I have an under-insulated roof above a "cathedral ceiling".  The shingles need to be replaced this year.  It occurred to me that this could be an opportunity to insulate from above.

Can it make sense to insulate above the plywood roof deck? Or to remove the plywood roof deck, add insulation, and re-install the plywood roof deck on top of that? 

Accessing the insulation from inside would require ripping down and re-installing a perfectly good ceiling.  I believe there's no more than 6-8 inches of fiberglass insulation in that ceiling, and in some places where there's an empty cavity, it appears the fiberglass has fallen out of position. 

The home is in Massachusetts.  I would love to hear from anyone on the theory and practice of doing this, and any real-world experience. 

Tags: Insulation, Retrofit, Roof

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Hi GW,

Is this a vented cathedral and if so where and how much venting?

What type of rafters are in there and how deep are they?

You suspect some of the insulation might have "fallen out of position", explain?  Did you look at this ceiling with an Infrared camera?

What problem/s are you addressing, ice dams, heat loss, or comfort?


Hi Bud,

The ceiling is not vented, so ice dams are occasionally a problem.  There are no soffits.

I'm not sure the depth of the rafters.  My best guess is 2 x 6". At one time, many years ago, it was probably an unheated room.

Insulation has fallen out of position over a closet and entranceway. There is empty, inaccessible space between the ceiling there and the roof.  I was able to see that when there was hole in the wall adjacent to it during an earlier renovation.  Snow melts very quickly above that area, and a little more slowly above the cathedral shaped ceiling.   The ceiling in the cathedral keeps the insulation above it in place.

As I say above, ice dams are occasionally an issue, but heat loss is the number one issue.  Comfort is not a problem, as the floors are radiant under a layer of 3/4" oak.

Hello Go.: you would only tear off plywood if it is rotted & needs replacement! The solution is to tear off the drywall, re-insulate if necessary, apply 2 inches of foam panel, and re dry wall. i've done this, so enjoy! t

I think it would be much less work to have this dealt with from above if that's possible. 

go. : right. ez way is to shoot foam using thermal detector. don't screw up venting. don't cause a "stephen girard" project like my builder did! you're right, t

I'm still not getting a good picture.  Are you talking about a flat ceiling at the top of the cathedral, the 6 to 8" of insulation.  And then 2x6 rafters which are only 5.5".

Here's what I'm looking for.  You should not have fiberglass insulation in an unvented cathedral ceiling, the rafters or attic for that matter.  Any air leakage or moisture migration will go straight to the bottom of the cold roof deck and cause damage.  If the space is truly unvented, then more information is needed to determine if there is a bigger problem already there.  In which case, now might be the best time to address the insulation and its installation.


The ceiling inside is at the same angle as the roof.  Sandwiched between the ceiling and the roof, and between the roof rafters, is a layer of insulation.  Those rafters are most likely 2 x 6. 

You are right... it is unvented, and I'm sure what you are describing is what's happening. 

From what you say, I will need to have venting added, though without soffits, I'm not sure how that might work.  If I were to have a ridge vent, moisture getting in there would not be vented out.  I suppose it might be possible to put soffit vents at the top of the wall, but I have never heard of that being done.


 Remove the shingles, drill a row of holes and use an 1 1/2 blowing tube to dense pack insulsafe fiberglass into the cavity, plug the holes add some foam board, add 1/2'" plywood over that and then reshingle with a light colored shingle. Hot roof, no air movement.

A minor note-'plywood' can be a used as a bit of a generic term for a number of products. I'd go with 5/8 OSB for the airsealing qualities. Every little bit helps.

Robert has it though I would lean more towards cellulose & don't forget to close off & seal the soffit vents

Scroll down about a 1/3 of the way & you will see that version with foam boards on the outside including a few best practices http://blog.sls-construction.com/2011/what-is-a-hot-roof 

One thing not listed there is make sure you install enough foam per code to eliminate condensation (806.4 in 2009 IRC) which looks like an R20 for you

Warm roofs only work if vapour is prevented from reaching the underside of the cold decking. Any gaps in the insulation or if it is porous will result in mould growth eventually if there is no ventilation. Plywood is a partial vapour stop so it is best located on the inside of the insulation. In the UK we fix insulation above the rafters using helical fixings and counter battens, and a breather membrane and battens under the roofing.

Insulating above the roof deck is the way to go in this case. I am completing a simular project on my house currently.

Remove the shingles, drill & fill the existing cavities with dense pack cellulose (3.5# cuft) or fiberglass. Air seal the plywood deck with tape (Siga, ProClima, ProtectoWrap, ect). If you do this it would be possible to use mineral wool above the deck, but not strictly according to the code.

Install a minumum of R-10 Rigid insulation (I seal the edges to each other with 1 part foam), more is better. Install 1/2" plywood, screwed to rafters below. (you could also add 2x4's flat for ventilation). Alternative is to buy nailbase (foam with 1/2" OSB on it). You can buy nailbase with a built in ventilation space. Venting the roofing would be preferable from several standpoints; reduce roofing temperature, and if (when it leaks it can dry out (and you can see it sooner)

Of course you will have to build up the trim on the edges (could use flashing depending).

On my house I left out the airspace, since I did not have the ability to add any more height due to another roof intersection. I also plan on painting the roof white.



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