I currently have pink fiberglass bats in the attic. I wanted to add blown in insulation.

My attic space is a bit unique. I'm used to a moderate sized attic that you can access. This attic has no access. There might be 1 - 2 feet of space between the ceiling and roof decking.

The one space I have to access any part of the attic is around the chimney. It isn't boarded up and I can shimmy straight up the side of the chimney to the attic. That's how I know it's only about 1 - 2ft of attic space.

I've read about standard insulation installation techniques such as: Not to push the insulation up against the decking or moisture will develop. Proper roof ventilation to avoid moisture build up.

There are no attic vents on the side. No soffit, no soffit vents. The decking just comes straight off the house. The only ventilation is a ridge vent.

I was going to cut into the ceiling and blow insulation in until the entire space was filled.

I'm afraid of trapping moisture, rotting the decking and creating a breeding ground for mold.

With a roof/attic system like mine, is it OK to just fill the attic with insulation or am I stuck at certain level of insulation? How much space must be between the decking and insulation?


  • Critical information: Current R-value, local climate extremes
    – isherwood
    Feb 3, 2016 at 20:30

3 Answers 3


Based on your picture and your description, you have what is known as a cathedral ceiling. The "1-2 feet of space" you mention leads me to believe that it's built with parallel chord trusses rather than solid rafters. Can you post a picture? Regardless, like most cathedral ceilings, yours seems to be insulated insufficiently and built incorrectly (ridge vents but no soffits vents? WTF?).

Right now your cathedral ceiling is semi-ventilated. If there is a way to add soffit vents, they should be added so that the ridge vent can actually do its job. This will increase the durability of the assembly substantially if you live in a non-desert climate. If you take this approach and build a fully-vented cathedral ceiling, you cannot fill up the extra space that you see with more insulation; the ventilation gap is critically important. How high the gap must be depends on the steepness of the roof. A very steep roof might be able to get away with a gap of only a few inches under the roof sheathing. It sounds like your roof has a shallower pitch, so 1-2 feet may be necessary.

The only safe and practical way to increase the insulation of your ceiling is to transform it into an un-ventilated ceiling. Step one is to remove the ridge vent. Step two is to either apply spray foam (open or closed cell; minimum 3" for closed cell or 6" for open cell) under the roof sheathing, or else remove the roofing material and then apply rigid foam insulation boards over the sheathing, usually 4"+ of polyiso. Step three is to fill the entire cavity under the foam or sheathing with as much as you want of whatever you want. Go wild stuffing as much insulation in there as you want. Step four is to make your ceiling as airtight as possible. Remove all can lights or replace them with LED models that have an airtight fixture, and caulk everything. You don't want inside air getting into your now-unvented roof.

The spray foam approach is likely to be more expensive, especially considering the access problems you mentioned, but does not require a re-roof; the rigid foam approach is better for the environment, will give you an ultimately higher level of insulation, and may be cheaper, but doesn't make sense if your roofing material is in perfect shape.

  • 1
    I think you're on target but I will get my butt up there and see how the roof is supported, parallel chord truss or solid rafters. Very good explanation on cathedral ceiling, vented and unvented. This helps explain the different systems. Feb 4, 2016 at 14:04
  • @MikeMarseglia - take your camera with you and snap a few photos. That will probably help get you a better answer.
    – FreeMan
    Feb 4, 2016 at 21:47
  • "built incorrectly (ridge vents but no soffits vents? WTF?" The OP didn't specify his Climate Zone nor the house's age. Occam's Razor would imply it was built to local standards for the time at which it was constructed. With older leaky houses, soffit vents weren't necessary as air leaks provided more than enough. Apr 5, 2016 at 4:30
  • Indeed. And that's an example of "built incorrectly." Most of those air leaks would come from the conditioned space of the house, wasting energy--evidently a major concern of the OP seeing as how he wants to add more insulation.
    – iLikeDirt
    Apr 5, 2016 at 13:03

The key question is whether you'll see a benefit from doing so. In the winter, you may see better insulation from full cavities. In the summer, you're going to have a very hot attic due to poor airflow. The cost to cool the home in summer may outweigh the savings in winter.

Moisture accumulation may also be a concern, though condensation is less when there's not much empty airspace.


From your diagram and description, it seems the ridge vent allows cold or hot air to flow below the insulation. In that case, the insulation will have little effect. Also, since there are no vents on the ends of the attic, the ridge vent will be somewhat ineffective, since there is no easy path for the air to flow. You might put in vents at the ends of the attic and move the insulation from attached to the roof to lying on the attic floor, with the vapor barrier closest to the floor of the attic (under the insulation). Also, check to see that there are no gaps that let warm air escape from the inside of the house into the attic.

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