How important are air barriers in a vented cathedral ceiling?
In August we had R60 of closed and open cell Demilec foam sprayed in our 1700 sq foot roof that unfortunately had a bad off gassing problem. After chemical (air and foam) testing and a long negotiation they agreed to remove the toxic foam. Removal was a long messy process but its finally done. Now we are looking to insulate with a non foam product which means converting a hot roof design to a vented assembly. We have 14″ TJIs to work with and the roof deck covered with vapor closed underlayment and vented metal roofing so at this time nothing can be added above the roof deck. Im inspired by Alex Wilson’s design https://foursevenfive.com/cant-vent-at-valleys-and-dormers-use-a-smart-vapor-retarder-not-foam/ Dense packed cellulose or Spider looks tempting but it sounds like it needs strong baffles and strapping installed to hold it up. The insulation contractors that looked at the job just wanted to put up cardboard or foam vent baffles and rely on sheetrock for air sealing. I’m tempted by Roxul as it would not press badly on the vent baffles or need strapping for the sheetrock and I can install it myself and save a bundle of money. After extensive reading here I’m leaning towards installing a vapor variable membrane (certainteed Membrain or Pro Clima DB+) above the sheetrock for air sealing but I am wondering how important it is to also air seal the 1 1/8″ vent channels. I was recommended to use a high perm rating roof underlayment and air seal it to the TJI flanges with caulk but that’s a lot of caulk! $750 for materials plus time. I am attaching a drawing of what I am thinking of installing. We are in central Virginia. Any advice is appreciated before we start purchasing materials!
Thanks
Ryan
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It looks like the PDF drawing does not preview so here is a jpeg of the system I am thinking of installing
Ryan,
I will respond to your post to give it a bump. Have you read Martin's article on insulating cathedral ceiling and building baffles? I will post links below just in case.
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
https://www.greenbuildingadvisor.com/articles/dept/musings/site-built-ventilation-baffles-roofs
Also... I can imagine why you don't want to take another chance with foam. (What a nightmare!) But your problem was likely caused by improper installation. A good installer -- and one of the HFO closed cell products -- might be worth considering.
It's best to have both sides of a partition air sealed - you don't want air coming from one side, passing through air permeable insulation, depositing moisture and then returning out the side it entered. Maybe someone has data about how important this is.
Ryan,
I think your approach will work. Mento 1000 is a good choice, unless you want to dense-pack against it, in which case you should use Mento Plus. Tapes are best (PC Vana, in this case) but caulking/sealant is significantly less expensive; PC Contega HF would work well for this. If you want to save time and money, 1/4" plywood with PL Premium adhesive would also be fine, and would hold up against dense-packed cellulose, especially if you provide spacer blocks down the center of the vent bay.
Although I avoid using foam, and understand why you would be leery of trying it again (what a nightmare!), I agree with Steve--one of the HFO-blown products would also be appropriate in this situation, and would give you the highest R-values. But in your climate zone, as long as your assembly is airtight, going down to R-45+ (cellulose) or R-50 (mineral wool) won't make much of a difference on your energy bills. If you wanted to boost performance, you could add an inch or two of rigid insulation (mineral wool or EPS foam) below the rafters. 1 1/2" insulation fits well between 2x horizontal strapping.
I guess what has me wondering about air sealing the baffle side is that in large open traditional attics with blown in or batt insulation there is no air sealing on the top side of the insulation but it seems to be recommended when installed in the small constrained space of a cathedral ceiling with only 1-2" of air channel. Is the wind washing that much of a problem? Michael I looked into thin plywood but its cost with glue is actually more than the Mento 1000 and from what I gather the moisture permeability is less. I have my hands on titebond weathermaster caulk which is about a 1/3 of the cost of the Contega HF caulk and stays super stretchy. Im leaning to using the Roxul batts (almost 1/2 cost of dense pack) which have a higher R value and mean that I can leave off the strapping on the underside which is a significant time and $ savings.
Thanks for all the input. After learning more about the pitfalls of foam we are certainly not going there again. I have since met homeowners with bad foam jobs done by all the major installers in this area such that I don't care to gamble with site applied spray foam again. I was getting nice headaches and irritated nose and eyes with ALL the windows in the house open for many weeks after the application. The list of chemicals found coming off the foam was pretty long and sobering even thought to the eye the actual foam looked perfectly good. I was the HFO closed cell foam that had the worst off gassing. Hot days were the worst but the smell when they removed in Dec-Feb was pretty intense as well.
Ryan, that all makes sense. Yours is the first bad installation I've heard of with the HFO foam--it's supposed to be more forgiving to install. I wonder what went wrong.
It's hard to quantify the effect of wind washing, because it depends on many variables. Typical advice is to say it reduces the efficacy of the top 1-2" of dense insulations such as mineral wool. It matters most where air velocity is highest; in an open attic that's just around the eaves, and it's easy to blow a couple of extra inches. In a compact roof assembly like yours it has a higher relative effect. So while air-sealing the edges of the baffle is best practice, if it's not your primary airtight layer, it's not a disaster if you don't get a perfect seal.
Interesting about cost of Mento vs. plywood, but it makes sense. I have no experience with Titebond Weathermaster but if it works for you, that's great. Contega is not inexpensive (though it is zero VOC, which matters in some cases.)
Thanks Michael, For the primary airtight layer is a good well sealed sheetrock job adequate for an air barrier or do I need a membrane like Certainteed Membrain or Pro-clima DB? The cost of the Pro-clima is giving me pause but it looks like better stuff. I don't want to cut corners if it causes moisture or performance issues! After the foam was removed we had to re-do over 1000' of damaged wiring so I took the chance to move as many fixtures to the walls and off the ceilings so we have a total of 8 penetrations in the entire roof. I have airsealed them all and would naturally caulk the fixtures to the sheetrock.
Ryan, drywall can make a good airtight layer, but each of your penetrations is a weak spot. Airtight fixtures are never airtight, but you can cover them with a purchased or site-built airtight cover.
If you want added insurance, the least expensive variable-permeance membrane is Certainteed Membrain. It's also the flimsiest to work with. I have not used Pro Clima DB+ in a large-scale application but it would work fine. Intello's numbers are better (lower dry cup, higher wet cup permeance.) Siga Majrex is another good choice. Whether you want a pair or two of suspenders with your belt is up to you.
Michael, All but one of the penetrations are 4" round J boxes that I have carefully caulked all of the holes and the wires. Rewiring was a neat test on the titebond caulk...it was quite hard to remove the wire from the J boxes! One was done with silicone and it was a bit too easy to remove so its tight bond for all the new ones. The big penetration is a panasonic bathroom fan which worries me the most. Its in a small dropped ceiling so I can run a membrane above it and air seal the 4" pvc vent pipe. I pulled the trigger and ordered the Mento 1000 today and roxul. If this was an unvented roof I would not bat an eye at the more expensive membranes but I'm thinking of going with the Certainteed and a careful sheetrock job.
Thanks for all of the advice
Ryan