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Community and Q&A

New addition with closed cell in rafters on top of current roof?

eoren1 | Posted in Energy Efficiency and Durability on

We live in a coastal town in MA and just started an addition on a 1940’s house. Currently have a simple sloped roof with blown in cellulose on floor. Two gable vents on either end and a few roof vents that we put in as well. The addition will be two stories and be a ‘roof on roof’ per the architect with a cut out to allow for electric/HVAC to be brought in from the current roof (HVAC is in this unconditioned space currently). The plan is to re-roof the entire house when the addition is put on. The addition will be unvented and have spray foam in rafters.

My concern is the junction between the old and new roof. If the old/main space is vented (two new larger gable vents and ridge vent per the roofer), will that create potential for ice dams where the unconditioned space meets the new addition’s conditioned roof?

If so, should I go ahead and do closed cell spray foam in the old roof rafters?

Would I then keep or eliminate the plan for gable and ridge vents?

Should I add a gable/ridge vent to the new addition roof?


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  1. GBA Editor
    Martin Holladay | | #1

    Your description is confusing.

    1. Does the old building have a gable roof or a shed roof?

    2. Does the new addition have a gable roof or a shed roof?

    3. Is the roof of the new addition co-planar with the roof of the old section of the house? Or does the roof of the new addition intersect the old section of the house in a way that is not co-planar -- for example, either above or below the height of the roof of the old section of the house?

    4. Is the ridge of the roof on the old section parallel to the ridge of the roof on the new addition, or perpendicular to it?

  2. eoren1 | | #2

    Sorry Martin. Here is an exterior elevation from the architect.

  3. GBA Editor
    Martin Holladay | | #3

    It's still a little confusing. The image is low-resolution, so I can't read the notations.

    In the upper image, is the addition the two-story section in the middle?

    What about the one-story section in the lower image? Is that a second addition?

    Basically, we need you to identify the old house and the addition in these images.

  4. GBA Editor
    Martin Holladay | | #4

    I'm going to assume that the center section in the upper image is the two-story addition.

    In order to avoid ice dams, you need to keep your roofing cold.

    In the older section with a vented attic, you have a lot of potential for ice dams -- because it sounds like you have a furnace and ductwork up there. The risk for ice dams doesn't come from the addition -- it comes from the location of your furnace and ductwork.

    The way to lower the risk of ice dams is to move the insulation in the old section of the house to the sloped roof -- converting the unconditioned vented attic into a conditioned unvented attic. This will only work if your furnace is a sealed-combustion furnace. If it's an atmospherically vented furnace, you'll need to replace it with a new sealed-combustion furnace.

  5. eoren1 | | #5

    Yes Martin - in the upper image, you are seeing a two car garage on the left, main house in 'back' and addition coming off perpendicularly.

    The older section just has an AC air handler and duct work. Heat in the house is forced hot water with boiler in the basement.

    Sorry for not specifying that earlier. I don't think the AC unit in the attic gives off heat in and of itself. Nor does it rely on any air for combustion.

    My main concern was having the addition being very 'tight' - zip system, closed cell foam in attic without venting VS the older attic being the opposite - blown in cellulose on floor, vented gables and ridge. Does the intersection of those two create a potential issue that would be fixed by using closed cell foam on old roof (interior) and closing vents? Or am I fine as planned?

  6. GBA Editor
    Martin Holladay | | #6

    As I said, the way to avoid ice dams is to keep your roofing cold.

    In the vented section, that means (a) making sure that there aren't any air leaks from the house below into the attic, (b) making sure that there is a thick enough layer of insulation on the attic floor, and (c) making sure that the forced-air registers aren't leaking air into the attic ductwork during the winter (a common problem).

    In the new addition, this means (a) making sure that the sloped roof assemblies have airtight ceilings, and (b) making sure that the sloped roof assemblies have a full R-49 layer of insulation rather than a layer of insulation that is less than minimum code requirements.

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