
  11. CMB2018

    CMB2018 New Member

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    Dana; A Few years ago I installed foam board underneath the overhang. You are right, it is a place that sometimes gets overlooked. The kitchen sink is installed over the overhang. The hot and cold water pipes extend thru a portion of the overhang and one winter it froze, was able to thaw it that day. Have not had a problem since .
    I will be looking at maybe improving it this spring, if needed.

    Is there a brand of EPS foam board that is better than others?

    Most homes around here that have added foam to the exterior seems to be pink or blue XPS.
     
  12. Dana

    Dana In the trades

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    EPS is a very competitive product, with many many quality vendors- it's a mature market.

    XPS offers a lot LESS drying capacity than EPS of similar thickness or labeled-R value, and in the end doesn't significantly outperform EPS from a thermal perspective, labeled-R notwithstanding. (Read the warranty)

    If you're willing to pay a premium for performance, graphite loaded EPS (eg NeoPor, or the perforated versions of SilveRBoard ) runs about R4.5- R5/inch with no long term degradation of performance, at the same vapor permeance of standard EPS.
     
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