GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Air sealing and insulation question

user-1139752 | Posted in Energy Efficiency and Durability on

I am buying a 1900 bungalow in the white mountains of NH, 10 foot ceilings 4 inch walls. The only insulation now is the attic is capped. The heating is hot air, with unsealed or insulated duct work. The house is one story. We are closing in October, so I’ll have no access until then. Where should I start?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    The ducts are in the basement, and not in the attic above the insulation, I hope(?).

    Is the basement sealed & insulated?

    Any fireplaces?

    Fixing the leaks at the attic-floor plane and at the basement o subdue the "stack effect" are far more important than leaks in between. Flues and plumbing/electrical chases that run from basement to attic are often huge thermal by pass stacks for moving air. Foundation sills & band joists are typically several times leaker than all of the window & door leakage combined.

    If you can't see any big leaks, it's probably worth doing an air sealing pass with a blower-door & infra-red imaging to map out big leaks though wall-framing etc to seal those up best you can prior to adding wall insulation.

    The wall construction type/ stackup may dictate how the walls get insulated.

  2. LucyF | | #2


    We have to know more details. Is this a year round home, summer home, winter skiing? How far are you willing to go to insulate the home? Tear out the walls, new siding, new roof? Is the duct work located in the insulated part of the house or outside of it.

    So first tell us what you hope to do with the house, then you can get much better advice.

    As you know, the first priority is air sealing before insulation.

    The White Mountains are beautiful. It would be lovely to have a house there.

  3. user-1139752 | | #3

    Dana and Lucy, Let me give more details. I live in southwest NH now, but my neighbor wants to buy my property, long story. This new house will be a year round house starting next June. This new area of NH, gets very cold -30 to -40, so air sealing and insulation will be important. All the ducts are in the basement. The construction type is balloon frame and from what I can tell 4 inch walls. The interior has plaster walls, the exterior is clapboard siding. The heat is oil via a Thermopride furnace. My old house had one of the same furnaces, but I heated with two Woodstock wood stoves and that's the goal here as well. Lucy asked how far I am willing to go. The house I am selling now has 2” of foil face urethane in the floors and 4” in the walls. I had 4” of spray foam put in the gable ends and the roof bays. We did a cold roof giving ventilation the entire way from soffit to ridge vent. This house has four peaks and would be a challenge to do that this time. The attic is a full walk-up, but I plan to keep it as an attic. There is one fireplace plus one other chimney, that serves the furnace. The basement is not sealed or insulated. The basement is stone with a granite foundation, it's a full basement. I am very familiar with air sealing thanks to this site and Building Science. I am always looking for a way to make an old house energy efficient. Thank-you for your help and advice.

  4. Expert Member
    Dana Dorsett | | #4

    There is no place in NH with 99% outside design temps anywhere near -30F (but if this is at 3000' of altitude that will be more than a once per decade event.)

    If the house has an uninsulated quarried granite basement it's worth putting up 1-2" of closed cell foam and a rock wool or unfaced fiberglass insulated studwall (with no vapor barriers-kraft facers would be OK.)

    The fireplace is a 24/365 air leak- even worse if it happens to be on an exterior wall. If you're planning to install a woodstove or wood burning insert in the fireplace firebox, it's worth doing some masonry work and installing a sheet-steel air-barrier where the new flue liner will drop through the masonry smoke-chamber to tighten it up a bit. While it's impossible to get a perfect air seal between the liner and the steel air-barrier, you can do pretty well with automotive exhaust sealants.

  5. user-1139752 | | #5

    Hi Dana, There are nights when it does get to -30F, but it seems like a nights few per season. The average low in Jan and Feb are 2-3 degrees F still much colder than where I live presently. Would you use closed cell spray foam on the basement walls or somehow use 2" urethane sheets? The stud wall would you use pressure treated lumber? Are there some links here on GBA you could point me towards for further study? Thanks for your guidance. Ken

  6. Expert Member
    Dana Dorsett | | #6

    With granite or fieldstone foundations it's harder to make permanent attachments with good air seals using rigid foam, whereas spray foam becomes a monolithic self-attaching monocoque. Even if it seperates from the stone and shifts a bit over time, it will remain air-tight. (With poured concrete or CMU foundations sheet-goods can work pretty well though.)

    Is there any sort of basement slab or ground vapor retarder?

  7. user-1139752 | | #7

    The basement does have a slab. but I doubt there is a vapor barrier in place. Would using 3 or 4 Rinnai wall furnaces in place of the ducted hot air furnace make sense? Therefor I could insulate the floor with ridge foam which would air seal the basement. We plan to add a gas range for cooking and for heating the hot water, so propane will be on site. We did it here in the current house and it works great. We heat with 3 cords of wood and use the Rinnais as backup only. I'm thinking of the cost of spray foaming an 8 foot wall on a 49x33 basement.

  8. user-1139752 | | #8

    I wanted to update on this thread and see if anyone had more thoughts. We are having the basement spray foamed in the next week with one inch of closed cell foam. They plan to spray the sills to the foot of the wall where there is a concrete slab floor. I did find several 1 to 3 inch openings in between the basement floor joists, where a former staircase ran. The openings went from the basement to the attic. I sealed it with 2" foil face foam and spray foam along the edges. While I was at it I sealed all the electrical and plumbing penetrations. Last week I started on the attic and found an area where the former staircase came out of the basement and first floor. I looked through the opening and could see my spray foam work from above. The opening was 40" by 70" the only thing covering it was 3 inch batts stapled to the ceiling joists. I filled in the voids with 2" foil faced foam sprayed around the edges. The joists are 8 inch I plan on filling the balance with Roxul batts. Now that I said all that what is the best way to insulated beyond the Roxul on the attic floor. Do I add 2 inches of foil faced foam then add the flooring back over it. I don't want to create a mold problem, with a sandwich of different materials, but I do want to eliminate the thermal bridging that is import to prevent.The attic floor currently has 2 inches of blown in rock wool. The walls I noticed are all back plastered against the exterior sheathing, but are empty otherwise.

  9. GBA Editor
    Martin Holladay | | #9

    Q. "What is the best way to insulate beyond the Roxul on the attic floor?"

    A. Blow a deep layer of cellulose on top of the Roxul batts.

  10. Expert Member
    Dana Dorsett | | #10

    Do I understand correctly that you have 3" batts between the 7.25" deep ( 2 x 8) joists, and there is flooring above the batts?

    First thing is to air seal the ends of the joist bays, and any electrical / plumbing / flue penetrations.

    If you're going leave the flooring in place you can blow cellulose over the existing batts, which will result in a center-cavity R of about R25, which is a huge improvement, but still sub-code. If you used Roxul over the existing batts you'd be at maybe R27-R28, a modest performance improvement if perfect, but unless you pulled the floorboards a perfect fit would be impossible, undercutting performance.

    To hit a code-min R49 you'd have to either build over the existing floor or pull up the existing floor to be able to install ~15" of open-blown density cellulose (cheaper than dense-packing). If you want to still have a usable floor for storage, install some 2x8s 24" o.c. perpendicular to the existing joists and blow cellulose up to just over the joist-tops, then rake or compress it flat when you re-install the flooring on your now elevated attic floor. (One of the guys in my office did exactly that a couple months ago to a 1920s bungalow he recently moved into in MA, pulling then re-using the old ship-lap floor planking.) By making the new joists perpendicular to the old rather than just sistering onto the old ones, the thermal bridging is dramatically reduced.

    If you are going to put foil faced polyiso on the exterior side of fiber batts, in your location (US climate zone 6), the foam has to be at least 50% of the total center-cavity R. (And with polyiso you need to de-rate it to R5/inch when on the cold side of an assembly to be valid from a dew-point control perspective, even though it outperforms that during the shoulder seasons.) In that of stackup the fiber needs to be tight to the foam too, no gaps. To g that route you'll need to fill completely up the joist bays or make the batts a compression fit (about R25-R28), then add a minimum of 5" of polyiso above that. Alternatively, fill the joist bays, add 2' of polyiso then add another R 15 of some other type of insulation on top of the polyiso.

    Blown cellulose is quite a bit cheaper than Roxul and it also fills in completely around all framing & other feature anomalies. Open-blown installation is an easy DIY with a box store's rental blower (usually free for 1-day of rental with some minimum purchase.) If the box-store cellulose contains sulfated fire retardents (most do), it's worth either pre-ordering a "borate only" version of their cellulose manufacturer's product, or chasing some sulfate free goods down through other sources and paying the rental fee. (Both National Fiber and NuWool products are all sulfate-free/borate-only. Kamco used to carry Nat'l Fiber, but now carry NuWool. The closest one to you may be in Portland or Lebanon ME, or Londonderry NH, which isn't exactly next door to the Whites- there are probably others that carry it too. )

  11. user-1139752 | | #11

    Dana Thank-you and Martin too for the excellent suggestions. The area that has the batts of fiberglass was empty because I removed them from that space. There was just air below, so there was just a void looking in down in some places 12 feet. I put some boards under the joist and laid the 2 inch poly between the bays leaving a small gap on the edges which I filled with canned spray foam. I plan on adding the Roxul over it. I'm doing it a little piece meal because the electrician still has to remove the knob and tube wiring that snakes all over the attic. Once he's done, hopefully by Christmas, I can get the borate cellulose and rent the blower to install it. Have a nice holiday.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |