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Community and Q&A

Closed-cell spray foam applied directly to underside of shiplap roof is OK, but it’s not OK applied directly to shiplap walls?

user-7128974 | Posted in General Questions on

I can’t be brief, so I guess I’ll have to be nice and polite.
I read two GBA articles hoping they could help me with my old house. iI was built just after WWII. I live in Canada in Zone 8.
The first article Insulating Walls in an Old House with No Sheathing applies to our house. The walls are ship lap with stucco on top. There is no house wrap. The article states reasons not to spray closed-cell foam directly against the inside of the wall (ie directly on the ship lap). The second article How to Build an Insulated Cathedral Ceiling states it’s ok to spray closed-cell foam directly on the underside of the sloped roof (ie directly on the ship lap in between the rafters) when creating an unvented roof assembly.
Question: if closed cell spray foam is not ok to apply directly on the walls why is it ok to apply directly on the underside of the roof when both wall and roof are ship lap? What component of the roof assembly would make it ok … what am i missing?
Stan W.

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  1. user-7128974 | | #1

    I want to add one more comment about the house...don't see an edit button for my original post though. Our house does not have soffits. The only way air travels into the attic currently is through roof cap vents and a couple gable end vents. This is why the insulation of the roof would need to be unvented. The cost of adding rigid board on top of the roof assembly is cost prohibitive so therefore not an option. Stan W.

  2. user-2310254 | | #2


    Can you provide your location? Zone 8 is pretty far north.

  3. GBA Editor
    Martin Holladay | | #3

    You describe your wall construction this way: "The walls are shiplap with stucco on top." That means that your walls have sheathing. The shiplap boards are the sheathing. The stucco is the siding (also called cladding).

    The article you referred to, "Insulating Walls in an Old House With No Sheathing," is irrelevant. That article is about a different kind of house -- not your house, since your house has wall sheathing.

    You can safely install closed-cell spray foam on the interior side of your wall sheathing if you want.

    That said, the fact that your house apparently lacks a water-resistive barrier (WRB) is unfortunate (though common in old houses). You wrote that the walls have "no housewrap." Of course, older houses used asphalt felt instead of housewrap -- perhaps your house has asphalt felt between the board sheathing and the stucco, and you just forgot to mention that fact.

    An old house often has water entry problems in walls, especially near the windows (since older houses didn't include window flashing in the rough openings). Adding spray foam can reduce the wall's drying rate, so you should assess the risk before proceeding. Wide roof overhangs are good, and so are exterior window trim details that keep rain away from the window opening. Shallow roof overhangs aren't good; nor is evidence of past water-entry problems

  4. user-7128974 | | #4

    Thanks for setting me straight. The photo in the article looks exactly like shiplap - so it threw me off.
    My house is a one and a half storey. There is no roof overhang. The roof ends at the wall. There does appear to be water stains in various places in the stud bays. They are not actively damp. At this time, I'm not able to say if the house does or doesn't have asphalt between the board sheathing and the stucco. Without ripping off the stucco is there a way I can check?
    I'm including a picture of what a stud bay in my house looks like. (Photo rotated counter clockwise 45 degrees when I posted it to your site, unfortunately.) Behind the plaster board is a heavy cardboard like layer followed by a thin black and silver layer followed by dead air space. (About 6 inches of wood chips that had settled in the bottom of each stud bay.)
    Based on your answer I'm now thinking of using roxul batts. How would you choose to insulate the walls? And would you use membrain over poly?


  5. GBA Editor
    Martin Holladay | | #5

    I have rotated your photo for you.

    You may be able to determine if you have any asphalt felt by very carefully drilling a hole in your wall sheathing from the interior -- in an inconspicuous spot -- to determine what, if anything, is between the sheathing and the stucco.

    The lack of a roof overhang, and the signs of water entry, are not good news. You need experience to evaluate the situation -- you don't want to make things worse. You need to determine how the water entered the stud bays and left the stains, and then you need to remedy the cause of the water entry before proceeding.

    We probably won't be able to evaluate the risks over the Internet. You may need to hire experts to visit your site and provide advice.

  6. user-7128974 | | #6

    Will do. Thanks.
    If things were perfect with no water entry, how would you recommend insulating? Just curious.
    Thanks, again.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "If things were perfect with no water entry, how would you recommend insulating?"

    A. The "perfect" situation for an older house would be:

    - The house would have generous roof overhangs.

    - The siding would be in good condition.

    - The house would have carefully installed asphalt felt between the siding and the sheathing.

    - The house would have new windows that were installed in rough openings that had been lined with flashing, or old windows that were carefully protected from rain by good head trim details.

    - There would be no stains on the sheathing indicating water entry.

    Under these circumstances, you could use almost any type of insulation between the studs that you wanted. If the stud bays are open on the interior, you can use the same range of insulation methods available to builders of new homes. The type of insulation chosen is less important than the care employed when installing it.

    I like blown-in insulations because they fill every nook and cranny in irregular stud bays. Either dense-packed cellulose or dense-packed blown-in fiberglass will work. So will mineral wool batts or fiberglass batts -- with the caveat that these types of insulation need to be very carefully installed. If I was worried about air leakage, I would consider specifying open-cell spray foam. Again, the type of insulation is less important than the care exercised during installation.

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