0 Helpful?

Interior retrofit of insulation in a cathedral ceiling

by Ken Ackerman

I have a 36 y/o home with 2 cathedral ceilings. A new roof was just put on (strip old roof, replace decking as needed, new 30# felt, new OC Duration energy reflective composition shingles (Shasta White). The re-roof process was on site inspected by an independent engineer to meet the Texas Windstorm Insurance Agency requirements. There was not enough time or money available to add Polyiso sheets over decking during the re-roof process. I live in Corpus Christi, TX, very hot & humid, no snow. I had continuous ridge vents installed and all old turtle backs and other roof vents removed. I am now starting to add Cor-a-Vent continuous soffit vents to balance the new ridge vent area.

1) Is it possible to remove the ridge vents temporarily and have an insulation contractor put liquid foam in each of the rafter cavities? I assume that due to the age of the home, that FG batts were probably installed originally. It's also possible to also open the soffits if that provides needed access. Just thinking about trying this approach gives me great reservations because it may actually block the venting cavities between the rafters.

2) If #1 above is not appropriate (more than likely not), then I assume the inside approach is the only one left. The current ceiling is sheetrock, is it better to take it down or just put polyiso sheets right over it? The diagram in this article above shows the polyiso put directly over the rafters. I'm assuming that taking down the existing sheetrock will more than likely allow the FG batts to fall out and prove to be a nightmare, unless the batts were stapled to the rafters.

3) What are the downsides to applying the polyiso directly over the current ceiling and then sheet rock over them, if any?

4) If the current sheetrock ceiling was ripped out and the batt insulation with it, would that allow an insulation contractor to liquid foam the back of the roof decking and still keep a vent channel to the ridge vents? If so, would it be necessary to put rigid foam inserts after the liquid foam is applied to insure the vent channel stays open?

OR install rigid foam sections one at a time prior to liquid foaming above it?
As you can see, I'm certainly not an expert in this area, but I'd like to do it once and only once, the right way, the first time.


Asked by Ken Ackerman
Posted Nov 1, 2012 6:35 PM ET
Edited Nov 2, 2012 6:08 AM ET


8 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Ken, I think calling contractors is what you should do. Find those that can show you having done what you are looking for.

You will want your ceilings to be permeable on the interior.

Answered by aj builder, Upstate NY Zone 6a
Posted Nov 1, 2012 8:04 PM ET


Either of your suggested interior approaches will work (removing the drywall and installing spray foam -- an approach that can be done with ventilation chutes in place if you prefer, or without -- or adding interior polyiso on the underside of the existing drywall). Your exterior suggestion will not work.

To me, the idea of ventilating an insulated cathedral ceiling in a hot, humid climate is a little strange. There aren't many benefits to venting such a roof assembly, and there are several possible downsides (added cost and complexity, and potential moisture problems from inviting humid air into your roof assembly).

Answered by Martin Holladay
Posted Nov 2, 2012 7:31 AM ET



It may indeed be strange, but that's the way this house was built. There are existing soffit vents on all eaves, but I do not remember now if there were any turtlebacks on that vaulted ceiling roof area. Now, however, there are ridge vents, put there at my request during the re-roofing.

My motivation for ventilation is based on hot & humid. From late June through October most of our days have heat indexes of mid to upper 90's & above! Sauna like actually. It makes the wheel on my electric meter go quite fast to the delight of TXU (the utility company), and the stinging pain of my wallet.

Other than the fact that there is no larger under roof cavity like the un-vaulted rooms, how would venting the cathedral be different in reacting to the same humid air that goes into the rest of the house? I'm more than a little fuzzy on that. That's why I'm asking questions.


Answered by Ken Ackerman
Posted Nov 2, 2012 11:06 AM ET
Edited Nov 2, 2012 11:07 AM ET.


All of the hot, humid air that enters your house will behave the same way. If any of that hot, humid air encounters a cold surface, the moisture in the air will condense. You don't want that to happen.

The best way to prevent problems is to have a good air barrier. Leaks of outdoor air into your home during the summer are not good, especially if any of the air contacts a cold air-conditioning duct or a cold register boot.

In most cases, inviting outdoor ventilation air into your attic or rafter bays doesn't cause problems, but it can, especially if there is an air pathway that allows exterior air to contact a cold duct or a register boot. These air pathways aren't always obvious, but if your house is depressurized due to an imbalance in your HVAC system or the operation of an exhaust fan, outdoor air can be pulled into nooks and crannies of our house where the air is not welcome.

Answered by Martin Holladay
Posted Nov 2, 2012 11:20 AM ET



Thanks for such a speedy reply. Previously I have downloaded & read a number of articles, including some from Building Science that relate to saving energy where heat & humidity are of concern. At this point, I'm very confused about how to proceed.

Perhaps my question now should be: How do I most economically add insulation to reduce air conditioning costs in a late 70's era single story ranch that was built with now compressed fiber glass batts in thin walls & roof (2x4's - EEK!)?

I understand the concept of moving the building envelope as far to the outside as is possible to mitigate the then common blunder of placing air handling ducts in the very small attic (close to a 4 pitch roof). Best done with foam insulation on the interior of the roof decking. I also understand it's important to increase insulation on the floor of "attic" spaces above non- vaulted ceilings too.

Should I do both on the non vaulted or just go with floor attic insulation increase? Should I leave the the vaulted rooms alone and seal up the soffit & ridge vents above them ?

Bewildered in Corpus,


Answered by Ken Ackerman
Posted Nov 2, 2012 12:07 PM ET


Before, you were talking about an insulated cathedral ceiling. Now it sounds as if your house also has an attic.

If your house has an attic that includes ductwork, the best approach is to install closed-cell spray polyurethane foam on the underside of the roof sheathing above the attic to create an unvented conditioned attic. More information here: Creating a Conditioned Attic.

To improve the performance of your cathedral ceilings, the best approach at this point is probably to install rigid foam underneath the existing drywall ceiling -- as thick as you can manage (the thicker the better) -- followed by a new layer of drywall.

Answered by Martin Holladay
Posted Nov 2, 2012 12:22 PM ET


You might also check out these two articles:

A Crash Course in Roof Venting; Understand when to vent your roof, when not to, and how to execute each approach successfully. http://www.buildingscience.com/documents/published-articles/pa-crash-cou...

By Dr. Joe


An Unvented, Superinsulated Roof. http://www.finehomebuilding.com/design/departments/energy-smart-details/...

By Mr. Holladay

Answered by Kris Knutson
Posted Nov 2, 2012 2:16 PM ET
Edited Nov 2, 2012 2:18 PM ET.



Sorry about the confusion. The house has 2 rooms (great room & mbr) that are vaulted. All the other rooms (Kit, DR, non - mbr- bedrooms, studio) have exceedingly small attics which amounts to a triangular crawl space.


Thanks for the tip on reading material. I definitely will download & read them.

Thanks for all the help guys!


Answered by Ken Ackerman
Posted Nov 2, 2012 5:49 PM ET

Other Questions in GBA Pro help

Are these calculations wrong?

In Mechanicals | Asked by Michael Grundvig | Apr 22, 18

Sloped ceiling insulation

In Energy efficiency and durability | Asked by user-7005570 | Apr 22, 18

Small addition with helical pile foundation in Toronto Canada

In Green building techniques | Asked by Lowell Lo | Apr 22, 18

Can Housewrap 'melt' under steel siding?

In GBA Pro help | Asked by user-7015584 | Apr 22, 18
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!