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My house has a false vaulted ceiling with a 4" gap between it and the roof. Unfortunately, whoever installed this ceiling neglected to install insulation in this gap.

However, I can access the gap from the attic and I do have enough clearance to slide things down the gap. My dilemma is how to best insulate this awkward area without taking down the false ceiling.

Here's a crude diagram of the location of the 4" gap (on the right):

Gap diagram

And here is a photo of the actual gap taken from inside the attic looking down the 4" gap:

Photo of the gap

The gap is 4" wide, but given the fact there are nails sticking through from the roof, the minimum clearance is probably only 3".

I've had a couple ideas for how to approach this:

  • Rent a cellulose blower from Home Depot. The end of the blower duct will be too wide to fit down the gap, but perhaps I could blow in some cellulose, pack it in with a long rod and then blow in more cellulose behind it...?

  • Cut sheets of foam board to fit and then slide them down the gap. R value won't be great and thick sheets might snag on the roof nails while sliding them down, but at least there would be some sort of insulation.

Any other ideas or recommendations?

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First, keep in mind that that venting in the roof rafters is essential to the ventilation of your roof. Essential. Do not fill that gap completely, you will be looking at future problems.

If the current gap is 3-4", the most you could do is insert 1.5" max of insulation. Panels of Polystyrene come to mind because their R-value is better per inch of thickness than roxul/pink stuff. You will, however, have to follow-up with "Great Stuff" foam to ensure filled gaps if you go that direction.

  • Upvoted, thank you for the reminder on the roof rafter vents. Without cutting into the roof, I have no way to ensure that the vents wouldn't get blocked by blown in cellulose, so that only leaves foam board insulation as a viable option. – Elliot B. Jan 23 '18 at 6:54

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