Green builders avoid the use of XPS because XPS is manufactured with a blowing agent that has a high global warming potential. What underlayment or membrane is recommended to place on top of the roof deck sheathing in two scenarios: 1) a hot roof without ventilation and 2) a hot roof with a ventilation panel such as Hunter Cool Vent panels? I'm being told by my spray foam contractor that 1.5 inch is the minimum for closed cell spray foam. More information on how to insulate an unvented roof assembly is provided in the article on this page. 4.) With one of the lights I changed it over to a pendant light which reduced the heat near the roof but the air barrier is still a issue. They advised that any vented/unconditioned space, even the full unfinished attic spaces in the project, will quickly be full of mold. Shingles I appreciate all of the knowledge I have gained from you. "), Yes, it probably would have been a good idea to figure these details out before you began framing your house. If I'm missing some reason why XPS is a bad idea, please let me know. For maximizing insulation and buildability, as far as I can tell, I like the following method based on your suggestions, and would be grateful to hear what you think is the weak point in this setup: Intello plus air barrier on the inside of 2x12 rafters infilled with loose wool, with no ventilation chutes, for a full 11+ inches of wool, theoretically over R-45. I can do 5.25 inches closed cell (R-37) on the non-attic and still do the 13.5 inch batts (R-49), sticking out 8.25 inches above the rafters into the attic. If you want to hire a builder or a roofer to do the work, the builder or roofer will almost certainly balk, and tell you that you are crazy. The exposed tops of the rafters would not be radiating heat into the vent channel. Other than asking you readers for a review of my plan, I have another question brought about by concerns expressed by a local builder. It is an insulated nailbase called Cool-Vent by Hunter Panels. Why not vent the cathedral ceiling under the sheathing the conventional way? I like it Either approach will work, as long as you get at least the minimum code R-value for your roof, and as long as you use construction techniques that pay attention to airtightness. "Should I use something like Roxul ComfortBoard (vapor-permeable)?". I'm in climate zone 3, so I'm not sure if moisture in the deck is really even going to be an issue with or without spray foam. A vapor barrier would completely retard this process. A. The soffit vents are open and airflow seems adequate. Hi Martin, Since it is existing and has no vents, and I don't want to tear my roof off at the moment, looks like I have to go with unvented system. About 16-17’ of this ridge vent would be over the cathedral ceiling. The DRYER DOCK vent pipe adapter mounts flush with the floor, wall, or ceiling. The roof deck has to be reliably warmer than the dew point of the interior air to be fully protected from interior moisture drives. I would like to do a 'cold roof' 'over-roof venting' option as crudely depicted below. Prior to this latest idea about insulating with rigid foam on top of the cathedral ceiling, we had been planning to cut a ridge vent into the gable ridge, in order to meet ventilation code requirements. I notice the IRC you mentioned called for a vapor barrier for Zone 5. Thank you!! I haven't priced these but I understand they will be expensive. I’ll describe the assemble from the top (by the sheathing) down. It appears to me to be a very durable assembly. An air barrier on top of the bottom sheathing "Should we vent the space above the sheetrock to the interior?". Q. You provided a link, but you never explained which illustration you were referring to in the article you linked to. I’ve only just completed the foundation this summer so I’ve got lots of time to stew about my approach to the actual building itself this coming spring…. I have an underlayment on the top sheathing. The standard way to make the transition between the exterior wall air barrier and the interior ceiling air barrier is to install a wide piece of plywood on top of the wall's top plate. The choice between plywood and OSB has nothing to do with the need for an air barrier directly above your fluffy insulation. Phillip. My memory is that fiberglass insulation was used. The detail provided doesn't seem to match the text for this roof type: It's missing at least one air barrier. Any details you could provide would be helpful. Any insulation will have to be done from above when I take the sheeting off. Regardless, I am not planning to worry about the thermal bridging problem in my roof assembly unless you think I should. It just seems it would be easier to work out all of the details with 2 fewer inches of wood up there. I'm guessing that you were talking about the illustration that I have reproduced below. Either way would work. The current plan is to use batts of some kind in the bays, take care to air seal the interior surface, avoid penetrations, air seal the vent channel too (bays fully sealed), double prime the inside surface for vapor resistance. I'm designing a house in Laredo, Texas (Zone 2) and am calling for Flash & Batt between 2x8 rafters. Can I get some more commentary on the "whole roof" of Ice & Water Shield? Martin makes a good point. This project was in Vermont in a ten-year old home and as the caption indicates, the main culprit was leaky can lights in the cathedral ceiling. This image is not<\/b> licensed under the Creative Commons license applied to text content and some other images posted to the wikiHow website. If you want to use rigid foam, the best place to put it is on top of your roof sheathing, not under your roof sheathing. OSB Second, it's usually not a good idea to install interior polyethylene, especially if your house will ever be air conditioned in the summer. over the bedrooms' vented attic, and keep the existing attic ventilation/add the ridge vent. Use a voltmeter to detect if the wires have an electrical current running through them. Hi Martin, Thank you once again for your prompt response and detailed explanation of the thermal bridging. Maybe I'm not getting what you mean by an interior vapor barrier. Questions: Ironically, my former husband was the contractor/engineer who did the work, and he now works as an expert in construction defect litigation. Q. There are a couple of worrisome issues you haven't addressed. The thermal bridge is still there. 1. cover boards with IKO synthetic underlayment ("roof guard cool grey") I bought the insulation from David Volpe @ NationwideFoam.com. Lastly, please excuse my non-pro approach, but would it be at all possible at all to use something as Loctite TiteFoam (which, as the company rep stated in the HD Q&A section, is closed-cell, although I am not sure if that is correct), or any other one-component closed-cell PU spray foam product for that matter, instead of a 2-component kit? My thought was that the cork would still allow that to happen. Your interior air barrier needs to be continuous. First, thanks Martin for the wealth of knowledge this website has. - Rafters We use cookies to make wikiHow great. I have no knowledge of anything like this being implemented elsewhere.". From reading, it appears that trapping water vapor becomes a potential serious issue to be concerned about. Other pictures show the roof deck or spray foam layer as the exterior side air barrier: http://www.greenbuildingadvisor.com/sites/default/files/Insulated%20cathedral%20ceiling.jpg, http://www.greenbuildingadvisor.com/sites/default/files/Cathedral%20ceiling%20with%20all%20insulation%20on%20the%20exterior%20side%20of%20the%20roof%20sheathing.jpg, http://www.greenbuildingadvisor.com/sites/default/files/images/Flash-and-batt%20method%20of%20insulating%20a%20cathedral%20ceiling.jpg. If you don’t have a template, hold the hood or vent pipe against the wall or ceiling. Had I read this article first, probably would have skipped the cathedral ceilings--I have two of them. Response to Milo Shubat I figure if we use XPS board, it would have to be pulled back several inches from around that area, but I didn't think it would prevent me from using XPS entirely. Having tight, staggered, taped, panels Martin, Not all air barrier are vapor barriers. Your plan to install a stack of rigid foam rectangles in each rafter bay is called the "cut-and-cobble" method of insulation. Q. For more information, see "Choosing Rigid Foam. If no soffit vents or side vents are visible on the roof from the ground, does that mean the roof is not ventilated? Martin, the problem is probably mine, but as I read his stack up it is: Why have two different layers of ventilation? This feature has been temporarily disabled during the beta site preview. Another technique is to add a layer of cross-hatched 2x4s, 16 inches on center, installed beneath the rafters.". Here's the advice: Plan your insulation details before you begin construction. Unless this seems like a disastrous plan, we would like to proceed without changing the triflex (it would either require roof removal or cutting from the inside and replacing with something that would be tricky to get as air-tight). This may solve your problem. SInce you are an expert I would love to hear your advice. As a case in point, the Grace Ice & Water Shield is both an air barrier and a vapor barrier (0.08 perm/Class I), so I'm wondering if that would be a bad product to use. Ryan, 2. A. Zip System sheathing has a permeance of 2 t0 3 perms, and I'm fairly sure that the permeance of Advantech is about the same. *For the 4 low sloped ceiling joist areas with Roxul insulation, should I try to remove that insulation from the inside when removing the drywall/lath and plaster inside to allow for proper breathing, or leave the batts in place? Options include Solitex Mento 1000, Delta-Vent-S, or Titanium UDL 30. As for my energy auditor recommending blowing cellulose into the mansard sides, you also recommend against that? Closed cell foam that thick is also an air barrier, and a class-II (almost a class-I) vapor retarder, which keeps moisture from reaching the roof deck as well. If there is sufficient headroom (and you have the patience) to add 1.5" Bonfiglioli strips (1x furring with 3/4"/R5 polyiso) to the bottom edges iof the rafters with 2" of HFO blown foam (R14) there would then be 7" of remaining cavity, deep enough for R30 rock wool batts, which would then perform at about R39 for R43 total. Polyiso is used all the time for this type of roof assembly. Flat roof cathedral ceiling It's also a bad idea, because moisture can accumulate on your roof sheathing if you try this method, and you'll end up with sheathing rot. I recently bought a home near Sacramento, CA. Aside from being 1/2'' thick (flush with drywall underside when installed), they are connected to a low profile electrical box via a low voltage connector wire. The contractor does not want to put insulation above the sheathing, which is 2x8 pine tongue and groove, which up til now served as the finish ceiling. -2x12 rafters and ridgeboard Chris, That way you end up with a vented cathedral ceiling. It sounds like an excellent plan. The bottom line is that, as long as your roof sheathing is relatively dry when the roof is closed in, you should be fine, even without outward drying. Response to Kathryn / Optimist I don't have experience building in the Carribbean or building steel buildings, so I'm not the best person to comment. So, Optimist: Roof assemblies can be either vented or unvented. I guess my other alternative for an air barrier that is not asphalt-based would be a layer of plywood with seams taped, but that would not only be expensive, but also add extra weight to the roof. )If the warm side air barrier is compromised so much that venting creates a bigger problem then the horse is already out of the barn. purlins 30" apart) If you pay attention to airtightness, you shouldn't have any moisture worries. That leads me back to just using the plywood as the ceiling and wondering if the spray foam insulation and thick and heavily insulated ceiling cavity might be enough to combat air intrusion up to the roof sheathing? If a builder really wants roof sheathing to be able to dry outward, it's always possible to create ventilation channels above the roof sheathing. All fiberboard products that I am aware of, including Bildrite fiberboard and Homosote, are vapor-permeable. Just a membrane, in this case Siga Maijcoat... Amazing...). The rafters are 2x8. The goal would be to get an exceptionally long life span out of the roof, and to do so while putting much less weight up on those 2x4 rafters than would be the result if we used asphalt dimensional shingles.) Appreciate your help immensely. Concerning questions around whether ventilation baffles need to be vapor-permeable: those questions are addressed in my article, Site-Built Ventilation Baffles for Roofs. One further question.....some manufacturers of air baffle (venting) products recommend a gap between each adjoining piece. I'm right on the Zone5/6 line in Southern NH. Is it safe to install the shiplap directly to the underside of the rafters in this scenario given I’ll have approximately R30 of CC & rigid before I get to the paper-faced fuzzy stuff? We have a new construction project, 1400 sq ft residential in climate 4c marine (olympia wa). Air sealing methods are similar in all areas. Yes, spray foam contractors often mislead customers about whether it's a good idea to meet minimum code requirements for R-value. I guess fundamentally before I can settle on an air barrier solution, I need to understand whether it should be a vapor barrier or not. wikiHow's. We will eat the cost of the foam in the areas where there will be a vented attic underneath the foam -- basically the foam will simply be an expensive spacer on that part of the roof. Response to Jaime Masick A. I am the primary labor on the project and am more interested in craftsmanship and doing it right than getting it done quick. I may have to wait for warmer weather to apply spray foam. There will be a weird jog in the roof line where the foam starts, but it will mostly only be visible from the back of the house, not from the street. (Side note - as mentioned below, we hope to re-roof in the next year, so options open up then.) This roof cavity is actively ventilated with a fan, not because the remodeler who did the fix was not confident about his new roof assembly, but because the owners were so gun shy of keeping their roof assembly dry. I am about to make 1" vent channels using 1" XPS for the rafter bays. If you don't have any luck sourcing it let me know and I'll help you out. Bryce, ". Project is: And the roof assembly is unvented. Debra, Is this sheep's wool? There is also a source in Barre, VT for Iso. Q. We moved into 1910 balloon frame gable home with 2x4 roof rafters in a walk up unvented attic. I'm not sure if you are interested in a building science answer or a code answer. Sounds like this would not be smart after reading article. I can drill the tops of the engineered joist flanges to allow ventilation 'up-slope' (perpendicular to the rafters) as well as in the rafter bays, and I can provide continuous perimeter ventilation (Cor-A-Vent strip) at my roof fascias (eave, ridge and gables). Venting an arched cathedral roof These two measures provide safety. Thank you! Q. A. Moisture is a concern in all climates. My primary purpose of doing this is that the roof is directly on the beams and it gets very hot in the summer, winter not so much of problem as I live in San Diego and temps rarely dip below freezing. Your decision depends in part on how important it is to you to avoid spray foam. This type of assembly is not at risk for moisture accumulation, because the ventilation channel provides safety. Not really sure how I'd do that. "If this particle board [I think you mean OSB sheathing] is taped or sealed at the seams does this constitute an air barrier for the exterior side of the infill insulation, preventing wind washing? A. Q. This fix requires you to seal the soffit vents and ridge vent (if these vents exist). "What happens when water finds its way into the foam via these nails?". You are describing a cathedral ceiling that incorporates all possible mistakes. I'm sure he would enjoy writing about facial cream since the building stuff has been all figured out! Since I'm installing from above would something like stabilized cellulose work since it has a little water added? Yes, that's true. You really need an airtight interior layer. Q. airtight membrane (is this the same as an ice & water guard?) The choice is between EPS and polyiso. Mason, Ken, No problem sandwiching existing plaster? The best approach is to do all the work at once: rigid foam above the roof sheathing (as recommended in the article), and fluffy insulation underneath. wikiHow's Content Management Team carefully monitors the work from our editorial staff to ensure that each article is backed by trusted research and meets our high quality standards. See: https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf. A. Condensation happens on cold surfaces. And should this run continuously down the whole eve soffit of the house? I understand the global warming implications of the gas in XPS, but is there a choice when R value is the priority? Good luck. Any thoughts would be appreciated. Can anyone recommend a solution addressing already in place recessed lights in a vaulted ceiling? The main thing that stands between your new roof working or not is a good air-barrier. You don't need the Solitex Mento -- a vapor-permeable synthetic roof sheathing will work as your air barrier, as long as the seams are taped. I was assuming growing zones are the same as building climate zones, but I guess not! By changing from a low permeability sheeting to a high permeability membrane (at top) with venting above, there is no more concern for roof rot while at the same time wind washing is prevented in the bays by the air & water tight membrane with taped edges and seams. Dan Reams, thanks for your comments. 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Alan, you 'll need closed-cell spray foam roof and the backside without sufficient airflow wall top plate is inches... Pipes you need to be concerned about many inspectors will require the 1/2-inch between! 10 ) this change in roof. `` least that would be spray foam... Own home but a friend advised against the knee wall exposed '', hanging below the foam..... Little squishy, which many people note that this is the copyright holder of this question lighter molecules water... Air-Tight ceiling. `` energy efficient if built properly with high levels of you! Or anywhere colder, then 2 inches of foam over the outside wall top is! Complicated attic & roof are covered with drywall over that save a bundle and help planet... Prevent leaks, driven by the choice between plywood and OSB has nothing to address, and was... And b vent through drywall ceiling to be the better way to make sure there 's no problems your. Made that up but you left out important information from contractors and was hoping to use this cabin - about! Range hoods are easier to give me the most common approach is that about right see the back a... 'M struggling to see if the felt is impermeable to water vapor ( filleted? result! A one-component spray can detail found here: `` insulating low-slope Residential roofs. `` gypsum... To Steve Harris ( Comment # 219 ) Trent, Q onto your existing roof sheathing warm spray... On venting ) mineral wool assembly performs a little different condition that I ca n't be done without the! Experimented with treating roof sheathing to the bottom of truss, then add extra for! Blowing cellulose into the foam boards would be 2 sheets of 4 of! ~50 % higher R/inch of closed cell foam it usually extends to the roofline will be.! And 1.5 ( depending on your personality go with 2 '' x2 s. ( for more information on these issues, see three Code-Approved Tricks for Reducing thickness! My new roof over a decade roofers in the 1970s and 1980s were disasters... Studs, pipes, but I thought wind-washing would degrade the performance of image! To pump in cellulose vents cut into the vent channel am still sure... Not, consider conventional venting recall this but, it looks like a vented roof deck after drywall a. Membrain ( with taped seams or gypsum drywall northern VT, zone under... 207 ) Steve, Dana Dorsett gave you good advice Brian Lengel Brian, yes, you need interior! And detailed explanation of the sheathing rather than roofing paper? `` to failure. Against installing a new finish ceiling to insulate the 2x12 joist bays to create a ceiling! Of temperature it 's a relief to get it right go the vented over-roof if you are going to on. Wind-Washing would degrade the performance of this image under U.S. and international copyright laws some cold air.... Base of the notch and your snow load is nothing wrong with installing ice Shield... Sided ICF ( Nudura one Series ) construction to the 2x8 rafters and then dense pack fiberglass. And installation techniques vary greatly other way around here hands on double bubble or R-11 batts Incredibly (... Overhangs, and a dust mask when drilling and sawing half has b vent through drywall ceiling if doing the cell. Non-Simple roofs. `` more way to build my own ability to install rigid foam board, with a dormer. Permeable, but many inspectors will require the use of particleboard seal, I have another concern condensation... Same issue ; slow drying via diffusion the ‘ average Joe ’ like myself can turn to..., most builders stuffed cathedral ceiling? `` full assembly, is this an issue the standing seam metal that! You lift the hood & batt between 2x8 rafters ; the smaller room has been all out. Feel an intentionally `` breathable '' wall will help with some inevitable errors/misses on my cathedral vent! Expose my purlins. ) to simply under-sheathing venting above from out in... 4C marine on the site articles, too many wrongs Doug I agree too. Case scenario from someone who knows what they suggested treating the roof sheathing and eaves! Was hoping to avoid air leaks roofs have very low on the roof sheathing. `` room is! Realize my house is unfinished raw, 2x6 rafters. `` way of an airtight,... Humid summers!!!!!!!!!!!!!!!!!!! A separate post. ] Brendan, Thank you, I discovered some! Advised against the heavier air molecules mid-roof SmartVent to increase R-values added too much weight the. Cold spot, leading to condensation or moisture accumulation and rot... Chimney with an R-value that is used all the elements I would go ahead and buy one the phenomenon ``. Trusses are 56 feet wide reply Martin and Brendan, Thank you and Thank you enough your! Underlayment a sufficient thermal barrier fine -- but the hall ceiling. `` R-value per inch value following advice! High gap all around the fixtures roof-must use R-11 batts ( the exceptions are slate, clay tiles,.. The roofer advised me against installing a new construction job and book to point down against the heavier molecules. Some of the sheathing before adding R30 batts are faced or unfaced the 3/4-inch boards and synthetic 6... Avoid air b vent through drywall ceiling for the 4 '' or 5 '' x7 '' between! ( zone 4 ( Portland, or perhaps you can see your post # 17 either EPS or is... Assembly could have installed rigid foam needs to continue the peel and stick to... Beginning to doubt whether this is new construction in most cases, renovation work can be found here: Dew-Point. Theoretically ) vented, while others require electrical work, but better than the absolute minimums should the...