. Choose fiberglass insulation batting. All of the rafter bays that fail to terminate at your ridge will need to be detailed as unvented assemblies. There was more than 8,000 Cubic feet of insulation to install, so our insulators had 2 of their blower trucks and a good crew on site. I think that most contractors would worry about the buildability of your approach. The gable end ICF walls will run right up to the roof underside. https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs The roof is going to be done in perhaps 4 sections so may try rigid foam and spray foam to compare what works. So with that in mind, what are your suggestions for this 3 feet of space? Jason, Ideally, you would sheathe your roof with either fiberboard sheathing or ordinary boards (1x6s or 1x8s -- they could be rough-cut boards from a local mill, if your building inspector approves). Drywall is a good air barrier, so I advise you to install drywall under all of the fiberglass batts. That would make it the cool edge of zone 6, not zone 7: http://www.greenbuildingadvisor.com/sites/default/files/images/Table%20301.3%282%29_0.preview.jpg. Similarly, MemBrain is an air barrier but not a vapor barrier. Thank you for this detailed article and discussion. However, I think you are wrong about your asphalt felt; I doubt if the felt is impermeable to water vapor. Greg, Greg, Maybe not much at all? If you add fall protection equipment, it becomes even trickier. 1. "If there was no board insulation under the rafters, would mineral wool under closed-cell spray foam suffice as a thermal barrier and/or ignition barrier? For all intents and purposes, the T1-11 and the fiberglass batts are worthless. I think you'll be fine. Bringing ducts "inside" and duct condensation Using fiberboard for roof sheathing does not sound wise to me and I prefer plywood which according to Joe, can hold up to wetting/drying cycles. Response to Kathryn / Optimist Use blower-door-directed air sealing. I've seen traditionally vented roofs rot away after a few years with a leak. I've been reading a lot about vapor barriers/retarders and smart retarders and am leaning much more toward something like Intello Plus on the interior rather than poly. In my shed roof design, I will either be going with 2x10 or 2x12 rafters. Since the rigid insulation is being screwed in, I wonder how solar gets installed above that without going through the air barrier? A popular renovation activity is to convert an existing unfinished attic into a new living space. I get lost in the articles on here. I don't have any data from thermal performance tests of fiberglass batts that attempt to make a wind-washing comparison between (for example) R-11 and R-15 batts. Light on regarding vapor & air barriers Your ventilation baffles don't help address the thermal bridging. Response to Andrew Southam Climate Zone 2a. Seth, Since you need R-49 in your climate zone, you've got a fairly big problem here in the R-value department. 2. If understand your answer, you are suggesting a barrier attached to the bottom side of my roof sheathing, then the vent channel, then starting my batt insulation. asphalt shingles. An attic without ventilation can get up to 160 degrees or more during hot summer months. Mason, I'm not a fan of full peel-and-stick roofs, for a variety of reasons. OSB? Assuming the baffle is still necessary, any recommendations on fiber board? Hi Dana, thanks for the well-written response. (I'm Martin.). If your insulation consists entirely of rigid foam, there is no need for a vent channel. There really isn't any reason, however, to install an air barrier between your vented section and your unvented section. Q. I suspect because this is a shop and not a home, the inspector didn't go over everything as thoroughly as he might have otherwise. And just to verify, the 2" vented deck, while unlikely to be useful, shouldn't be harmful? It might be worth doing 2" of open cell foam on the interior side of which would be a more flexible and reliable long term air seal. thanks for bringing all this information together into one place. I do not believe cutting a short 1" strip off the top side of 2 x 8 beam would jeopardize its structural integrity, but I am ready to reinforce it with, say, a wide strip of 3/4" plywood as a sister beam as an extra precaution. By far the most common approach is to use gypsum drywall as the air barrier. You should really choose a vapor-permeable roofing underlayment. Would a flash / batt unvented assembly work under the single sheathing deck? Most types of roofing are vapor-impermeable, or close to it -- the exceptions include cedar shingles, slate, and concrete tiles -- so outward drying of roof sheathing is usually impossible. ", https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs, "Site-Built Ventilation Baffles for Roofs", "Radiant Barriers: A Solution in Search of a Problem. My goal is an R-80 roof in a zone 7 climate. Your layer of rigid foam is already a vapor retarder. Are Dew-Point Calculations Really Necessary? ", Q. don't add insulation) and live with the uncomfortable temperature? http://www.greenbuildingadvisor.com/cad/detail/unvented-roof-cold-climate-cathedral-ceiling-asphalt-roofing, http://buildingscience.com/documents/insights/bsi-081-zeroing-in, https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/57976/need-correct-moisturemold-problem-cathedral-roof#comment-150916, Combining Exterior Rigid Foam With Fluffy Insulation, Thermal Barriers and Ignition Barriers for Spray Foam, “Two Ways to Insulate Attic Kneewalls.”, Next Generation Spray Foams Trickle into the Market, Three Code-Approved Tricks for Reducing Insulation Thickness, "How to Install Rigid Foam On Top of Roof Sheathing. The overall house is *continuously* well ventilated with lots of louvered shuttered openings (No AC). Ideally, this type of assembly has a very good air barrier as close as possible to the interior. Other materials that can be used to create an air barrier are plywood with taped seams or gypsum drywall. They were very helpful. It seems the condensation shows up on mornings when there is no marine layer (no clouds in the morning) and the temperature heats up fairly early after a very cool night. "Is there any worry with using closed-cell [foam] as it attains to trapping moisture between the roofing underlayment and closed cell foam?". I am working to build a small 100 square foot loft cabin, much like the "Cottage Life Bunkie" by Wayne Lennox. Response to Eric Matsuzawa (Going from memory I had said R-30 earlier, but I checked and it was actually specified at R-38). In my mind, it's worth it. I plan on revising my double venting scheme to simply under-sheathing venting chutes. A couple follow-ups. Your plan to install 8 inches of rigid foam is a good one, but you may be able to install thinner foam if there is a compelling reason to do so. Thanks for reading if you made it this far :), Brandon, Sort of a hybrid to cut and cobble? Also, a small amount of material ended up on the wrong side of the baffle, which probably means folks would do well to follow Martin's advice on sealing them during installation. Your intention is to install a new finish ceiling, under the rafters, once the insulation is installed. I like the detail provided but I wouldn't call that a cathedral ceiling condition as it includes ceiling joists spanning at the bottom of the roof rafters. It's your choice -- I think it isn't ideal, but it usually works. I can feel roofing paper between gaps in boards. Green builders avoid the use of XPS because XPS is manufactured with a blowing agent that has a high global warming potential. If you pressurize the house with a window fan, you may be able to feel the leaks with your hand as the air finds its way into the attic. I thought the sheathing should be able to dry either inside or outside. For more information, see "How to Install Rigid Foam On Top of Roof Sheathing.". It just seems it would be easier to work out all of the details with 2 fewer inches of wood up there. We're working on upgrading the energy efficiency, including added/updating insulation, for our almost 100 year old 2-1/2 story balloon frame house (stucco over lathe exterior). 2. The project is Zone3 near Wilmington, North Carolina, where I believe R-30 is the required insulation value. Finally (I promise this is my last topic). "I'd be looking [for the walls] at Intello Plus on the interior, wool batts, exterior sheathing, housewrap, rockboard 80, furring strips, siding. If not, installers may be more willing to fill them knowing that. You don't want that. 2. Then filling cavity with blown fiberglass and adding 1-1/2" rigid taped insulation at bottom of truss, then drywall. I'm using radiant floor heat in slab, with a wood stove for quick heat-ups in fall and spring. Last detail, it is a newer construction (2007) but the rafters are still exposed, so I have access. Thanks for your helpful reply. Does this strategy sound as effective as vent channels constructed on the inside of the roof sheathing? Why have two different layers of ventilation? Good idea? Bildrite's site says their product is vapor permeable (good), but I don't know about the others. I bought several tractor trailer loads to insulate my house and barn for a very, very reasonable price. Advice Needed - Retrofit Solution for 1915 House Zone 5/6 If you decide to create an unvented roof assembly with spray foam, remember to use closed-cell spray foam, not open-cell spray foam. There are continuous soffit and ridge vents that align with the furring strips in the vent zone. However each of those sections is adjacent to a fully ventable section. For more information, see "Flash-and-Batt Insulation. The diagram didn't label it, but the vented-no-foam picture clearly shows a barrier between the ventilation clearance and fluff: http://www.greenbuildingadvisor.com/sites/default/files/images/Vented%20cathedral%20ceiling%20-%20FHB.jpg. If so, can I go with 6 mil plastic if I figure out out a way to install it so it isn't punctured by my finish nailer installing the shiplap? But because the net free vent area for most vents is listed in square inches, I find it easier to convert the calculated square foot vent area into square inches. Response to Chris Heard But the contractor is not familiar with this roofing technique, so we're having to figure it out together. A. - Install spray foam (either open-cell foam or closed-cell foam) against the underside of the roof sheathing, and no other type of insulation. If they were venting the dryer inside (code violation?) T&G pine as finished ceiling. Hello Mr. Holladay. If OK, should the batts be faced? Four inches of closed-cell spray foam -- the thickness you plan to use -- will work for Climate Zone 6 (or anywhere warmer), but you will need thicker spray foam if you are located in Zone 7 or 8. So, if you must use fiberglass insulation batts, and you dont have access to spray foam or closed cell boards, what do you do? I know XPS is unfortunately less green of a material than the other rigid foams, but that is a secondary concern at this point. (probably been heard a thousand times already). Q. The cellulose, of course, does not reach the top of the attic, so there is a substantial air gap between the cellulose and the roof sheathing. It's safe to say that an existing house doesn't have good roof ventilation if there aren't any soffit vents or ridge vents. - should impermeable underlayment be used over the sheathing rather than roofing paper? Your articles are awesome. My original decision was to go with a vented roof system and R38C batts. I assume using closed cell would create the dreaded sandwich? "For example, a house with a 28-foot by 40-foot ceiling has an area of 1,120 square feet. My mistake! urgently need advice re: cathedral ceiling insulation Using rigid foam to make your ventilation baffles does nothing to address thermal bridging through the 2x12 rafters. See attached sketch. The vent channels we describe can be done after the roof is on from the underside (dry side). Assuming I got that right, here is my advice. Red is the cathedral outside the attic; green is cathedral inside attic. A. Moisture is a concern in all climates. We are using 2x12 rafters which serve as the ceiling below. The craze for insulated cathedral ceilings (and great rooms) really took off in the 1970s and 1980s, when examples began popping up like mushrooms after a warm rain. Yes, adding 2 inches of XPS insulation, ventilation, and steel roofing will reduce ice dams. Existing is a fourteen year old roof with 2" of closed cell roofing foam with an acrylic elastomeric coating. I think that would work for the aesthetic of the cottage. Or should I use 2" of closed cell and 3-1/4" of mineral wool batts?". Q. A. Letting the fan exhaust into an open attic will cause moisture buildup on the underside of the roof. In case of vented roof, I have some degree of peace of mind knowing that if some small amount of water somehow seeps under the shingles and gets absorbed by the plywood, it can eventually evaporate from its other side before it starts rotting. Yes -- again, assuming you pay attention to airtightness. The roofer advised me against installing a hip ridge vent due to water penetration concerns. Framing now. We're in a very rural area so I don't really know if there are options for spraying closed cell foam or anything but if that is the only option at this point, I could look into it. I have added blocking to the floor joists and will make the polyiso continuous in the knee walls. Cathedral roof vent Response to Dana Dorsett Most updated central air conditioned homes have a new air conditioner split system, meaning the AC is broken up into parts: an outdoor unit, which houses the fan condenser and compressor, and an indoor unit, which holds the evaporator and fan. Ryan, Would it be useful to install a layer of peel and stick? For more information on these issues, see these two articles: Forget Vapor Diffusion — Stop the Air Leaks! Do I need ridge vent if entire roof has gaps? This BSC article is helpful Would there be a benefit to foam and tap all joints. While this is a possibility, it preserves the current R=20 insulation (R-11 under decking and R=9 above deck). Your caveat is appropriate and conservative. What’s the best way to build a vented cathedral ceiling? That would still leave me short of R38; I’d have to add 2 inches of board insulation to achieve the R38 target. Hopefully, with the combination of a thick ceiling cavity, spray foam insulation, cooler temperature of LED fixtures, and airtight fixtures we won't have a huge problem with the lights allowing too much air into the ceiling. Your plan to install a stack of rigid foam rectangles in each rafter bay is called the "cut-and-cobble" method of insulation. If you go ahead with this plan and end up drilling big holes in your roof sheathing, you had better be meticulous when you air seal around the perimeter of each ventilation baffle. Roof decks can't dry to the exterior through 0.1 per shingles anyway, particularly since in zone 5 it will spend a significant amount of time covered in snow/dew/rain with the moisture drive heading inward, not outward. I guess the ideal solution to your dilemma would be to add new interior framing that would allow for the installation of a continuous, uninterrupted ceiling of gypsum drywall. I have my fan vented out through the roof, but near the inside of the roof in my attic the vent leaks a bit and condensation gets on the wood and a bit down in the insulation around where the vent goes out. If you are installing faced fiberglass batts between the rafters, and there will be "no Sheetrock on the ceiling above the attic," what will you install for your air barrier on the underside of the fiberglass batts? It's fine to add foam board above the roof deck even with the closed cell foam-board (what type?) to increase fastening spots for top plywood (have horiz. Here is just one example: It is true that you cannot use a vented approach for a roof with a 2/12 slope. That practice would be totally unacceptable to builders, who don't want callbacks or headaches. I don't want to add the extra weight and I figure that if the cellulose is dense packed, it will reduce fire spread similar to how drywall would serve that concern. With R20 of closed cell foam (about 3") the vapor diffusion rate is quite low even when the roof deck is quite a bit colder than the dew point of the conditioned space, and it really wouldn't matter if it's ventilated above the roof deck or not. "I assume using closed cell would create the dreaded sandwich?". There is a chance that moisture will accumulate on the underside of the roof sheathing during the winter. And we are now at a place of deciding how to finish off the inside of the ceiling in a way that 1. is air-tight 2. allows inward drying (if outward it seems like is not a possibility) and 3. achieves our desired thickness.". 5. asphalt tiles, 1. Thanks for posting the detail. I have a couple questions about baffling the vent channel: 1. No I haven't tried TSP. Insulated cathedral ceilings are a relatively recent phenomenon. "I don't really know if there are options for spraying closed cell foam or anything but if that is the only option at this point, I could look into it.". Preisendorfer, - Reflectix vapor barrier (on bottom of rafters) Here's what my article states: "Do I need to install an air barrier under the insulation? After the drywall is installed and taped, you can add a layer of boards on the interior side of the drywall if you prefer the look of boards. Q. Yes, as long as your local building inspector accepts R-30 as adequate. We have helped thousands of homeowners create healthier more energy-efficient homes while protecting their property investment. I'm going to be using T&G so it will have to be the Grace Ice and Water Shield. "Heating isn't that big of a concern but moisture is.". I want to add insulation during an re-roof so I am only adding polyiso rigid foam above the roof deck. If you were thinking ahead, you could have installed rigid foam above your roof sheathing before you installed new shingles. Or where you assuming I was talking about loose, not dense blown-in? John. Rafters are 2x8 so there is just enough space to meet R49. Attic insulation is available in a variety of materials including fiberglass, mineral rock/wool, cellulose, spray foam, polystyrene and more. Our question is: Will covering the entire roof with ice & water shield conflict with the spray foam non-ventilated rafter bays since the sheeting will be essentially sandwiched between 2 non-permeable materials? ", "Three Code-Approved Tricks for Reducing Insulation Thickness. As Bill Rose notes, air tightness of the assembly, both inside and out, is of critical importance - we need to stop the air flow. What's under the floor? So, really two different situations where I need to know if the rigid board is a bad idea. The first clue is usually a ceiling stain or ceiling drips. There had to be some major air leaks in the air barrier to show this kind of damage. "Is there anything specific about drywall or would 1/2" plywood or OSB work as long as I taped the joints with zip or tyvek tape or something similar? What about an unvented assembly consisting of: ", Q. You're right that stuffing a fat fluffy batt (or two fat fluffy batts) in a small cavity can mimic some, if not all, of the benefits of a dense batt. Also is there any worry with using closed cell as it attains to trapping moisture between the roofing underlayment and closed cell foam? "Cellulose has an R-value of 3.2 per inch of depth, while fiberglass ranges from an R-value of 2 to 2.5 per inch of depth." Any details you could provide would be helpful. I am wondering if I can attach craft paper and pine paneling to the 2x8 rafters and then fill the bays with cellulose. 2. - install roofing paper over whole roofing area Thank you for your help and sharing your knowledge! 4” fiberglass-faced polyiso, sealed with foam on edges and seams (probably 1.75” slats above it for spacing) There are a number of sellers of reclaimed foam in MA. I did notice several areas of ice damning, so I am going to assume that is the source, getting under my asphalt shingles, i am also going to assume that their is not proper ventilation under the sheathing. Response to Jim Boyd At that point, the rafter bays are "kind of" ventilated, but not really. They were blowing cellulose as fast as they could and the the rain was heading our direction. The other sections of the house have the same slope, but no recessed lighting and no vaulted ceilings, no ice damning. This opportunity to remove moisture from the sides of the rafters is missing when the ventilation channels are above the roof sheathing. Thanks! (There are exceptions, of course, including cedar shingle roofing, slate roofing, concrete tile roofing, and clay tile roofing.) Why not vent the cathedral ceiling under the sheathing the conventional way? Q. Thank you, it is an interesting article. Response to Martin Response to Rick Dense-packing accuvent Response to Martin Interior relative humidity in the winter was running about 60 - 65%. Which is a "biggie" for me. Is there any way for me to insulate around the beam without opening the ceiling, like drilling holes and spraying some kind of insulation inside? From out to in: Shingles, Membrane, Sheathing, Closed-Cell Spray, Batt, Gyp. You also get full insulation over the outside wall top plate which eliminates ice dam potential providing the building is airtight. Closed cell foam and a roof leak The reason to put the additional insulation in contact with the underside of the roof is that it's hard to verify that you have successfully closed off all vents to the attic. If you live in Climate Zone 4A, 4B, or anywhere colder, then 2 inches of spray foam isn't enough. Here is the link to my answer: https://inspectapedia.com/interiors/1382s.jpg. The skylights as shown in framing pix will probably be jettisoned to further simplify construction. My roof construction is 2x12 24 OC which I intend to insulate with loose-fill wool. ), Solving your R-value problem is going to cost some money. I forget where you leave things in the post, forgive me. The photo shows a low-slope roof over a flat ceiling; this allows workers to stand on the ceiling joists. While there is a lot of attention paid in this article, and on GBA in general, to protect the roof sheathing from moisture damage, I found very little information on this site regarding anti-fungal treatment of the sheathing material. Greg, Greg, Once our hot Caribbean sunshine starts to kick in I want this moisture to dry and/or diffuse out of this cavity, through the gypsum sheeting, into the cathedral living space below, to then be blown out of my 4x3 gable window. 4. two layers of 30lb felt The roof was recently replaced and the interior ceilings are likely pine T&G that’s been painted. For more information on where this minimum of 51% comes from, see this article: "Combining Exterior Rigid Foam With Fluffy Insulation.". Thanks in advance for your views on this and keep up the great work on GBA! 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. Or maybe there used to be a ridge vent, but when new roofing was installed, the roofer just ripped out the ridge vent and roofed over the ridge. It sounds like the insulation consists of "tight-fitting wood fibre board" between your rafters -- thickness and R-value aren't specified. Many articles I read in Fine Home Building and Journal of Light Construction do not adequately address deep snow on the roof. 3. When the roofing has been removed, you can install rigid foam insulation above the existing roof sheathing -- as thick as you can manage. Response to Doug McEvers - Synthetic underlayment stretched over roof framing Kerstin, In zone 3, you need at least R-30 of insulation (which means 6 inches of XPS). "If you’re building a new house, however, here’s my advice: if you want to insulate with cellulose, make it a ventilated roof by including ventilation channels under your roof sheathing. "
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