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Updating attic/roof deck insulation

Schmidts | Posted in General Questions on

Good day all,
I have an older home (circa 1905) remodeled and expanded in the 1990’s. During the remodel the previous owner finished the attic and added mechanicals. They insulated the roof deck underneath with fiberglass insulation and the floor of the upper now attic with blow in insulation. Not sure what the r value is but not good. They also did not insulate the duct work which runs around the outside of the living space in the eaves. 

I would like to increase the insulation in the attic to increase the efficiency of the home. I will of course be insulating the exposed ductwork to remedy the lack there of and hopefully regain some of the heat/cold that is likely being lost. The more complicated part is the roof deck which I want to insulate. I want to use rock wool insulation between the rafters with a minimum of R23 and 2 inch polyiso or similar over that to prevent thermal bridging. The spaces that need to be insulated are not vented from what I can tell as there are no external soffit vents. As this is essentially a conditioned space I feel like I need to air seal and insulate as best as possible the roof to the interior as opposed to the attic to the living space. Are there any further considerations that I should account for in this project.

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  1. Expert Member
    BILL WICHERS | | #1

    Note that you can't combine R values of floor and roof insulation in an attic, so you don't usually want to insulate both areas. Right now it sounds like you have a bit of a hybrid attic, with some insulation on the floor, and some under the roof sheathing. With mechanicals in the attic, it's often best (but not always practical) to insulate the roof only, to bring the attic inside the building envelope -- a fancy way to say the attic becomes conditioned space just like the living areas.

    The tricky part becomes avoiding moisture issues in that roof insulation. Ideally, you'd have a bunch of rigid foam on the exterior of the roof, but that's usually only possible if replacing the roof. Next easiest is to put in vent channels, then add batts until you get up around code minimum R level for your climate zone (which is now R49 is much of the country, but I don't see you mentioning your climate zone). If you can't put vent channels, then spray foam is usually the best/only option to avoid potential moisture issues.

    If you can put in vent channels between the rafters from the eave (where they would tie in with soffit vents) to the ridge (where you would need a ridge vent), then you could use mineral wool batts and polyiso, although with the combo you mentioned, you'd only be at around R36, and you'll probably need more than that.


    1. Schmidts | | #2

      Hi Bill,
      Thanks for your reply. I am in Ohio outside Cleveland which puts me in zone 4 or 5 I think.
      You have brought an interesting point I hadn't thought of. That is, keeping moisture build up off of the inside of the roof deck. I can add baffles between the rafters to allow air flow under the roof deck. However if there are not soffit vents will I have enough air flow to keep the moisture from condensing? If I need to I would fur out the rafters to allow for an increase in mineral insulation and then cover with the polyiso. I want to get the insulation up to code if possible but am not 100% sure what the best approach is at this point.
      Also the roof is at least 25 years old. Not ready to be replaced but when it is time I would like to add insulation above the roof.

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