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Question on air sealing attic

humm9er | Posted in Energy Efficiency and Durability on

Hi Guys,

Home in zone 5A built in late 17th century with major renovation done in the early 90s. Major insulation projects completed at that time…all walls are insulated and have vapor barriers, new portion of attic has batts + 14″ of blown in cellulose, etc. As detailed in other posts I have been very busy air sealing home itself, insulating rim joist, sealing basement windows, sealing recessed light cans, etc.

On to my specific question:

There is a 20 x 15 attic over the original portion of home. All post and beam late 1600s construction. Ceiling of room under the attic is hand hewn timbers with drywall retrofitted between the joists in such a way that about 60% of the timbers are exposed to the room (very nice looking). Drywall joints are caulked to the joists but given the age of the home and the hand finished nature of the beams there are some small air gaps i’m sure. There are no light fixtures, smoke detectors or electrical holes in this ceiling. Above this ceiling is 1.5″ foam board fit between each joist cavity tight to the drywall, followed by tongue and groove pine planks tight to the foam board running perpendicular to the joists. These pine planks run underneath major 12″ x 12″ framing timbers — getting them up appears to require cutting / major work.

When I moved in I air sealed all areas I could access with spray foam — the top of the wall plate and all attic corners and walls. I also air sealed a major 4″ x 36″ air gap around the chimney with aluminum and fire rated caulk. I then rolled out r-30 batts perpendicular to the joists over the pine boards. I also installed 2″ foil faced poly-iso on attic access door and along staircase walls. And air-sealed attic door with rubber gaskets.

I do not imagine they spray-foamed the seams where the foam board meets the floor joists. I am very reluctant to pull the floor to do so, and really don’t want to get into spray foaming those old timbers if possible.

I am now second-guessing whether I should go back in and seal all the cracks in between the pine boards? If so, could I use a tape product? Or is the best option a small bead of great stuff?

Or are we thinking it’s not worth the effort?

Thanks for all your input. I am new-ish to all this and tend to overthink things!

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  1. GBA Editor
    Martin Holladay | | #1

    The best way to assess leaks in your home's thermal envelope is with a blower door test. I suggest that you hire an experienced blower door contractor to evaluate your ceiling / attic floor.

  2. humm9er | | #2


    I appreciate that assessing homes over the internet is near impossible.

    Previous owners recently had a blower door test performed. I can barely read the results but it appears prior to some air sealing the contractor performed, the blower door test was 4490 (or 44% hard to read), and after the work dropped to 3490 or .34% again hard to read). Not sure how to interpret those numbers.

    I had a "energy audit" performed by Mass Save. That individual told me my house was tight and he recommended no significant improvements, making me ineligible for insulation rebates.

    I subsequently called a few providers who quoted me $500+ for a blower door test. $500 goes a long way in common-sense DIY air sealing and insulation improvements, so I wanted to get as far as I can through self-education and my own work rather than hire a blower door tester. Furthermore, and correct me if I am wrong, most energy auditors aren't too interested in just being hired for a blower door test ... they want to be hired for at least a day or 2 of insulation work.

    Thanks everybody for your input.

    PS. Am I mistaken or can air leaks be spot diagnosed with a hair dryer?

  3. Expert Member
    Dana Dorsett | | #3

    How would you possibly be able to detect or tease out leaks with hair dryer?

    With a large window fan or high cfm whole-house ventilator you can chase leaks by pressurizing/de-pressurizing the house with the fan and using smoke-pencils or a cob-web dangling from your finger to look for moving air around suspected leak areas.

    3490 cfm/50 isn't a super leaky house, (especially for a 17th century antique) but it's not super tight either.

  4. jj1 | | #4

    Hi Justin: Gary Reysa at has developed a fully functional and very useful blower door device and test protocols for approximately only $30 in parts cost. Check it out at:

  5. humm9er | | #5

    Thanks Dana for your reply.

    I appreciate a hair dryer cant find large leaks behind walls or top plates, but don't see how it can't tell me if a door is sealed tight to the frame, or if a recessed can is tight to the attic, or if for instance those drywall seams to the joists are leaking to the attic above?

    I'm all for getting another blower door test done, just weighing the value. Is $500 the going rate? Will providers be interested in just doing the test, or are they only interested in doing the test than getting the air sealing work too?

    That prior blower door number of 3490 is prior to air-sealing the entire rim joist with 2" xps and foam, air sealing all known plumbing penetrations to basement and attic, sealing basement windows with shrinkfilm, gasketing basement door to house tight, foil-taping all recessed openings in my 36 IC light fixtures, stuffing with rockwool and retrofitting with gasketed LED inserts that mostly air seal to attic, gasketing attic door, caulking/foam gasketing/child-proof plugging all outlets, and air-sealing attic as best I could including as I mentioned closing a massive 6" x 36" opening around chimney.

    Just trying to determine whether I make my money back on another blower door test.


    PS. Taking out the blower door issue for a second, could one use foil tape to seal joints in a tongue and grove attic floor to be extra vigilant? Or is foam the only answer.

  6. GBA Editor
    Martin Holladay | | #6

    Your description of the air leaks through your ceiling / attic floor are hard to follow. Only you have a good idea of how big these cracks are.

    If you don't want to pay for a blower-door test -- and I understand your reluctance -- you are going to have to make a judgment call based on your understanding of the cracks.

  7. Expert Member
    Dana Dorsett | | #7

    Foil tape probably isn't the right thing to seal t & g planking seams and can-foam probably isn't either. You can do a lot with fiber-reinforced duct mastic, which remains flexible when dry, has a bit of stretch capacity to deal with seasonal dimension changes in the lumber from humidity & temperature shifts, and it sticks to just about everything. (Makes a great hair setting gel, as long as you don't plan on combing or brushing that hair any time soon! :-) )

    I still don't understand where & how a hair dry could be useful for figuring out air leaks though. It doesn't pressurize anything, just blows a stream of air (hot or cold) at some modest cfm. What you need is a method of pressurizing & depressurizing at least a room, if not the whole house.

    A decent sized reversible window fan like a Lasko 2155A can be had for under $100, and can be quite useful for pressuring & depressuring a room or house while on the hunt for leaks, and at your not huge leakage rates it'll probably work pretty well. You may have to close doors to rooms your not actively sealing to help keep the pressure differences as high as possible on the rooms you're working on ( it doesn't need to be the room with the fan in the window.) IIRC everybody from box stores to Staples & Wal-Mart etc carries that series. Be sure measure your window openings to ensure that the model you buy at least fits in at least some of your windows if buying online.

  8. humm9er | | #8

    Jan thanks for the link to the DIY blower. Dana, I like your plan of a powerful window fan to depressurize one room at a time. Can I depressurize the attic and check for leaks being pulled up through the ceiling or better to depressurize the house and check for leaks down coming into the house? I assume incense can help find leaks -- any other pointers for pinpointing leaks?

    Once I assess how leaky the ceiling is I will consider mastic for the attic floor grooves. Closer examination of the ceiling has led me to believe the air leaks aren't massive, but I will test to determine.

    Also, I assume I should gasket/air-seal my basement door into the house to reduce that opening as a conduit for pulling air from the basement. Correct?

    Thank you everyone as always!

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