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Community and Q&A

Air sealing and roof venting in kneewalls

squeakjr | Posted in Green Building Techniques on


Six years ago I bought a 1 1/2 story house in western Pennsylvania which was built around 1973. The energy audit recommended airsealing the outside of the kneewalls (the interior side already had fiberglass batts), rim joists, and crawlspace under an addition. That was done with closed cell spray foam. Of course, the spray foam closed off the soffit vents and blocked any venting into the top half of the roof structure, leaving the ridge vent disconnected from the kneewall and on its own. So my knee walls are considered “conditioned space” even though they aren’t connected to the airflow in the rest of the house, except for a minimal amount of leakage around the access panel and maybe through the ceiling drywall.

I am getting a metal roof in a few weeks and the roofer is concerned about venting. He is talking about moisture and mold and maybe using box vents. I don’t remember what the energy auditor said about moisture in the upper part of the roof or the need for venting it. I hope that someone on this board has experience with this type of situation and can give us advice on how to deal with moisture issues and venting, if we even need to. I really hope that I haven’t damaged my roof by air sealing only the bottom half of it.


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  1. GBA Editor
    Martin Holladay | | #1

    It sounds as if your contractors insulated the sloped roof assembly behind your kneewall. This is the preferred way to prevent heat loss behind a kneewall.

    Hopefully, the contractor who installed the closed-cell spray foam did a good job. The usual method is to spray the insulation against the underside of the roof sheathing, creating an air barrier, vapor barrier, and insulation layer with this single layer of spray foam. This approach works well. This is called an unvented roof assembly.

    Some spray foam installers fail to install enough insulation -- so you might want to investigate the thickness of the spray foam. In your climate zone, you need R-49 insulation in this location. That means either 8 inches of closed-cell spray foam, or 3 inches of closed cell spray foam accompanied by 8 inches of air-permeable insulation (fiberglass batts, for example) directly up against the cured spray foam.

    For more information on insulated sloped roof assemblies, see How to Build an Insulated Cathedral Ceiling.

    I'm guessing that the attic above your second floor has a different approach to insulation. There, the insulation is on the attic floor, I bet. If that's the case, it should also have an R-value of R-49 or more. Usually, this top attic is vented, but venting isn't that important -- as long as your ceiling doesn't leak air.

    For more information on this issue, see All About Attic Venting.

    Assuming that the work was done as I have described, you have nothing to worry about. Your roofer can go ahead and install new metal roofing without further worries.

  2. squeakjr | | #2


    Thanks for the quick response. But I think that I might have confused the issue. Let me clarify. I have a 1 1/2 story Cape Cod. There isn't an attic. The second floor is finished and has the sloping ceiling that ends on either side with walls about 4 feet high and the kneewalls outside of that. The spray foam was put on the sloping sides of the kneewalls, directly on the sheathing and, I assume, blocking the soffit vents. They did nothing with the top half of the roof, over the ceiling. Those bays are just as they were before the spray foam (I don't know how they are insulated, I assume pink stuff.) You can tell where the spray foam stops, because the snow melts pretty quickly from the top half of my roof but doesn't from the bottom half.

    I worry that moisture is getting into the roof sheathing and not getting vented away without the flow from the soffit vents and the kneewalls. Then, the addition of the metal roof and the synthetic underlayment and ice guard should be adding a mostly impermeable layer on top of the sheathing and would prevent the kneewall part of the roof from drying to the outside. It already has an impermeable barrier on the inside with the spray foam.

    The roofer is concerned about how to vent and I am worried that, when he pulls the old shingles off, he will find a rotting roof.

    The spray foam is there and we need to deal with it, so how do we properly protect the roof from moisture? I've done a lot of reading on this site and I still don't know what to do.

    Thanks, I really appreciate any insight or advice that I can get.

  3. GBA Editor
    Martin Holladay | | #3

    Re-read my answer. I think I got it right.

    One possible confusion: you talk about "the sloping sides of the kneewalls." But all kneewalls are vertical.

    Here is the terminology: a kneewall is a type of wall. It is by definition vertical.

    Behind a kneewall you can usually find a triangular attic. I think I understood your situation and my answer is relevant.

    Above your second-floor ceiling (presumably a level ceiling) is another attic, which you say is insulated with pink stuff.

    I discussed both types of attic in my answer: the triangular attics behind your second-floor kneewalls, and the third-floor attic above your second-floor ceiling.

  4. user-1072251 | | #4

    There is a lot of moisture inside a home - from breathing, cooking, showering, and often a moist basement. That moisture travels on air leaking out of the house, much of that air warms up and rises into the attic through tiny openings. The roofer may be concerned about all that moisture condensing on the bottom of the metal and damaging the wood, but there is a much better way to deal with it: air seal your attic. Have the batts pulled out and the whole ceiling area spray foamed, then add a lot more insulation and an insulated, gasketed hatch. The contractor should do before and after blower door tests also.

  5. squeakjr | | #5

    Martin and Bob,

    In trying to be concise, I may have been too brief.I apologize. But I have a little trouble using the word "attic" when the top ("third floor") attic is only about a foot high. I grew up in a ranch house where you could stand in the attic. :-)

    There are actually 5 attic areas in my house. The 2 kneewall attics on either side of the original house have batt insulation on the kneewall and floor and closed cell sprayfoam on the sloping underside of the roof. This would have blocked off the venting from the soffit vents. I assume the batt insulation continues up the roof, between the ceiling drywall and the roof sheathing. I have no idea what is in the top attic. At most it is about 18 inches wide and tall. I don't kow how they were constructed 40 years ago, but it is open to the ridge vent. When I bought the house someone said that they had added 2 layers of rigid foam board under the roof. When my roofers were looking at it, they didn't see any evidence of that and didn't want to add any rigid board under their roof.

    There were 2 rooms added on after this part of the house was built. The dining room attic was previously open to the garage. That has been sealed with a firewall and also that roof sheathing was spray foamed. The last room in the front of and connected to the dining room attic has a very shallow attic area and has can lights. They sprayed as much of the underside of the roof as they could reach. I don't know if they did any airsealing around the can lights since they were in the conditioned space. Both the soffitt and ridge vents in these attics would have been sealed. One of the kneewall attics was also connected to these attics when they knocked a hole in what was the outside wall. That allowed access to the area over the bathroom and the replacement of the bathroom ceiling vent fan and the addition of a hose so that the fan was vented outside.

    If an unvented attic is OK in this situation, should I have the roofers close off the ridge vent in the older part of the house and not install box vents? Should I use some sort of vapor retardant paint on the ceilings to try and further seal moisture out of the attics? The energy contractor who did the original work, with before and after blower door tests, was happy with the air sealing.

    Thank you for all of your comments. I am learning a lot.

  6. GBA Editor
    Martin Holladay | | #6

    There is nothing wrong with an unvented attic, as long as the correct type of insulation was installed, and as long as the installed insulation has an adequate R-value.

    An unvented attic is insulated along the sloped roof assembly. This sloped roof assembly can be either vented or unvented, even if the attic below is unvented. (I know that it's confusing.)

    A conditioned attic shouldn't have any holes that allow exterior air to enter or leave the attic space.

    However, it's possible to create a sloped roof assembly that includes a ventilation channel (usually from the soffit vent to a ridge vent) even when the attic below is unvented. Does that make sense yet?

    If spray foam has been installed on the underside of the roof sheathing in your house, that type of assembly is called an unvented insulated sloped roof assembly. It's acceptable -- as long as the R-value is adequate.

    For more information on these issues, see Creating a Conditioned Attic.

    For more information on insulating and air sealing the triangular attics behind kneewalls, see “Two Ways to Insulate Attic Kneewalls.”

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