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Insulation for Sloped Ceilings Question

Christopher Tomasso | Posted in Energy Efficiency and Durability on

I’ve been searching the net in general for some answers but I am having a hard time purely because there seems to be quite a bit of conflicting information. I have a 65 year old cape style house with 2 dormers on the front side of the house and shed roof on the back side of the house. The roofline is such that it ends directly on top of the exterior walls (no soffits). Ventillation is currently handled through 2 undersized gable vents and 2 small circular vents near the peak (not whirlybirds). I live in climate zone 5, ~6500-6700 heating days.

When we bought the house 100% of the insulation was up in the rafters. It was all sagging and there were clear indications of mold growth and dry rot happening between the insulation and roof deck. 2 or more really long sections of craft faced cotton insulation was run from the ridge of the roof all the way to the top of the exterior walls.

In the attached picture all of the insulation that was in the rafters has been removed, mold remediated, and insulation replaced with 18 inches of fiberglass batts on the attic floor (shown in green) after air sealing everything in sight (electrical, light fixtures, the works).

On both the front and back sides of the house there are 2 very long crawlspaces/knee walls (shown in blue), roughly 4ftX3ftX12ft. There are doors leading into these areas but they are poorly built (no insulation or air sealing). Of the areas highlighted in blue neither the vertical wall (wall shared with 2nd living space), nor the floor (shares ceiling with 1st floor) are insulated in any way. The knee walls on the front side of the house are all accessible. On the back side of the house, one such area is non-existent because the builder choose to turn the area into a closet by sheet rocking all the way out to the exterior wall.

As of this moment, everywhere where the old batts would continue beyond the area marked in green into the knee walls on the second floor have been left alone (nailed directly to the roof deck), regardless of whether I have physical access to them or not.

My area of concern is the area marked in red (A to B), due to a lack of clear insulation retrofit and ice dams.

My original plan was to take a hole saw to the floors marked in blue and blow in cellulose and line the vertical sections in blue with batts. Glue foam board to the door and airseal the entry point. After this I had planned to place obstructions (probably foam board) sealed (with spray foam) and strapped with rood for support into the bays at point A. Go into the attic (marked in green) and blow in cellulose at point B till full and then cap them off using the same method point A. The idea being that I would not only fill these areas to capacity but seal them off as well (since the house has no soffit venting to speak of).

I cooked this plan up based on information I received from a representative at GreenFiber insulation who stated it would be perfectly fine to blow in insulation and have it come in direct contact with the roof deck.

Then I read over the following links:

And now I’m just really confused.

All of the sections of sloped ceiling in the picture are fairly long (in excess of 5ft) and in a very cramped area. I have my doubts that I could secure anything (foam board, air baffles, etc) to the roof deck without extensive demolition. I have no mudding and taping skills which is fine for me, but not fine for the wife (which means bringing in a contractor to clean up my mess).

I am trying to determine if there is a retrofit solution that will keep the mold/dry rot at bay, keep the heat in the house, and keep the ice dams from building up and screwing up my brand new shingles.

Will some or any of this work? Is there a better way this can be done? I’m primarily looking to do the work myself to keep the costs down. We plan to be in the home for less than 10 years but plan to hold onto the property to rent it out once we are gone.

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  1. GBA Editor
    Martin Holladay | | #1

    To do it right, you need to either install ventilation baffles plus air-permeable insulation, or spray foam insulation (for an unvented approach). If you need access, you'll need to take down the drywall ceiling.

    A third approach is to install all of the insulation (rigid foam) on top of the roof deck. This approach requires new roofing.

    For more information, see: Two ways to insulate attic kneewalls.

  2. Christopher Tomasso | | #2


    Thank you for taking time out to read over my question and provide me with some much needed information.

    I forgot to mention in my initial post that while I was installing batts in the area marked in green I discovered that almost none of my joists were 16" or 24" on center. Some were as small as 12 inches, others as big as 22". Almost all of them taper in width across any given 10 ft length by as much as a few inches. I was buying insulation meant for 24 on center and custom cutting each peice to fit in any given bay.

    If I were to pick a baffle size that would fit within 95% of my bays without modification (e.g. a 14"x4' baffle) is it essential that the entire width of the roof deck have ventillation (e.g. a 22" wide bay needs a 22" wide baffle) or can the baffle width be shortchanged? If so are there any guidelines that cover this?

    If this isnt something I can short change I may be forced to rip some long pieces of OSB and use standoffs to make my own baffles sized to fit each bay. Or look into the options that will force me to bring in contractors (demo the sloped ceilings, spray foam contractor or roof contractor).

  3. GBA Editor
    Martin Holladay | | #3

    It isn't necessary for each baffle to extend the entire width of the rafter bay. But if many of your rafter bays are difficult to ventilate well, it raises questions about whether you should even be pursuing a vented solution instead of an unvented solution.

    For more information, see How to Build an Insulated Cathedral Ceiling.

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