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Adding Exterior Rigid Foam to a Common Vented Attic

gman571 | Posted in Energy Efficiency and Durability on

I live in the greater Boston area (square in climate zone 5) in a 1978 vintage colonial. Like most of us, I’m on a budget. One part of the house is about 1200 square feet of living space over a single floor (and basement) with a hipped roof. The attic has 10″ of fiberglass in the floor between the ceiling joists and is vented with a few box vents near the peak and 2.5″ round soffit vents placed every 3 bays. It shows no adverse signs from condensation.

I need a new roof and would like to take the opportunity to add some additional insulation. I’m planning to add 2″ of XPS to the original roof sheathing (taping seams). I’d covering it with plywood, tar paper, ice and water shield, and applying the new asphalt shingles. I’ll keep the soffit vents and either re-cut holes for the same box venting near the peak, or add a ridge vent for aesthetic reasons.

Is this acceptable? I’m a bit confused over the vapor issues, but believe that since the original roof and ventilation performed adequately, my new system will as well. The sheathing and rafters, if they become wet, should dry to the inside. Sealing the attic (non-venting) seems expensive (CCSF) or difficult (not a lot of room toward the eaves).

I’m also concerned over the cost effectiveness, since the ventilation will mitigate any R-value gains. I think it’s prohibitively difficult to accurately model this and get real answers, but someone must have done this before me? I don’t expect miracles, but would like to see some decline in heating/cooling costs, and a commensurate improvement in living comfort. Can someone share some wisdom?

Sorry for the long post. I spend a lot of time reading on this website and find it very informative.

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  1. Riversong | | #1


    Insulating above a "cold" vented attic will add nothing at all to the efficiency of the house, while subtracting considerably from your pocketbook. Either the roof is insulated and the attic sealed or the attic floor is insulated and the attic is vented.

    The most cost-effective solution is to add 10"-12" of cellulose on top of the fiberglass, making sure that there are insulation dams or vent baffles in place to keep the soffit vents clear and allow air flow over the insulation. Since you're replacing the roofing, I would replace the box vents with a continuous ridge vent with exterior wind baffles (such as AirVent ShingleVent II or Lomanco OR-4). I would also put a round soffit vent in every bay.

    Since there are apparently no moisture problems currently, there should be fewer with additional insulation that better controls air flow, but it would be a good idea to lift the fiberglass and use canned or gun foam to seal any penetrations in the ceiling to minimize convective heat losses and moisture flow.

    P.S. Tar paper is fine as a roofing underlayment. Unless you're getting ice dams (which will be reduced by increasing the insulation), there's no need for a self-adhering membrane which only prevents drying upwards.

  2. 2tePuaao2B | | #2

    When you say" in floor between ceiling joists" does that indicate that there is plywood or something installed as a floor in the attic?

  3. GBA Editor
    Martin Holladay | | #3

    Robert is right. You have a vented, unconditioned attic, so you don't want to add insulation to the roof planes.

    If you want to improve the thermal performance of the existing insulation, just add more insulation to your attic floor.

  4. gman571 | | #4

    Thanks Martin, Roy, and Robert; I appreciate your responses, especially over the holiday weekend.

    I guess I should have specified, but the attic has a pull down stair and is used for storage, so adding insulation there is not a good solution for me.

    I know that ventilation is recommended by the industry and required by code, but I'm under the impression that it's for condensation issues. I believe insulating the top of the sheathing would keep its underside warmer, effectively raising the dew point and reducing condensation compared to what I have now (assuming other variables remain the same). If condensation proves to be an issue, I can either seal the attic or increase the ventilation (which will mitigate the new insulation.)
    In my view the rigid foam will help thermally isolate the attic from the great outdoors (to some degree).

    As I understand it, as long at there is a delta T (thermal gradient) there will be heat transfer. Currently the attic does not track the outside temperature very well. For example, it gets hot on sunny afternoons. So I'd be inclined to believe the foam would help my summer AC bill by keeping the attic cooler. In contrast, when winter brings temperatures in the 20's, the attic hangs around 45 degrees. As so, I have a 1400 square foot radiator trying to warm the atmosphere. I believe insulation would help this scenario also.

    Thanks again, guys. I want to assure you all that I'm not trying to be argumentative, but am looking for a solution that will work for my situation and a bit more depth than what code mandates.

  5. GBA Editor
    Martin Holladay | | #5

    If you seal up your attic vents, then adding rigid foam on top of your attic will probably lower your energy costs. If the foam is thick enough, there's no reason to vent the attic.

    However, such a hybrid insulation approach -- with some of your insulation above the roof sheathing, and some on the attic floor -- is not ideal. It would be better to add a significant amount of foam over your roof sheathing and go for a real conditioned attic, if that is your goal.

    You can read more on the topic here: Creating a Conditioned Attic.

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