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Community and Q&A

Air sealing and insulating finished attic space

Cherylann128 | Posted in GBA Pro Help on

Hi. I’m new here and would like to start by thanking everyone who shares their extensive knowledge with homeowners like me. I read last year’s thread on retrofitting insulation to a 3rd story finished attic with great interest as I have the exact same problem. However, I didn’t understand some of the technical aspects and a definitive solution was never determined. The link to that thread is

We live in an old stone/stucco (100 years) house outside of Philadelphia. The third floor is essentially a finished attic space with three rooms and a bath. There are 5 large dormers and several large closets built around the kneewalls. I have no reason to believe that the dormer walls, kneewalls or sloped ceilings in the finished areas are insulated. The attic floor above the finished space has loose cellulose between the rafters, installed around 10 years ago. There is also an airhandler and flexible a/c ductwork in the attic. Like the previous poster, we too have limited access to the space behind the sloped ceiling, dormer walls and kneewalls. The proposal I received was to seal all leaks, ducts and registers then bury the ductwork in more cellulose which would also be dense packed into the slopes and kneewalls. I have no idea how the roof is vented but adding to the complexity is an 18″ stone wall that goes through the middle (sort of) of the house right up to the roof. My guess is that the roof it has a ridge vent.

I would like to insulate with spray foam but I know my husband would never agree to remove the existing plaster/lathe walls and ceilings in order to apply it everywhere. Would it be feasible to sprayfoam the underside of the attic roof and insulate the ceiling slopes, dormers and knee walls with either fiberglass or cellulose? If not, what would be the best way to insulate the whole space. The roof is only 8 years old (I wish I knew then what I know now. We could have avoided this mess). I’m really concerned about all of the energy we’re wasting and possible damage from ice dams (though the winters have been getting milder). Thanks for any advice you can offer.

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  1. GBA Editor
    Martin Holladay | | #1

    Cheryl Ann,
    Q. "Would it be feasible to spray foam the underside of the attic roof and insulate the ceiling slopes, dormers and kneewalls with either fiberglass or cellulose?"

    A. Probably. But your question is difficult to answer over the Internet; an insulation contractor needs to visit your house to assess questions about access and to make recommendations.

    If you are seriously interested in insulating your attic, then you should contact three or more insulation contractors to obtain bids. When these contractors visit your house, they will provide you useful feedback on your plan.

  2. Cherylann128 | | #2

    Thank you, Martin. My HVAC company did any energy audit and made recommendations for adding more cellulose, burying the ductwork in it and dense packing the slopes and kneewalls. I'm sure they subcontract the work so I have since made an appointment with an insulation company to come out and take a look at our situation. I was just hoping to have some knowledge before we meet so that I can tell whether what they propose makes sense. As you know, many contractors aren't really up to date on the latest recommendations coming out of organizations like Building Science (although these guys seem to be on the same page). After I meet with them, I will come back with their proposal and run it by you guys. Thanks again.

  3. user-788447 | | #3

    I work for a diagnostic testing and insulation outfit and encounter situations similar to yours in a cold climate on a regular basis. There are only definitive answers if you have an unlimited budget, otherwise the situation you find yourself in is always a bit of a puzzle.

    The way you describe the energy audit you had performed with its recommendations gives rise to one concern - only adding more cellulose insulation (even densely packed) will do little to reduce air leakage at the attic plane. A good audit should quantify how leaky your house is and estimate its potential for reduction. Infrared imagery is often used to locate specific areas of the attic assembly the contribute to significant leakage.

    Scopes of work for your situation can range from thousands of dollars to add more insulation with limited air sealing to tens of thousands of dollars to do comprehensive air sealing and detail the roof in a way to prevent ice dams.

    In finished attics with complex roofs another strategy to air seal inaccessible building assemblies is to remove the roof and spray foam from the outside. Of course this is much more cost effective when done while replacing old shingles. However spraying from the outside is comparable if not cheaper in many instances than demoing and replacing interior finishes. Spraying low perm closed cell spray foam to the back side of dormer walls, slants walls and ceilings from the outside also places the vapor retarder where you want it and allows for the possibility of venting the roof decking.

  4. Cherylann128 | | #4

    J Chestnut

    All good points. After spending $50K to put on a new roof 8 years ago, there is no way I can justify insulating the exterior. Hahaha. As I said, wish I knew then what I know now. If it were up to me, I'd take out all the plaster and lathe walls/ceiling and spray foam the roof with closed cell but I know my husband would never go for it; he does want to retire some day, poor guy. I know anything can be remedied if one is willing to pay for it and I don't want to go cheap on this. At the same time, I don't want to break the bank.

    I have infrared pics of where I'm losing conditioned air; all the typical offenders, hatch doors, closet doors, outlets, windows, a/c registers etc. and the proposal included sealing air leaks and ducts. I'm going to wait till September to have the spray foam company come out when it won't be so brutally hot in the roof and we'll see what they recommend. I'll report back then.

  5. markgroupusa | | #5

    Good Morning Cheryl Ann,

    I work for Mark Group down at The Navy Yard in Philadelphia. We are the leading provider for home energy efficiency solutions in the Delaware Valley. I was able to obtain what I think is a great solution to address your home comfort needs. Please let me know if you're interested in scheduling a Home Energy Assessment at no cost to you, and I'll put you in contact with one of our Home Performance Advisors.

    Here is Mark Group's response to your post:

    "This is a common issue in older homes and/or homes that have been re-modeled. A lot of factors come into play during the evaluation period with regards to access, roof venting, soffit venting, and existing insulation conditions. The most challenging part to insulate is the sloped roof - starting from the top of the knee wall up to the peak.

    For the sloped ceilings, one option to increase the r-value and stop air infiltration is to install a self leveling open cell spray foam into the ceiling cavity. You would drill ½” diameter holes as needed on the ceiling. The foam would be injected and expand to fill the space at a rate of 50 times the original volume. As you can imagine, this must be completed by a trained applicator that is familiar with the process. This will minimize the chances of bowing or blowing off any interior finishes. Once completed, the open cell foam completes a bond with the interior and exterior finishes and air movement is eliminated. This method is not invasive and the holes can be easily patched."

    Dana Pulli

    [email protected]

  6. Cherylann128 | | #6

    Thanks, Dana. I have a question about open cell foam. It is my understanding that using open cell in roof applications requires a vapor barrier. I have to admit that there are many elements in the articles I have read that I don't really understand. Could you explain to me why open cell would work in this application? When we had the new roof installed, I didn't pay any attention to the how's or why's. As far as I know, it is a standard asphalt shingle roof. Any information that can help me better understand these insulation properties would be really welcome.

  7. Cherylann128 | | #7

    Oh. Ok. Thanks, AJ Builder.

  8. wjrobinson | | #8

    Cherylann, Your finished walls and ceilings are your vapor retarders.

    Your most welcome. White water playtime next. Happy trails, life is but a dream.... enjoy every second.

  9. markgroupusa | | #9

    Hi Cheryl Ann,

    I wanted to write you before I left for the weekend. I spoke with our head of Operations here at Mark Group about open cell spray foam application and your vapor barrier concern. Here is Mark Group's response to your post:

    "The building code requires a Class II vapor barrier in climate zones 5 and north, even though the spray foam stops all air movement and convective loops. In these case Sherwin-Williams has a product that can be applied to the drywall or plaster to create the required vapor barrier. The Delaware Valley is in climate zone 4 (south of climate zone 5), so this requirement would not be applicable."

    I hope this helps to better answer your question.

    Dana Pulli

    [email protected]

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