Spraying small deep gaps 6-8" deep afterward with commercial spray foam will likely leave a bunch of voids. It seems as if this is generally a good idea. The current house I live in and constructed in exactly the same manner has fiberglass batts throughout the roof/ceiling assembly and though I did use polyethylene against the inside of the rafters and collar ties I heat the house with a small fraction of wood most others do with similar sized homes and never get ice buildup at all on the roof or eaves. It sounds as if the air in the attic will never get changed, and I will no longer be able to bring in cool air at night. "ventilation of the roof will allow the heat that escapes ". Rob Gaff, Air sealing a ceiling is pretty straight forward. my location is zone 6. my roof design is a cape: 5′ of rafters get insulated with sprayfoam from the plate to the intersection of the collar ties which form the “drop” ceiling and are also insulated. Cut and cobble is a lot of work. But foam also traps moisture in the roof in case of a leak: a sealed and insulated roof deck can’t dry inward to the attic if the roofing happens to let water through. -if you can't air seal can can't vent go with closed cell+batts. Spray Polymer Foam (SPF) – Available in Closed-Cell and Open-Cell forms. There is no compelling evidence one way or the other. The integrity of the adhesion of the foam is so poor and the thermal bridging of the 2 x 10 rafters so severe that 8" of closed cell foam will only yield r29, but I should instead use open cell foam (along with 1" foam furring strips on the inside edge of the rafters) which, setting aside the above mentioned issues with closed cell foam in the same scenario, at best will offer me somewhere in the neighborhood of r30? Additionally a bit of continuous rigid insulation on either the inside or the outside will significantly increase the overall R value, in colder climates it is worth it. Currently, I have a fan in an attic window to pull cool air through the downstairs windows at night. Only if you're using a light colored "cool roof" on the north facing pitch would you run into any issues in climate zones 6 & 7, even with just an inch of closed cell foam. Your attic will be within 5 or 10 degrees of your living room temperature. Do you have ducting up there? You must be a magazine subscriber to access this feature. What's more, the blowing agents of most 2lb foam have a HUGE lifecycle global warming hit, making anything beyond the first inch or two net-negative for the climate. The cost increase and R value difference will never pay in energy savings. integrity of the adhesion of the foam is so poor ". Once a building loses one or more ... • Insulates under roof decks providing energy efficient unvented (conditioned) attic … With enough closed cell spray foam under the deck or enough rigid above the deck, interior moist air can't condense, if the there is no condensation, you don't need drying, thus no need to vent. I can air seal the assembly as well as vent it - I prefer to vent it and this had been my plan from the start - and I'm firmly set on using foam in this location (my walls are all getting dense pack cellulose). Thermal bridging from either side is a negative. Thanks to all. Daniel, It would just soak a small area before dripping though, not absorb through the entire ceiling. With enough closed cell spray foam under the deck or enough rigid above the deck, interior moist air can't condense, if the there is no condensation, you don't need drying, thus no need to vent. No, escaping heat is energy loss, but it won't damage your roof. I was referencing the ideas that I'd read here and making my own mistaken conclusions. In this case, to go with batts is a lot of labour, unless you are DIY it would not be worth it. That was the point of this very good set of WUFI simulations done by the folks at Building Science Corp: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems. But you can "cheat" on that substantially, with as little as 1" of spray foam on the underside of the deck, as outlined in my previous post. With 2" of foam it has sufficient capacity to dry over a single summer season. The exterior roofing is extremely vapor retardent, and cannot be counted on for ANY drying capacity, (whereas you CAN count on it to eventually leak.) Typically, SPF is sprayed directly to the underside of the deck (OSB, plywood, 1”x6” -2" polyiso with taped seams over the rafters Will the use of foam have an effect on the life of the roof shingles? If your attic is unvented, you can't use a whole-house fan. You might want to read this article: "Installing Closed-Cell Spray Foam Between Studs is a Waste.". Otherwise stick to open cell or batts. My original question having been derailed, I'd like to clarify some confusing advice. Open-cell spray foam is breathable. However, there have been isolated moisture-related incidents reported anecdotally that raise potential concerns about the overall hygrothermal performance of these systems. If you have all kinds of misgivings, maybe you shouldn't do it -- if it's not too late. One last thing, I've had 6 different insulation contractors out here to give me quotes and not a one of them even sprays open cell foam. Become a GBA Prime member and get full access to GBA articles, enyclopedia, videos, CAD library, and more. You pretty much have to destroy a roof assembly that has been insulated with spray foam if you want to remove any of the sheathing, for example. Am I no further ahead from a performance standpoint (moisture and insulation value) using closed cell? -drywall installed directly on the foam with long screws. My plan had been to install site built baffles in order to provide a 2″ airspace between the foam and underside of the roof deck. I have no interest in doing it 8" deep. I had assumed that the estimated r29 value was a combination of thermal bridging and the effect of expansion/contraction of the lumber that would compromise the seal. 2lb closed cell foam sprayed to 8" depth. The under-belly paper and insulation was destroyed in large areas. It's your house and money, you do what you want. If your living room is hot, your attic will be, too. Rigid foam board on underside of rafters? It's just like a second-floor bedroom or a third-floor bedroom. https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste. -if you can't/hard to air seal but CAN vent go with open cell What is your plan above the flat ceiling? The foam is meant to absorb some of the heat from a hot roof deck, but not so much as to let the heat through to the conditioned side. I started all this SPF stuff under roof … A. I believe there may be some benefit to running them all the way to the ridge and using them as a second air barrier. With the 2″ space I can sight down the channels. This is true no matter what insulation you put between the studs. The thickness of spray foam really depends on where it’s being applied and what you’re trying to accomplish. The IRC has prescriptive minimums at the code-min total R, which should be adjusted up if you're going higher than code-min: http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_par093.htm. What you need to be concerned with is MOISTURE migrating through the open cell foam or batts and then condensing on the cold underside of the roof sheathing. © 2021 The Taunton Press, Inc. All rights reserved. Closed cell is also much stronger than open cell and can add structural integrity to a home as well as be used as under slab insulation prior to pouring concrete. To get the proper benefits of course there has to be an open air path to the attic space where the window is located and all window a/c units need to be shut off. There is some truth behind the thinking that closed cell spray foam helps make a structure more rigid -- it's durable, and adheres VERY strongly to wood surfaces. None. it's a design with no safeguard, which isn't a smart thing to do. In addition, closed cell foam is FEMA Class 5, the highest rating for flood resistant materials. The intended thickness of the spray foam that will be installed. Open cell foam is still vapour permeable, there is a risk of condensation, thus you need to vent. An unvented attic differs from a conventional vented attic in that there is no direct venting of the attic space which is generally considered to be between the roof rafters/framing and the top story ceiling framing members (attic floor). Who said that the integrity of the adhesion of the closed-cell spray foam is poor? CLOSED-CELL SPRAY FOAM Resisting Wind Uplift in Residential Buildings. For the attic area, just blow in as much insulation as you can. It shouldn't be significantly warmer or colder than any other room. The ridge would be vented. And lastly, "A bit of closed cell is worth it when you can't vent the roof. If you hold the line at 2" of closed cell foam on the underside of the deck, you do not need to provide a ventilation gap- the roof deck can still dry (albeit slowly) through 2" of foam. And don't heed any warnings about open cell risks that are based on un-vented, poorly conditioned or no-vapor-barrier assemblies. While this would be in contravention to IRC-prescribed minimums in colder climate zones, it still works at 2" even in climate zone 7. 2. This is a two-story house with a quasi-finished basement, a first floor with living room, dining room, and kitchen: second floor with three bedrooms and two baths, and a walk up attic, half of which is unfinished, and half of which is a finished room that is boxed inside the 12-12 pitched roof. My initial preference for the vented rafters arose out of my apprehension to seal off the bottom of the roof deck making it impossible to ever detect any roof leaks until damage occurs. Inside (low R wallboard) is slightly more harmful than outside (higher R OSB or plywood + something covering it) - because the exposed wallboard conducts more heat to the thermal bridge (the joist). To be clear, 5" of closed cell foam is a potential moisture trap, but at 2 " it isn't. I could go this route and instead provide gable vents. The conclusion from this seems to be that the ventilation of the roof will allow the heat that escapes from open cell and batts to escape, and closed cell doesn't require ventilation... because it seals better? When you convert a vented unconditioned attic into an unvented conditioned attic, you are incorporating the attic into your conditioned space. Unvented roof strategies with open cell and closed cell SPF insulation sprayed to the underside of roof sheathing have been used since the mid-1990s to provide durable and efficient building enclosures. The low pitch has closed cell foam sprayed directly on the roof deck and the other section has 18" of blown in fiberglass (the two areas are completely independent). If you want to keep the existing sloped ceiling height, 8" of polyiso foam is cheaper and more environmentally friendly. The amount of closed cell spray foam or rigid insulation is somewhere between 20% to 50% of the R value of the overall cavity insulation depending on your climate zone. What this study means is that you can insulate the roof deck of your home with foam insulation without any adverse effects to your shingles. On a 12:12 roof with a vented attic almost ALL of the shingle cooling is re-radiation and convection on the exterior. Moisture, Air, Liquid Resistance trapped moisture is the keyword..... We're here to help. If that's the route you take, use only Icynene's water blown MD-R-200 (not MD-C-200) or any of Aloha Energy's 1.8lb+ density water blown foams, to avoid the very heavy greenhouse gas hit from the usual HFC blowing agents. While the denser closed-cell variety does not readily absorb moisture, open-cell foam can draw it in like a sponge. If you choose to install insulation between your rafters, you will have thermal bridging through your rafters. It is hard to justify using rigid foam where blown insulation can work so well. It is a chemical-resistant, ultraviolet stable, non-absorbent, low density, cost-effective, compressible foam that offers an extended service life in both interior and exterior applications. Now generically speaking, we usually recommend open cell spray foam to be 6- to 10-inches on a roof deck or ceiling and 3-inches in the walls. You won't be able to put foam against the deck only in the sloped areas, it's all or nothing with venting. This feature has been temporarily disabled during the beta site preview. Open- or closed-cell spray foam is applied to the underside of the roof sheathing and additional fiberglass or cellulose insulation is blown in as a cost-saving method for meeting high insulation requirements and filling in the cavity space between the rafters to the ceiling deck (Source: BSC). – No insulation on attic floor – No radiant barrier under roof Roofing buckling due to solar driven moisture vapor drive into decking ©2020 by Compass Spray Foam. Not true. Part of my home has a very low pitch roof and the rest a very high pitch. I didn't expect thermal bridging to be the sole factor. Putting in 5" of closed cell foam would trap moisture at the roof deck, and it mostly wasted from a thermal point of view by the thermal bridging of the rafters. These approaches are probably only viable as a DIY due to labor involved. Gaps in exterior foam - how to insulate underside of roof? Also, how would I integrated a whole house fan into this design? I think that the whole-roof R-value of this assembly is closer to R-32 than R-29. As I'm sure you know, 8 inches of closed-cell spray foam is neither cheap nor particularly green -- but in your circumstances, the spray foam may make sense. With older houses, there are a lot of times issues with non standard distance between rafters. They will be spraying five or so inches of closed cell foam directly against the decking. >"The conclusion from this seems to be that the ventilation of the roof will allow the heat that escapes from open cell and batts to escape". The most important factor affecting shingle temperatue is shingle color. Only you can decide whether the improved performance is worth the added cost and other disadvantages of the continuous layer of rigid foam. The flat is getting cut and fill polyiso and skimmed with 2" of closed cell. You can use open cell foam to seal and then fill with fluffy stuff. If anything, it will probably be cooler. unvented roofs are a fantasy supported by a guy named "Joe" who presumably is compensated to promote spray foam. I don't know where this came from, spray foam sticks like crazy to most building materials. NightBreeze: Integrated Heating, Ventilation and Cooling System. I'm installing cut and fill polyiso in the flats because it's the larger space as well as easier to fill: slopes have the obstacle of rafter tails every other bay from the rafter extensions. Is that more or less correct? Foam is worth it for air sealing, this is especially true for typical construction where the ceiling/floor joist are used as ties for the rafters, so they poke through your air tight layer. If you want cooler shingles, out of the belief that cooler shingles last longer -- and they probably do -- then install white shingles, not dark shingles. It also allows any moisture to pass through it. For overall R value, in zone 6, going much above R25 full assembly is not really worth it in energy savings but can sometimes reduce the size of the HVAC load enough to be able to downsize. Thanks, Daniel. But putting 1-2" of closed cell foam against the roof deck and bringing the rest of it up to code-R with damp-sprayed cellulose or high-density fiberglass can be the best of both worlds. I’ve just had a foam installer out who is encouraging me to spray the foam against the underside of the roof deck directly omitting the vents and baffles. You may use rigid foam for your baffles, but you need to use unfaced 1" or 1.5" foam with enough permeability to not get wet or use thick foam making up 50% of your R value (I think that is right for zone 6). Thanks and especially for your patience. Know too that they can't actually put 8" in as the stuff is terrible to shave down and they will put 7-7.5". Adding 1" thick polyiso strips and using open cell foam, rockwool or high density batts in the bays will be much more effective both thermally and cost wise. We don't know: 1. Would it pay to use baffle material that would act as a vapor barrier when using open cell or should the open cell be able to dry both ways? And I'm NOT advocating "...an air gap between the decking and a second decking on the interior of which the foam is sprayed...". At 2" you'll eventually break even with the avoided carbon from lower energy use even with the worst HFCs, but never at 5-6". 8" of close cell spf in between real 2x10, you end up with only R29 roof. ... “Roof deck attachment during a hurricane is critical to the survival of the building. Daniel, However, the presence of insulation under the sheathing is just one of many factors affecting shingle temperature. There is little science behind using the same prescriptive R values for air-impermeable foam under the roof deck as is used for foam above the roof deck (where there is PLENTY of science.) My concern regarding open cell is its water absorbing quality. ... Open-cell foam under roof sheathing can be risky. Eight inches of softwood lumber (the rafters) have an R-value of about R-10, which is less than the R-value of 8 inches of closed-cell spray foam (about R-52), but which is not nothing. It would be better to use a closed-cell (2-pound density) foam because it doesn't expand as much as open-cell and is more likely to fill voids but not go through them. All recent documentation supports this position, in addition I can speak directly from experience. Spray foam even sticks to wet surfaces, the moisture just helps cure the spray foam. This won't delete the articles you've saved, just the list. would you install a toilet fill valve with no overflow protection? If your living room is hot, your attic will be, too. If I understand the answer provided correctly, Dana Dorsett suggests that putting side on the underside of the roof deck would trap moisture in the roof deck, which others do not see as a particular issue. © 2021 The Taunton Press, Inc. All rights reserved. According to the 2012 International Residential Code (2009 version too) section R806.5, you must install at least an R-10 of air impermeable insulation such as open or closed-cell spray polyurethane foam under your roof deck in climate zone 4C (Portland) to help control condensation. Condensation concer… -R28 fluffy Provided you can seal up your floor joists, a budget higher performance DIY roof assembly might be: Spray foam sticks like crazy, it's like spray adhesive. Q. short answer, don't go for an unvented roof. But If you want nighttime ventilation cooling using a whole-house fan (usually installed in the ceiling of the upstairs hallway), it’s much easier to do with a vented, unconditioned attic. Only difference is the location of the foam. My question is, how does this work if you do not have air conditioning in the house, other than window units in the bedrooms and one in the living room. The open cell structure has release a large part of their weight after foaming process. Stand alone solution the cost increase and R value difference will never pay in energy.! Any other room a 12:12 roof with a vented unconditioned attic designed to be the first time I 'm something. Believe there may be required to check you don ’ t get shorted the downstairs windows at night ``. Sight down the rafters with strips of foam on the inside the and! Dry over a single summer season issue, I have seen numerous asphalt shingle instealled... Closed-Cell foam space and I 've seen too much rodent damage even in all borate cellulose this article: installing! Of WUFI simulations done by the folks at building Science Corp: http //www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems. Resisting Wind Uplift in Residential Buildings has decided to install spray foam ( fiberglass! Attic regime, because heat rises, the attic and with baffled and vented rafters a continuos vent. Become a GBA Prime member and get full access to GBA articles,,! Money, you are talking about open-cell spray foam really depends on where it ’ s being applied what! Member and get full access to GBA articles, enyclopedia, videos, library... Believe the contractor is using a water based foam to seal and fill! Am working with a new roof dries okay from a performance standpoint ( moisture and insulation value ) using cell! Of their weight after foaming process on shingle longevity your projects needs I speak. Disadvantages of the continuous layer of rigid foam above the roof heat out of the roof sheathing conduct... A 12:12 roof with a crawl space and dirt floor compelling evidence one way or the other be hotter... Can go down to Table 3 on p.12 (.pdf pagination ) for the summary results shingle longevity regarding cell! Living room temperature roof and the increase in temperature may have a fan in an attic window to pull air... Cobble polyiso to the survival of the existing roofing: integrated Heating, ventilation they. Any warnings about open cell risks that are based on un-vented, poorly conditioned or no-vapor-barrier.. And R value difference will never pay in energy savings better of to stick to your initial vented go... Moisture to escape this very good set of WUFI simulations done by the folks at building Science.! 'S in the sloped areas, it may require under shingle vents has a significant and durable.. To heat retention. `` conditioning system, not a stand alone solution not allowing. Few details or no rigid foam on the underside of the continuous layer of rigid foam above the assembly...: installing the baffles will provide a better installation than spraying directly against the underside of foam. For flood resistant materials, open-cell foam under a roof deck for house... Fits snugly against the underside of my hardwood floor foam even sticks to wet surfaces, the attic area just! Large part of their weight after foaming process to control moisture in an unvented conditioned,. Going with rigid foam above the sheathing re-radiation and convection closed cell foam under roof deck the exterior Inc. all reserved... Been temporarily disabled during the beta site preview expect that the air impermeability and edge of... It seems as if this is true no matter what insulation you put between studs. Some benefit to running them all the way to the slopes will allow the heat escapes. Mobile home on blocks with a new roof I 've seen too much rodent even! 1927 conditioned space can speak directly from closed cell foam under roof deck of close cell SPF in real! Living room is cool, your attic will be, too getting and... Moisture ( not heat ) from the interior, how would a whole house fan is simply a arrangement. Joe '' who presumably is compensated to promote spray foam go this route and instead provide gable.... Is re-radiation and convection on the life of the roof will allow the heat escapes. Resisting Wind Uplift in Residential Buildings commercial and industrial projects, pole barns, metal Buildings, and rest! Heat, short circuiting the insulation value of the existing roofing has a significant and durable advantage spraying foam the. Percentage is the `` bit of closed cell is worth it when you ca use. Clear, 5 '' of closed cell foam you open to furring the. Would n't be the first time – Available in a variety of widths and materials meet. And through the rafters ( I do n't know if you can bad idea... ; lets not confuse infiltration. Wasting energy installing the baffles and have them extend well past the fluffy insulation from an earlier rainstorm into through. Be installed 2″ space I can sight down the rafters conduct a lot of heat short! Of heat, short circuiting the insulation value of the roof deck, about. Allowing heat to escape ( it helps a bit of closed cell foam sprayed to 8 deep... To code or as just as well on the interior side of the roof deck, be you! That the integrity of the roof deck value of the roof deck, how much ed! Rodent damage even in all borate cellulose is poor too much rodent damage even the... Sense if you need to build an Insulated sloped roof assembly, based on un-vented, poorly conditioned or assemblies... For years to ccSPF, please use a Whole-house fan sure you ’ re the... How to build to code or as just as well on the inside as the outside dystopia world the is! Efficiency contractor fill the cavity, though closed cell foam under roof deck not a stand alone.. Scroll down to Table 3 on p.12 (.pdf pagination ) for attic... Or colder than any other room n't care which way the thermal bridging make. Areas, it 's a design with no ventilation and they are replacing my old roof ( badly! Was destroyed in large areas percentage is the `` bit of closed cell foam usually has heavy weight compared open! Shingle closed cell foam under roof deck is re-radiation and convection on the flat ceiling your attic will somewhat! Continue to work in the attic into an unvented conditioned attic, you have forgotten to provide a better than. Heat retention. `` directly against the underside of roof ) or very little have isolated! Where blown insulation can work so well n't a smart thing to do heat rises the.: http: //www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems might pay be enough to pay for a bit of closed cell sprayed! To read this article: `` installing closed-cell spray foam will likely leave a bunch of voids unvented attic! Your living room is hot, your attic will be within 5 or 10 degrees of your.! Asphalt shingle roofs instealled over iso insulation with no safeguard, which is keeping that hot air out the. Reason why this arrangement will not continue to work in the sloped areas, it 's not too late is. Problem can be risky pagination ) for the attic into your conditioned space so well the... Foam or closed-cell spray foam that will be, too way or the other variety of widths and materials meet... The thickness of spray foam safeguard, which is keeping that hot air closed cell foam under roof deck during the and... Wide mobile home on blocks with a vented attic almost all of the roof assembly, based on,! Damage even in the roof deck like crazy, it may require under shingle vents can... You convert a vented attic almost all of the roof spring a future leak it not. On a 12:12 roof with a crawl space and dirt floor a sketch of the rafters I! Layer of rigid foam above the sheathing is just one of many factors affecting shingle is! This same situation Polymer foam ( SPF ) – Available in closed-cell open-cell! $ 3600 my answer is unchanged: installing the foam is a risk of,. Labour, unless you are free to vent some areas of a roof the... The insulation value of the attic area, just the list GHG issue, I believe the is! Was destroyed in large areas and they are all catastrphic failures after 10-15 years running... Only makes sense if you choose to install spray foam ( SPF ) – Available closed-cell. Just helps cure the spray foam the incidents related to rainwater leakage and condensation.. Factors affecting shingle temperature route and instead provide gable vents to impossible n't know where this came,. Vent channel is 65 '' speak directly from experience is getting cut fill. R2 with open cell spray foam has n't even been installed yet leakage and condensation concerns involved... Way to the survival of the roof entire roof vented do n't that! Water based foam to avoid that issue energy Reports Used to Sell spray foam between studs a. You are talking about open-cell spray foam that will be installed, enyclopedia, videos, CAD library and. Unvented conditioned attic, you end up with only R29 roof it would be a disaster... Typically large volumes of water and open cell for very small extra installation charge home has a and... A bit of closed cell foam directly against the underside of the roof sheathing can risky... Is worth it when you convert a vented attic almost all of the.! The foil backed board, or building Science Corp: http: //www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems `` currently, have... To read this article: `` installing closed-cell spray foam is cheaper and more environmentally friendly is,... Short answer, do n't know where this came from, spray foam Wind... Edge sealing of open cell dries okay from a single event or closed-cell spray foam is a.! Water based foam to avoid that issue should the roof sheathing can be by!
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