0 Helpful?

Critique my cathedral roof insulation

Hello GBA -

I live in a very wet climate in Alaska (Kodiak Island) where our mean winter temperature is around 32 or so degrees. It is almost always wet. I just purchased a new place and am going to take advantage of a state provided program to help out w/ subsidies to weatherize places.

I have a single slope cathedral ceiling with a brand new rubber type roof. My plan is to install polyiso beneath the ceiling to help insulate it. Here is a picture of my plan.

The big x's are the rafters and the polyiso would go below the rafters.

In order from TOP to Bottom
* Pink Board ( polystyrene) -
* Polyiso (4.5" R - 33) -
* Vapor Barrier -
* sheetrock -

Here is a picture of my roof:

I have a few questions.

1. What affect will the polystyrene (pink insulation) have? Is it worth the investment to put 4" up between the rafters (above the Polyiso?). There would be an air gap above the pink board allowing the roof to vent (I think I have 2 x 8 " up there..so roughly 4" of airspace)

2. Do I need a vapor retarder/barrier between the polyiso and the sheetrock? From the literature I am reading I do not (so long as I tape up the seems of the panels well.

3. If I do not use a vapor retarder/barrier how do I airseal where the polyiso/sheetrock meets the walls?

Please critique my design!

Asked by Kevin Lauscher
Posted Jan 29, 2013 9:57 PM ET
Edited Jan 29, 2013 9:58 PM ET


9 Answers

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

Your plan will work.

Q. "What effect will the polystyrene (pink insulation) have? Is it worth the investment to put 4" up between the rafters (above the Polyiso?)"

A, The "pink foam" in your sketch (I assume that it is XPS manufactured by Owens Corning) will add valuable R-value, as long as it is installed in an airtight manner. It is fussy work, but you can do it. You'll need to install 1"x1" sticks at the sides of your rafter bays; these will act as stops so that you have something to hold the XPS at the right location. Each piece of XPS needs to be installed in an airtight manner. That means that the perimeter of each piece of XPS needs to be sealed with caulk, canned spray foam, or high-quality European tape.

Q. "Do I need a vapor retarder/barrier between the polyiso and the sheetrock?"

A. No. The rigid foam is already a vapor retarder.

Q. "If I do not use a vapor retarder/barrier how do I airseal where the polyiso/sheetrock meets the walls?"

A. The perimeter of the polyiso -- where the polyiso meets the walls -- should be sealed with canned spray foam or high-quality European tape. The Sheetrock should be sealed with paper tape and drywall compound -- assuming that your walls are finished with Sheetrock, too. If your walls have some other type of finish, you can caulk the gap between the Sheetrock and the walls, and then install some molding to cover the seam.

Answered by Martin Holladay
Posted Jan 30, 2013 6:53 AM ET


Kevin I like your plan, it's creative and seems like it could work well, but I wonder if you've considered applying all of that rigid foam on top of your roof? As Martin says, your plan involves a lot of fussy detailing between each rafter. But for such a simple single slope roof, maybe its worth considering using your new rubber roof as an air/vapor barrier and layering all your foam on top of that (PERSIST style) then apply 2x4 furring and new roofing of your choice. The downside is maybe the expense of the 2nd roof, but you could get exposed beams inside and simpler installation if you went this route.

Martin, if you think Kevin's plan is workable, can you discuss the risks with this type of assembly? I'm interested in this approach as it seems less expensive than spray foam and working from the inside could allow you to keep a recently re-shingled roof. I have a 1.5 story home with a recently re-shingled roof and I'm having a hard time accepting the idea of tearing it all off and applying foam on top of the roof deck, but I can't see how else to achieve at least R-49 for code compliance (I'm in Zone 7) However Kevin's idea here has me wondering again - do you think there is any limit to the thickness of the rigid foam on the underside of the rafters? What are the disadvantages in placing the roof trusses outside of the thermal enclosure?

Thanks, and good luck Kevin!

Answered by Mark Fredericks
Posted Jan 30, 2013 2:48 PM ET


I agree with you: installing one or more layers of rigid foam on top of the existing roofing would be simpler and better. However, it's probably not cheaper.

I don't think Kevin's approach is risky, assuming that the air channels under the roof sheathing are vented. At worst, the air seals around the rigid foam inserted in the rafter bays won't be perfect (especially if the rafters expand and contract with changes in temperature and humidity -- which they will). However, the continuous layer of rigid foam under the rafters should provide a good air barrier, limiting risk.

Answered by Martin Holladay
Posted Jan 30, 2013 3:04 PM ET


Thanks for the well laid out responses.

I have not looked into replacing the roof, but I don't think it's in the budget.

I'm new to any kind of work and I am curious as to what types of screws I should use?

To secure the polyiso to the rafters do I need a screw w/ a plastic cap on it?

Any place where you could recomend buying screws/fasteners? Everythign needs to be shipped/barged in.



Answered by Kevin Lauscher
Posted Jan 30, 2013 7:58 PM ET


Rigid foam can be installed with cap nails or with long screws and roofing buttons. You can also use long screws and furring strips.

Cap nails are available in a variety of lengths, up to 8 inches long. Many manufacturers sell 8-inch and 9-inch screws. Links to the web sites of cap nail manufacturers and screw manufacturers can be found in this article: How to Install Rigid Foam Sheathing.

Answered by Martin Holladay
Posted Jan 31, 2013 1:38 PM ET
Edited Jan 31, 2013 4:20 PM ET.


I have no idea what you mean by High Quality European tape. I googled a bunch and found nothing. Can you give me an idea as to what you mean?



Answered by Kevin Lauscher
Posted Feb 27, 2013 7:49 PM ET



I think Martin is talking about tapes like Siga which can be obtained on the west coast from Small Planet Workshop. You can read about this kind of tape on their website at www.smallplanetworkshopstore.com. These tapes are reputed to have good track record of longevity. I think that is why Martin recommended the European tapes.

Answered by Lucy Foxworth
Posted Feb 27, 2013 10:46 PM ET


The Polyiso showed up yesterday. It got absolutely mangeld in the shipping process and many pieces have a few chips in them. Joint sealing may not work as well considering what happend.

Would putting a layer of polyethylene below the polyiso accomplish the joint sealing of the polyiso just as well?

If I were to tape, the euro tapes are not in the budget right now. What other tapes would you recommend?

Sorry for all the questions but again, this is my first time and I'm looking to do it right.

Answered by Kevin Lauscher
Posted Mar 6, 2013 11:31 AM ET
Edited Mar 6, 2013 1:29 PM ET.


If the polyiso has foil facing, you can tape seams with housewrap tape. If it is recycled roofing foam, it's going to be hard to tape -- so you should seal the seams with canned spray foam.

In your climate, it's perfectly acceptable to install a layer of interior polyethylene. Interior polyethylene can work as an air barrier, as long as you pay meticulous attention to sealing penetrations, as long as it doesn't get ripped, and as long as seams are sealed. (Seams should occur over framing members and should be sealed with Tremco acoustical sealant.)

Answered by Martin Holladay
Posted Mar 7, 2013 6:21 AM ET

Other Questions in Plans Review

Roof / wall connection showing vent above structural sheathing

In Green building techniques | Asked by Ethan T | Jun 15, 18

Building Code Reqs for Stacked Window Headers, Bracing, Stucco over Foam

In Building Code Questions | Asked by Zane Bridgers | Jun 17, 18

Room over garage

In General questions | Asked by Jay Raja | Jun 16, 18

Do I need an HRV/ERV for a house built in 1947?

In Mechanicals | Asked by Will82 | Jun 14, 18

Ceiling insulation for a room addition to my house

In Energy efficiency and durability | Asked by Howard Gentler | Jun 16, 18
Register for a free account and join the conversation

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