Unvented Cathedral Ceiling Cellulose Insulation

[scootermcrad]

Hey everyone!

I currently have a steep 12-12 pitch roof structure that is 2x10 construction. It is an unvented structure (for architectural reasons). We're ready to insulate and had actually planned from the beginning for this to be insulated with a spray foam system. Unfortunately Covid kind of through a wrench in that plan.

After extensive research on the best method/practice for our particular structure we found that 2" of HFO closed cell up against the decking and the rest filled out with batting to reach our total R value and then topped with a smart vapor membrane and then drywall was the best system. And it took quite awhile to find a reputable company to install that system. NOW that reputable company has completely gotten out of spray foam. I'm left to find someone (again) to apply this system and it's expected that the cost will be double the original quotes due to the times we're living in IF I can even find someone to put down HFO closed cell.

This brings me to the question or maybe just a concern. The reputable company I had chose said they have been doing damp application dense-packed spray-in cellulose for cathedral ceilings for many years. They essentially put a permeable membrane/net up and then they just spray it and build it up solid just like they would with spray foam and it gets in every nook and cranny. This is not like your big box store loose blow-in stuff. It's much more dense. They first use a single part spray foam to get any cracks and crevices to insure no leakage. The trick to making an unvented system work is to simply have no voids or chance of air making it's way in to the system. They tell me this will hold true with their cellulose application methods.

This company has been around a long time and has a great reputation for what they do. The price is SIGNIFICANTLY cheaper and I know this is a healthy product that won't off-gas or create other greenhouse type issues. My concern is the packed cellulose eventually breaking down and settling. I know it will have a slightly lower R value, but I'm not terribly worried about that and it meets building code. The price is extremely attractive (probably less than HALF or maybe even a THIRD of what spray foam costs will be at this point) and they are available to start when I'm ready.

Has anyone had experience with this cellulose method in a sealed cathedral ceiling? I'm always questioning what a "salesman" tells me and just want to do my research. There's not a whole lot out there on this method. So any experience from the pros would be appreciated. THANK YOU!

 

[jar944]

From buildingscience.com

"Now, for the bad stuff. As fantastic as the dense pack approach is for walls, it is a pretty dumb approach for unvented cathedral ceilings and flat roofs. And, no matter how hard some of us work to try to stop people from going ahead and doing it, they persist on doing it. First, it is a building code violation. Second, it violates the physics."

BSI-043: Don't Be Dense—Cellulose and Dense-Pack Insulation | buildingscience.com

www.buildingscience.com

And the corresponding discussion of that article on GBA

Joe Lstiburek says, Don't dense-pack unvented cathedral ceilings - GreenBuildingAdvisor

It's not really news, but it's worth reading: Joe Lstiburek says that unvented cathedral ceilings shouldn't be filled with dense-packed cellulose insulation. Read about it here: http://www.buildingscience.com/documents/insights/bsi-043-dont-be-dense/

www.greenbuildingadvisor.com

 

[scootermcrad]

@jar944 Thank you for that!! I appreciate the links. That seems to kind of confirm my suspicions and gut feeling. Dammit. Back to plan A. Wish me luck finding someone around here that will do the work for me. Probably going to cost me a fortune. This is why my gut told me to ask, though.

It's kind of further proof that the statement, "We've been doing it this way for years without any issues" can also simply mean that they just have discovered the long term side effects of installing a system like this.

If anyone in the Charlotte/Concord, NC area can recommend someone that will install a couple inches of HFO closed cell (instead of HFC), let me know.

 

[Voi]

Yikes, I hope you get more responses.

I was involved in a project a few years ago. Unvented 10/12 pitch. I-joist rafters, plastic (non-smart) moisture barrier. Tongue and groove directly over plastic and onto I-joists. So no drywall acting as air barrier. [EDIT] Blown in fiberglass was the insulation.

Part of sheathing had to be replaced due to massive rot this past spring. The damaged I-joists had plywood sistered onto both sides for repair. House is only a few years old.

Blame was put on large hail breaking the shingles cap. This is certainly possible but it's also possible moisture from the inside caused some of the rot, thereby making the hail damage more likely if the shingles were unsupported.

I would do this right.

 

[scootermcrad]

Yikes, I hope you get more responses.

I was involved in a project a few years ago. Unvented 10/12 pitch. I-joist rafters, plastic (non-smart) moisture barrier. Tongue and groove directly over plastic and onto I-joists. So no drywall acting as air barrier. [EDIT] Blown in fiberglass was the insulation.

Part of sheathing had to be replaced due to massive rot this past spring. The damaged I-joists had plywood sistered onto both sides for repair. House is only a few years old.

Blame was put on large hail breaking the shingles cap. This is certainly possible but it's also possible moisture from the inside caused some of the rot, thereby making the hail damage more likely if the shingles were unsupported.

I would do this right.

Yeah! That exact situation is was I don't want to run into! That's terrifying!!! I just can't risk it.

I'm throwing darts at the phone book, now. I have a guy coming out at in a couple hours to look at the project and talk to me. He's the owner of this said company. I was pretty specific in my wants/needs and he seemed to understand what I was after. Fingers crossed this guy "gets it". Fingers crossed he blows HFO type. I've really had a hard time finding people that do HFO closed cell. Most people install HFC. This company boasts being "green", so we'll see.

 

[FMB4]

It is an unvented structure (for architectural reasons).

It must have vents at the end of the gables. Period. That said, I'd consider the installation of 1 or more attic fans that are controlled by temp, humidity, and manually.

 

[jar944]

It must have vents at the end of the gables. Period. That said, I'd consider the installation of 1 or more attic fans that are controlled by temp, humidity, and manually.

Vents in the gables of a hot deck cathedral ceiling would be holes in the wall of the room.

What you described only works with a attic, and doesn't actually "work" there either.

 

[FMB4]

What you described only works with a attic, and doesn't actually "work" there either.

Vents in the gables of a hot deck cathedral ceiling would be holes in the wall of the room.

What you described only works with a attic, and doesn't actually "work" there either.

I didn't realize that it was a 'hot deck'. And I find difficult to believe that anyone would build such thing (and meet code). As for your 2nd statement; you do not know what you're talking about.

 

[jar944]

I didn't realize that it was a 'hot deck'. And I find difficult to believe that anyone would build such thing (and meet code). As for your 2nd statement; you do not know what you're talking about.

Saying gable vents in the attic are needed to meet code, when there is no attic would indicate someone else doesn't know what they are talking about, or isn't reading.

Saying you didn't realize it was a hot deck, when by definition a hot deck roof is unvented also makes me wonder.

If you don't believe me about gable venting, look up the current building science take on it.

I'll even provide a link

Lstiburek’s Rules for Venting Roofs - GreenBuildingAdvisor

Building Science Fundamentals: Roof, Part 1: Ventilation By Dr. Joseph Lstiburek Dr. Joseph Lstiburek talks about the not-so-controversial ways to maximize the efficiency and airflow of your roof and attic. Video Transcript: There’s been so much stuff said about roofs that you sometimes lose...

www.greenbuildingadvisor.com

 

[billconner]

A lot of the Lstiburek quotes seem to be for heating climates, with moisture from inside getting into building cavities. There are climates where it's warmer and more moisture in the air outside. One solution does not fit all.

Whether or not you can achieve and maintain as close to perfect as possible barrier on the inside is key. The pressure of the warm air in the house (there I go having always lived in heating climates) is significant.

I have the attic vent concerns mentioned - it works against air sealing. In addition, the biggest advocates are the people who make, sell, and install the soffit and ridge vent products. They also pushed for it's inclusion in the building codes. Always makes me pause.

 

[scootermcrad]

The last thing I'm going to do is debate rather or not building a sealed/unvented/"hot roof" structure. People build unvented roof structures like we have built all the time for "conditioned spaces". As long as the structure is insulated properly and the air and vapor is controlled properly for the application and climate, these types of structures have no problems. And saying they are not allowed by code is absolutely untrue.

Most people are familiar with vented type roof structures and they are more common. If someone would like to start a thread on that debate, that is fine. That is not the topic of discussion here. This is a sealed roof structure design and it was built that way and it's not getting changed.

I think we have the answer on Cellulose at this point. Spray foam is the direction for this project. The company that just came out doesn't like the 2" of closed cell up against the deck. They use open cell, which has its arguments, as well. So I'll have to do some more research on that system. They have been doing it for 18 years that way in the Carolinas with no issues. So back to researching the "Plan A" I mentioned in the first post (for those following along) versus the open cell. That's the third company I've had out here that says they successfully do unvented cathedral structures with open cell. Interesting.

 

[jar944]

The last thing I'm going to do is debate rather or not building a sealed/unvented/"hot roof" structure. People build unvented roof structures like we have built all the time for "conditioned spaces". As long as the structure is insulated properly and the air and vapor is controlled properly for the application and climate, these types of structures have no problems. And saying they are not allowed by code is absolutely untrue.

Most people are familiar with vented type roof structures and they are more common. If someone would like to start a thread on that debate, that is fine. That is not the topic of discussion here. This is a sealed roof structure design and it was built that way and it's not getting changed.

I think we have the answer on Cellulose at this point. Spray foam is the direction for this project. The company that just came out doesn't like the 2" of closed cell up against the deck. They use open cell, which has its arguments, as well. So I'll have to do some more research on that system. They have been doing it for 18 years that way in the Carolinas with no issues. So back to researching the "Plan A" I mentioned in the first post (for those following along) versus the open cell. That's the third company I've had out here that says they successfully do unvented cathedral structures with open cell. Interesting.

If I recall correctly there was some discussion by Martin holiday about a hot deck with cellulose provided you had a certain amount of foam (sprayed or cut and cobbled) against the sheathing.

From what I recall basically you needed enough r value in foam to ensure there was no condensation effect possible on either side of the foam. I haven't looked for the discussion/ article but it may be a alternative option.

 

[scootermcrad]

If I recall correctly there was some discussion by Martin holiday about a hot deck with cellulose provided you had a certain amount of foam (sprayed or cut and cobbled) against the sheathing.

From what I recall basically you needed enough r value in foam to ensure there was no condensation effect possible on either side of the foam. I haven't looked for the discussion/ article but it may be a alternative option.

Yeah, I feel like I ran across that somewhere also. Can't seem to find it now.

I'm going to get at least one more company out here to see if I can get someone to quote me what is supposed to be the right way to do this, if the science is understood for a zone 4 installation. Seems like this method also yields the highest R value for a 2x10 rafter as well as protect the decking.