Insulating exterior of 70’s-built T&G cathedral ceiling, Zone 4a — Q1: Vented or Unvented?
Long-time reader, first-time writer. Thanks in advance for your time, expertise, and consideration.
INSULATION DESIGN AND APPLICATION QUESTION
In Zone 4a, which insulation method is preferred for T&G cathedral ceiling?
4″ vented polyiso (~22-24 LTTR)
• 1st layer: 2″ Atlas ACFoam III
• 2nd layer: 3.5″ Atlas ACFoam CrossVent (2″ polyiso, 1″ vent, 7/16″ OSB)
• 2′ staggered seams per Atlas recommendation
-OR-
5″ unvented polyiso (~28-30 LTTR)
• 1st layer: 2.5″ Atlas ACFoam III
• 2nd layer: 3″ Atlas ACFoam NailBase (2.5″ polyiso, 7/16″ OSB)
• 2′ staggered seams per Atlas recommendation
Replacing shingle roof in middle Tennessee (Zone 4a). Mixed roof design with 12:12 exposed-T&G cathedral and 3:12 over ceiling-insulated attic. Most of 2×6 T&G roof has no added insulation, inside or out. (Two portions covered by attic; see BUILDING DETAIL below). Insulation cannot be added to interior to preserve the appearance of the exposed wood. Desire to gain better thermal performance, mainly to increase comfort in the living space. Existing chimney height limits roof envelope increase to ~5.5″. Local residential roofers seem uninformed and resistant to the idea of exterior insulation, though it seems common in local commercial/industrial buildings.
BUILDING DETAIL
I am replacing a roof in Zone 4a on a late 70’s-built home in middle Tennessee. The family room is in the middle of the house and has a 12:12 cathedral ceiling constructed of exposed 4×6 rafters (48″o.c.) and 2×6 T&G. There is a loft on one gable end and a brick chimney on the other. The ceiling is the roof: underlayment and shingles are applied directly to the T&G where exposed outside the envelope. There is no additional insulation except where enclosed by “attached” attics.
The rest of the house, on either side of the cathedral room, is ranch-style construction. Each side has 3:12 roof with attic typical for age and location (ridge vent, soffit vents, gable vent on open end, and blown insulation). The lower slope roofs are built onto/over the steep roof. In other words, there is a portion of the cathedral ceiling/roof that is part of each respective attic. There are two valleys where the rooflines intersect (4x valleys total).
During summer (cooling), the loft is unbearably hot in the making it unusable, though there are two supply ducts that “service” the space. The rest of the family room easily sits about 4° above the thermostat during the hottest part of the day. The rest of the house remains relatively comfortable or too cool being better insulated with the single stage HVAC running constantly during the day.
During winter (heating), the loft is tolerable whereas the rest of the family room is cold and drafty. The rest of the house remains mostly comfortable.
Along with insulation, air barrier improvement and mini-split are being added during this renovation to help improve the comfort and performance of this living space.
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Replies
Hi Todd,
Being a long-time reader of GBA, you've probably already discovered some of the blogs related to roofs, but in case you haven't, here are a few of the ones covering cathedral ceilings:
https://www.greenbuildingadvisor.com/article/all-about-roof-venting
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Climate zone 4 will have different roof R-values depending on which code cycle your area is currently on. 2018 and earlier versions of the energy code require R-49 for roofs. The 2021 upped the requirements to R-60. The 2024 version has a new option, "insulation entirely above roof deck". This only requires an R-30. I've had success in convincing code officials to allow me to use the updated versions of codes if I submit them the code reference. You're 5-inch design comes close to meeting the new code.
Probably the most critical part of the assembly is getting the air control detail right. My suggestion is some type of peal and stick membrane on the T&G ceiling paneling before you begin the insulation process. You'll want to extend the air barrier over the ends of the T&G, there are lots of small air pathways that could still affect the air tightness if they are not sealed.
As far as venting the roof, that's up to you. If you are in an area with some snow and you are worried about ice dams, venting will help with that. Another reason to vent is if you don't have confidence in the ability to achieve an effective air barrier at the T&G. Venting can help clear any moisture that is moving into the roof assembly by way of air leaks.
Hope this helps, and good luck with your project.
Randy
Hi Randy,
Thanks so much for your reply! After reading and rereading so many of these articles, there have been a few details I have struggled to resolve. Your points bring assurance to the unvented design.
You may have answered the question I planned to post next: Peel-and-stick or house wrap over the T&G. I get tangled up with the specificity of word meaning sometimes — sorry if my asking for clarification sounds like I'm being a smart aleck. You wrote:
"Probably the most critical part of the assembly is getting the air control detail right. My suggestion is some type of peal and stick membrane on the T&G ceiling paneling before you begin the insulation process. You'll want to extend the air barrier over the ends of the T&G, there are lots of small air pathways that could still affect the air tightness if they are not sealed."
Am I safe to assume you mean applying peel-and-stick on the exterior side of the T&G? This will act as WRB, air barrier, and vapor barrier. Concerns for vapor related rot on the ridge beam, rafters, and T&G are greatly reduced since they remain inside the conditioned envelope. Concerns for vapor related rot for the OSB nail base is reduced since effectively should be no vapor drive from the interior. The current plan is to install ice-and-water shield under metal in the valleys to address the places most susceptible to damage from ice dams.
Do I need to tape the seams of the polyiso if I am using peel-and-stick?
Thanks again!
Todd
T&G leaks not just up through the boards but also along the T&G grooves. Where the T&G goes over your outside wall, all those small gaps between the boards add up to a pretty big air leak. Common to see spider colonies between the boards in that location as there is so much airflow.
With a retrofit, the simplest way to seal it up is to drill a hole above the top plates (can be done from the inside if careful, easier on the outside) and inject flex foam in the gap. See picture of where you need to air seal.
If using peel and stick, no need to tape the seams of the rigid.
I would go with an acrylic based peel and stick such as VP100 or VaporDry SA , the common ice and water with modified bitumen can smell and sometimes it can interact with sap in the wood causing it to ooze.
Anyone know how to get accounts fixed? My account won't let me post new QA questions. Grrr.
I'm having the same problem
There was a big spike of spam recently. I believe the moderators disabled new posts until they figure out a workaround.
Hi Todd,
You are correct that the installation of the peal and stick membrane should be above or exterior side of the T&G paneling. If you have any lighting that is going to penetrate the membrane, you'll want that to also be sealed to the membrane, caulks, tapes, or small pieces of the membrane that cover the electrical penetration all work. Akos's solution for the wall to T&G connection is a great suggestion, T&G paneling creates lots of three-dimensional air pathways that are difficult to seal. Sealing from the roof side will stop air and vapor from entering the roof assembly, but you still have the potential for air to move into the conditioned space from the gaps in the T&G paneling where the paneling meets the exterior wall.
Randy
The replies above are all excellent, especially avoiding any bitumen-based membranes directly on the T&G. And in addition to sealing the air leaks where the T&G goes from inside to outside, you should take a careful look at all penetrations of the exterior shell for sealing: beams and purlins are always biggies. Pay attention to where the 12/12 ceilings intersect with the 3/12 roofs - the portion of 12/12 inside the other roofs can be tricky, especially at the overlaps. Also, if the interior space feels drafty in winter, that could mean there are significant air leaks down in the living space as well. Since you're doing this project, you should make it complete.
And just a quibble/clarification with your post #2 where you say the peel&stick will be air barrier, vapor barrier and WRB. The first two are correct, but this is NOT your WRB. The WRB is whatever underlayment you use under the shingles on top of the sheathing. That said, I've done roofs where the peel&stick acted as a secondary WRB when the primary one was damaged by high winds.
Good luck on the project.
Hi All,
These details and clarifications are so helpful! Thank you, thank you. I will inject foam from the top as recommended. No bitumen/asphalt-based peel&stick, copy.
Regarding the acrylic-based SA membranes (eg: VP100 or VaporDry SA), do I need to be concerned with vapor drive since the perm rating for these is higher. As I search the web, I have not found an acrylic-based peel&stick that is considered a vapor barrier.
On top of the peel&stick there will be 5" of polyiso >> 7/16" OSB >> synthetic felt >> asphalt shingles with no air or vapor vent. Have I considered everything to limit vapor/moisture entrapment?
First, the membrane does not have to be self-adhered. The self-adhered is just easier in some applications, also more expensive. Second, you do not want vapor permeability here in most weather. You want a class II (semi-permeable) membrane. Best is one of the variable-permeability membranes like Siga Majrex, Membrain, or similar. These have very low permeability when they are dry. This keeps water vapor from the house out of your insulation sandwich and that's a good thing. If you do get leakage or condensation in the sandwich and the humidity in/near the membrane goes up, the membrane pores open up so they become vapor permeable, allowing drying through the membrane. In a roof sandwich you won't get much drying until spring, but most materials don't risk moisture damage when they are cold and wet, so spring drying is generally OK. Of course, it's best if they never get wet in the first place. With a smart membrane and staggered seams in the rigid insulation, you should be good to go.
Thanks again, Peter! It's been a lot to ingest without the application and quantitative experience that you and this community readily share. I definitely feel better informed. Now to make sure the subs install it to plan.
All the best,
Todd
Since the foam layer itself is a vapor barrier, I don't see the benefit of another vapor barrier layer bellow. The peel and stick can be either permeable or non-permeable, both will work as well. There are some acrylic based vapor barriers out there but might be hard to get (ie SOPRASEAL STICK AVB AT or IKO AcrylicStick SA).
Something like Membrain works well on a ceiling when applied from bellow but would be near impossible to walk on without tearing. You want something that will hold up to foot traffic and won't be slippery.
Regular roofing underlayment is hard to get air tight unless the installer is diligent with taping all seams, I would go for peel and stick for simplicity.
Make sure whatever you go for is either available at your local roofing supply or you source and deliver to the site before the install. You don't want the roofer chasing after non-standard bits.
Also pick an insulation thickness that works with standard roofing deck screws. You want to pick a thickness where a standard length screw does not poke through the T&G. Make sure the installer has the right size screws as well!