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Best Practices for Insulating a Cold-Climate Attic

Anna_CB | Posted in Energy Efficiency and Durability on

Thanks in advance to all of you who will finish reading this post 🙂 This is my first remodeling in my first house, so I appreciate all help.

We live in Boston metro area (zone 5) and in a 1901-built house. The top floor has cathedral ceilings (probably converted from the attic in the 1960s or so – we are not sure). Right now it serves as our bedroom with master bathroom + exercise zone. It is crazy hot in the summer and surprisingly tolerable in the winter. We finally have some money now to insulate it, and so I started my research.

As I found, there’s about 3-inch fiberglass in the ceiling – all in poor condition, missing in several places + the kneewalls are not insulated and provide nice heat pockets so that the second story of the house right under gets hot and non-livable as well. The rafter space where insulation is placed is not vented – there are soffit vents (installed to pass some brisk inspection maybe?) but they are covered with wood from inside so not functional, and no rigde vent either.
All in all, any project would be an improvement 🙂

Now, our initial plan was to remove the ceilings and install closed-cell foam (a la “unvented attic”), however, the rafters are only 2×6 and hence we cannot pack more than 5.5inches of foam there. That would give us about R35 – but not R45 required by code. We could add extra strips to the rafters to allow more foam but it is not an option in some areas like bathroom – the space is very tight; even 2 inches of ceiling would cost us a toilet :(.

Our contractor says that since the shallow rafters are “grandfathered design” we would be waived the code requirement, but I am worried if he is right. I called the city permit phone line but they were not helpful. So if someone could give me an idea how to approach this potential problem and whether to dismiss it and start the project, please do. Also, my contractor said that if we do add strips to the rafters to fit more foam, that would not be original design anymore, and no waivers would be applicable. Does that make sense to you?

BTW, I also want to add more insulation to the kneewall area that we know gets the most heat – just in case, you know. That shouldn’t hurt.

To sum it up – if I choose this option, I will get R35/R35+ in the roof, about R20+ spray foam in the walls (there’s none now), extra insulation and renovated/optimized closet space (that we would like to re-map, so this is a great excuse).

Alternatively, I was recently advised to not touch the interior at all, but to dismantle the shingles on the roof, add rigid insulation there, and finish it again. I am not sure if that’s the best idea – true, spray foam is expensive, but having >R40 worth of rigid insulation would require us to deal with a huge overhang (that needs to be covered with trim, new fascia, extended molding etc + lots of labor; the price tag for such project would be way more than simple roof replacement. We have a relatively new roof (installed 2009) in good shape, relatively light color, so replacing it now is totally impractical. The benefit is that I don’t have to deal with interior demo/restoration mess (huge!).

I am inclined towards the first option – if the code conflict is resolved one way or another; then wait and watch, and maybe add more rigid insulation to the roof when we replace the shingles next time.

What would the experts say? I would appreciate your help tremendously.

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  1. Anna_CB | | #1

    (cannot find the edit option so adding info here)
    In fact, I am not sure where R-45 as a target value came from; that's what my contractor gave me. I am totally confused now; here's the IECC 2015 code requiring R-49 (?) but saying "where Section R402.1.2 would require R-49 insulation in the ceiling, installing R-38 over 100 percent of
    the ceiling area requiring insulation shall be deemed to satisfy the requirement for R-49 insulation wherever the full height of uncompressed R-38 insulation extends over the wall top plate at the eaves."

    Here's Massachusetts code saying that I only need R-37 (if I interpret it right) or R-30 (same reasoning as above). This code book is 2007 but it is still listed as current on the website + amendments that I didn't find to be referring to this article.

    I also recall seeing R-49 number on this website in the context related to our situation but I cannot find where - most likely in one of the excellent Martin Holladay's articles.

    Also, I was reading his article on insulated cathedral ceilings ( and this option jumped at me:
    "Another way to add R-value to your roof assembly is to include one or two layers of rigid foam in the roof assembly — either above the roof sheathing or below the rafters. In addition to improving the R-value of the roof assembly, a layer of rigid foam has another benefit: it interrupts thermal bridging through the rafters."
    Is it easy to install those over the spray foam? Any special techniques? My uncle (an engineer, but not a building engineer) suggested that in that case there should be an air space left between the spray foam and the rigid foam, vented into the living space "to prevent condensation". To me, it rather sounds like a way to deliver condensation into a perfectly isolated area and NOT a good idea - is that so? (also, I believe it's against the code to vent the closed cell spray foam-filled space into living area)

  2. GBA Editor
    Martin Holladay | | #2

    Your contractor is probably right. In most retrofit projects like the one you describe -- even ones that involve furring down the rafters to deepen the available rafter thickness -- there is no need to meet code requirements.

    If it matters, and I don't think it does, I think that the code generally requires R-49 in your state, but allows R-38 for cathedral ceilings if the insulation extends over the top plates of the exterior walls.

    Here's my advice:

    1. Installing rigid foam above the roof sheathing is certainly the best approach, but it's an expensive approach.

    2. If you can't afford to install rigid foam above the roof sheathing, you should go ahead and install spray foam insulation between the rafters. Make sure that you get closed-cell spray foam, not open-cell spray foam. If you can manage to install 5 inches of closed-cell spray foam -- usually, there is a small air gap between the spray foam and the drywall -- you'll end up with about R-32. If you can manage to install 5.5 inches of closed-cell spray foam, you'll end up with about R-35 or R-36. That's much, much better than what you have now.

    1. woobagoobaa | | #4

      I'm also west of Boston and had a nearly identical situation as you described. We went with Martin #2 ... 5.5 inches of closed cell between the rafters. Not perfect in that we still have some thermal bridging from the rafters, but much much better than previously. #1 would have meant substantial expense and changes to the exterior trim.

      I am told some of the local towns (thankfully not mine) are very strict in that any mod to the premises requires full code upgrade. Do check with your local building department, or make sure your contractor has.

  3. charlie_sullivan | | #3

    Thanks for the complete description of the situation and for doing some research first.

    On the option of adding foam boards below the rafters, to get as good performance as spray foam, and assuming you left the fiberglass in the rafters, you would need something like three inches of polyiso foam board. The nominal r value would be lower, but the lack of thermal bridging would help, so you would end up with similar actual performance. You would have less space left in the room, but it would be a faster cheaper project and you would avoid the climate impact of ccSPF, as well as avoiding the risk of a spray foam job going bad, which is very rare but ruinous when it happens.

    That design should ideally include ventilation above the fiberglass, just as your installation now should ideally have that. But you can probably continue to be OK without it.

    Doing the foam boards over ccSPF would not be my first choice. Your uncle's plan probably would not cause condensation, because the ccSPF would be thick enough to keep that space warm enough to avoid condensation. But if there's too much air circulation there, it would undermine the effectiveness of the foam boards. And even with out deliberate ventilation, that gap is not ideal.

    So I would instead install 4.5 inches of mineral wool or cellulose between the rafters, and then maybe 3" of polyiso board below, if you open the ceiling.

    You can opt to insulate the knee wall space on the wall and the floor, or along the roof, and there are some important details there that need to be done right. But that decision comes after deciding about the.main ceiling area.

  4. MichaelAspyr | | #5

    The extra insulation will be helpful in winter, but it may push the room temperature up.
    There are many ways to insulate your home without blocking air circulation so that you have more comfortable living conditions during cold spells of weather like rain or snowfall! One way would involve installing something called "foam board" below ceiling heights which can provide an effective barrier against heat loss when paired with fiberglass batts installed within these walls - this particular material has lower nominal r values (but higher actual ones), meaning less space needs filled with additional materials. Georgia Roofers

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