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Community and Q&A

Exterior rigid foam options

PA Marcotte | Posted in Energy Efficiency and Durability on

Oh, building sciences… a week ago I didn’t know you existed and now you make my head want to explode! Seriously though, thanks to all of you who have contributed to this amazing resource at GBA.

For my new old house renovation in central Massachusetts (zone 5), mistakes have been made and I’m trying to figure out best way forward to get siding done before winter. Bungalow is a mix of old and new 2×4 studs, covered with 7/8″ shiplap, then 3/4″ T-111 except gables are still shiplap only. Inside stud insulation ranges from none to a very little bit of fiberglass except for the areas I had to gut when we found leaky windows with mold in the walls. There we added R15 Roxul after removing the old materials and professionally treating the mold areas (cause mold triggers a lung infection in me).

All brand new windows, but despite my telling contractors there’d be rigid foam under the vinyl siding, they didn’t bump them out on the install. Have 1 1/2″ from sheathing to outside edge of window frame. Siders were going to put 1/2″ rigid when I learned that would nowhere meet the minimum R5 recommended for my zone for exterior rigid foam so could trigger condense problems. Been to every building supply place in the area cause they all told me on the phone they carry EPS. In every case it was either XPS or polyiso.

The big box stores have a R-Tech insulfoam poly faced with a R3.85 per inch in 1″ or 2″ thick. 2″ would recess windows tremendously when 3/4″ vinyl added. Best idea I’ve come up with is to go with house wrap, 1/4″ fanfold XPS for R1, then the 1″ R-Tech for R3.85 (though it says it performs to a higher R-value as it gets colder), followed by the vinyl siding and window trim. That gives me R4.85 with a depth of 2″ so windows only recessed 1/2″.

But despite all my recent research, feel like I don’t know what I’m doing. So any advice or better options would be welcome. In particular, not sure about the facing on the R-Tech. Some places seem to recommend peeling off the facing(s). Other important factor is there are times in the winter due to health issues I have to run a humidifier to bump humidity up into the 30 – 40% range, at least for a few days at a time.

Thanks for any help.

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  1. Expert Member
    Dana Dorsett | | #1

    In central MA you have multiple options for using reclaimed or factory seconds foam at WAY below box store pricing for virgin-stock goods. The two biggest are Nationwide Foam in Framingham, and Green Insulation Group in Worcester.

    There are a few others, some of whom advertise here:

    Don't peel the facers- the assembly is set up to dry toward the interior, and even through unfaced foam the drying rates toward the exterior are compromised. Peeling the facers off low-denstity EPS like R-Tech would give you some drying capacity, but it's impossible to do that without shredding the somewhat fragile EPS (only fragile at that low density- the facers help protect if from damage in handling.)

    If you can't get at least R5 on the exterior you may have to hold the line at 30%RH max in winter rather than min, or install smart vapor retarders on the interior side of the wall assembly assuming the wallboard isn't already up. (An 8' x 100' sheet of MemBrain is about $100, and cheap insurance if you only have room for R4.) Using cellulose rather than rock wool or fiberglass would also give it a bit of protection if going slightly low on the insulting sheathing.

    Box stores usually sell 1" foil-faced polyiso, which would be sufficient dew-point control for a 2x4 wall. The labeled R would be R6 or 6.5, but it's average wintertime performance would be about R5 in that stackup & climate but almost R7 during the shoulder seasons.

    In a tight home you would usually have to ventilate to bring the interior humidity DOWN to 30-40%, so concentrate on air sealing the structural sheathing when you still have access to it.

  2. GBA Editor
    Martin Holladay | | #2

    It's always hard to give advice to someone who is in the middle of a project -- especially when mistakes have already been made.

    Ideally, you would take a deep breath and remove the windows. If you want exterior rigid foam, do it right. It will never be easier to fix this error than right now.

    So, the rigid foam goes in first. Then the windows. That gives you an opportunity to properly flash the window rough openings, and to install the windows in the right plane. It also allows you to make sure that all of the window flashing is integrated with your WRB.

    It's no fun to remove windows that have recently been installed, but it's a lot easier to do that now than later, after you realize that your windows were installed wrong, and that water is entering your walls.

    For more information, see these articles:

    How to Install Rigid Foam Sheathing

    Installing Windows In a Foam-Sheathed Wall

  3. PA Marcotte | | #3

    Thank you Dana for your advice, especially the info on other sources of foam. I'll be contacting them Monday. I've only ever lived in old houses so bringing up humidity as been the major battle. Good to know if I make some improvements it could be less of a fight. Bringing the humidity down is easier than bringing it up!

    With my window problem, polyiso is the obvious choice for it's higher R value per inch. But I was scared off by the "in cold weather R5 EPS beats R6 polyiso" info I read on GBA and elsewhere. I assumed an R6 polyiso wouldn't necessarily provide my dew-point control. I'd be a little more comfortable with the R6.5 polyiso but it still could be really borderline. But in your opinion it's acceptable, especially if I keep the indoor humidity down?

    Going with the 1" polyiso would give me a window recess of only 1/4" that I'm more comfortable with because with a good bead of caulk it should shed water ok (I hope).

    Unfortunately the mineral wool is a done deal too, even for filling more walls as future work is done. I do some audio recording projects and come to find out there is an extreme noise problem with the very near neighbor. I had several houseguests get headaches from the noise. The sound blocking properties of the Roxul have been a blessing and I may not need to build a sound proof room for recording as a result. But I had figured out that in a borderline dew-point setup it is not the first choice. It's another reason I was concerned about the cold weather decrease in effectiveness in the polyiso.

    Thanks so much for the help.

  4. PA Marcotte | | #4

    Thank you Martin for your input. I wish resetting the windows was an option. But even if I could afford it, time is the deciding factor. I have a limited timeframe with the contractor before he has to move on to other jobs and because we already encountered several other problems (bad attic ventilation, rotting walls...) there quite simply isn't enough time to redo the windows. I understand the ideal, but am living in the land of best possible compromise : (

    So I know it is a poor situation with limited choices. I'm choosing to think of it that I'm lucky to have prevented them from making my house worse. Any gains I can make are good, even it not what could have been achieved if this project had been tackled correctly from the start. Not a single building or siding contractor or building supply person I've spoken to knows any of this info on proper use of rigid foam for exterior insulation. Any only one or two expressed any interest in learning it. We homeowners can't make the right choices without the right info!

    I do appreciate you feedback and the many articles you wrote that I have read. I'm now on Team Exterior Insulation Public Relations and hope to help others get it right from the start.

  5. Charlie Sullivan | | #5

    Other options to consider:

    1)Graphite infused EPS insulation, usually sold under the brand name Neopor, a trademark of BASF. That boosts it to at least R4.5/inch or more which might give you a little more margin on condensation potential. And like EPS, it has some vapor permeability so that would give you just a bit of capability for drying to the exterior, and the R value only goes up at lower temperatures.
    Some manufacturers:
    The latter has versions that are specifically designed to fit under vinyl siding.

    2) Mineral wool rigid insulation boards. These are very vapor open, and so you don't have to worry about minimum thickness.
    A challenge might be finding them in 1" thickness.

    3) I haven't heard of doing this, but you could use fiberglass rigid insulation boards the same way. As long as they are unfaced, they will also be vapor permeable, and so there's no minimum thickness.

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