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How To Insulate A Cathedral or Vaulted Ceiling




Carmen of Sussex NB Canada wrote:

I have my insulation, vapor barrier and my true vents installed. My question is how far up from the bottom of the eaves do I start my insulation? Do I join the Insulation at the peak of the house? First time insulating.... 

We suspected Carmen was referring to a vaulted or cathedral ceiling so we emailed in response to the question and confirmed it was. Insulating a vaulted or sloped ceiling is a little trickier that a flat ceiling, but it is pretty much the same concept. 

First, you need to prep the cathedral ceiling to be insulated. Grab a caulk gun, a ladder (or scaffolding if needed) and seal any areas where wiring or plumbing may be coming through the joists or wall plates. 


Also seal around any ventilation PVC pipes. This will eliminate air infiltration and will also help as a fire suppressant.

Next you need to install baffles, sometimes known as vent chutes. The baffles will ensure that you have adequate air flow for ventilation purposes. These come prefabricated to fit the width of your ceiling joists. I prefer the cardboard variety as they are easy to install and are cheap. Locate the vents at the eaves and grab a staple hammer. Starting where the vaulted ceiling joists meet at the exterior wall plate insert the baffle between the sloped ceiling joists and staple to the inside of them leaving a 1" space underneath the plywood roof sheathing. Make sure the baffle is placed completely over the wall plate area. Continue this process until all of the lower vents have a baffle installed.

Oh boy, now the fun starts - we're gonna insulate that ceiling! Put on some protective clothing and a dust mask. DON'T forget to wear goggles or the dust from the insulation above WILL get in your eyes if you don't wear them. You may also want to wear a hard hat to protect your head from protruding nails. Here are two options:

1. Insulating a vaulted or sloped ceiling with kraft faced batts.  I like to start at the top. The reason is I know I'll have to make cuts to make the insulation fit. I would rather do that at the bottom as I get near the wall plates. It's easier to make the cuts when your lower to the floor. Press the insulation batts between the joists. The kraft paper side should be facing the room. Make sure you leave at least a one inch space underneath the plywood roof sheathing for ventilation. Staple the paper flanges to the bottom of the joists, about every 8". Keep the insulation at least 3" away from heat sources such as recessed lights, metal chimneys and other sources of heat. 


2. Insulating a cathedral or vaulted ceiling using unfaced fiberglass batts.  These are easier to work with as they cut easy and fit using friction. The only draw back is they don't provide a vapor barrier. Check with your local building code people to determine if a vapor barrier is necessary in your area. If so, you could staple plastic sheeting to the bottom of the joists after the insulation has been installed. Take the same steps as above and omit the instructions regarding stapling the flanges as there are none. If you have a run between the joists that is greater than the standard 23" on center you may have to staple twine in a criss-cross pattern to hold the insulation up until you have drywalled the sloped ceiling.

That's it. Take a look at your work. Do you see any gaps? If so, cut some of the scrap insulation that's probably on your floor and use it plug any gaps. Finally, get out of those clothes and head for the shower! You'll need it, believe me.

Related Tips and Advice:

Blown In Insulation
Fiberglass Batt Insulation
Insulating Access Covers
How To Insulate Attic Drop-Down Stairs
How To Install A Radiant Barrier In A Attic


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