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spray foam insulation for underside of roof

Last Post 01 Jun 2016 06:26 PM by Dana1. 201 Replies.

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jacksond

New Member

Posts:5

13 May 2010 08:41 AM
I don't understand why an interior side vapor retarder is necessary here. Seems to me the attic space is going to be essentially conditioned space and vapor retarders are not typically put on ceilings. Can you provide a little more detail? Thanks

sgo70

Basic Member

Posts:125

13 May 2010 09:56 AM
I'm just getting ready to drywall now and I'm starting to have some second thoughts. The walls are Sips and everything else is spray foam, roofs R40, basement walls R21, everywhere else R28. I used 2lb Walltite for all of it and I wasn't going to use an interior VB even though code here says you have to. I have the roof vented by using purlins and doing a second layer of sheathing along with soffit and ridge vents. Now lately I just heard of a friends house that had all kinds of mold on the ceiling drywall in between the insulation and drywall. Somehow moisture was getting in there and condensating on the back of the drywall. I imagine this would be from lack of ventilation but I'm not sure and I'm starting to wonder if I should use the VB. Has anyone ever heard of this happening and the causes to it? I have four bath fans, a 600 cfm range hood, and even though Radiant heat is our primary I have a modulating furnace to run on the low speed fan for air exchanges. I may add an HRV later if necessary. Any input would be great as I start drywalling tomorrow and have to make a decision. Thanks, Sean

egouin

Basic Member

Posts:126

13 May 2010 10:04 AM
Sean, It sounds as if you've got a great - exhaust - plan, but where is all that air going to come from? You are going to create a negative pressure environment and suck unconditioned air into the structure any way possible. If I were you, I'd add that ERV right away. Just my twe cents. Good Luck, Ed www.GouinGreen.com

http://www.GouinGreen.com<br>Superinsulated SIP/Modular House (HERS = 30)<br>GSHP w/SCW, ERV, Passive Solar, Solar HW

Dana1

Senior Member

Posts:6991

13 May 2010 10:45 AM
Posted By jacksond on 13 May 2010 08:41 AM I don't understand why an interior side vapor retarder is necessary here. Seems to me the attic space is going to be essentially conditioned space and vapor retarders are not typically put on ceilings. Can you provide a little more detail? Thanks They SHOULD be typically putting vapor retarders (or vapor retardent paints) on ceilings in cold or very-cold climates, to reduce the potential for condensation within the attic insulation or attic (even in a ventilated attic.) Highly retardent poly vapor barriers are required in ceilings by code in Canada (even when they arguably shouldn't be, as in the closed-cell foam-insulation case.) With an insulated roof deck vapor retarders are it even more important because a roof isn't exactly a tilted wall- it's water-saturated on the weather side for significant amounts of time, and even when it isn't, it often undergoes nightly radiational cooling below ambient, increasing the number of hours when it's temperature below the dew point of the conditioned-attic air. The roof deck can only dry toward the interior when it's temperature is above the dew-point of the interior air, which can be zero hours weeks or months in cold climates. (The dew point of 50% RH 68F air is ~49F, an outdoor temperature not seen for months in many places, and certainly not experienced by the roof deck when it's covered with snow.) When it's temp is below the dew point, the wood absorbs water vapor from the insulation, which drives further diffusion into the insulation from the more-humid room air like a slow sponge. With more condensing hours & fewer drying hours than wall assemblies in the same structure, the roof deck is at higher risk of rot than wall sheathing. Humidity that finds it's way in has to wait for dryer warmer weather in order to be able to dry. A rain-wetted or snow covered roof deck has ZERO drying capacity toward the exterior, whereas the wall sheathing never experiences those conditions and will have some drying capacity even on the coldest or wettest days. But vapor diffusion is still a slow process, and air leaks through the insulation reaching the cold roof deck are much more likely to cause a problem. It's much better to put at least some insulation (or a ventilated nailer-deck, or both) on the exterior of the roof structure when you can- the roof deck stays warmer(=drier.) A common misconception is that the benefit of ventilated roof decks are to keep the roof deck & attic cooler, but that effect is very slight even with best-case ventilation schemes. The far larger benefit is to keep the roof deck & attic DRY in colder climates. (Attic ventilation is of dubious value in hot humid climates, since it only cools slightly, but draws large amounts of moisture IN.)

jacksond

New Member

Posts:5

13 May 2010 04:43 PM
But how exactly is the roof deck getting wet to begin with? If the roof has a vapor barrier, the only place it could absorb water from is the conditioned air in the house. And that air has to go through 8+ inches of spray foam, which is impervious to air, right? I guess I just don't see how much moisture would be able to penetrate into the wood. But I'm no expert...that's why I'm asking.

Dana1

Senior Member

Posts:6991

13 May 2010 06:45 PM
Posted By jacksond on 13 May 2010 04:43 PM But how exactly is the roof deck getting wet to begin with? If the roof has a vapor barrier, the only place it could absorb water from is the conditioned air in the house. And that air has to go through 8+ inches of spray foam, which is impervious to air, right? I guess I just don't see how much moisture would be able to penetrate into the wood. But I'm no expert...that's why I'm asking. The roof doesn't have a vapor barrier (at least with most conventional roofing). Roofing felt is highly permeable to water vapor, and the shingles are not air-tight, which allows the roof deck to dry toward the exterior. Liquid water doesn't penetrate, but water vapor (gas) diffuses quite readily through the felt layer. And even if AIR can't get through the foam, and even if it's waterproof to liquid water, WATER VAPOR can (and does) get through, but far more so with half-pound foam than with the 2lb density stuff. If you have 8" of closed cell foam, don't sweat it, but half-pound/open-cell goods are likely what you're talking about at 8" of depth (~R30-ish, not R50-ish, is that right?). While an excellent air-barrier, half pound foam is fairly permeable to water vapor, whereas 2lb foam is semi-impermeable to water vapor and considered a vapor barrier for all practical terms at thicknesses greater than 2". You need about 3-4 FEET of half pound foam to equal the vapor retardency of 8" of 2lb foam (which is still more permeable than 0.006" thick polyethlene sheeting.) If you're not sure what you have, 8" of 2lb foam costs ~$8-10 per square foot, whereas 8" of half-pound runs ~$3 per square foot. Did it break the bank yet? Read this to see if you need any sort of interior vapor retarder with open cell foam in your climate zone. (Look for the Full depth ocSPF column on the charts on p.11)

sgo70

Basic Member

Posts:125

14 May 2010 09:46 AM
Okay Dana, Now I have cathedral ceilings with the 2lb sprayed on the inner sheathing of the roof deck (then 1.5" air gap and second sheathing layer, grace ice and water sheild, shingles), so the only way I can see condensation getting in is from the inside. So is a vapor retardent paint a good idea or the 6 mil poly, or nothing at all and I shouldn't have to worry???? It is a heating dominant climate for sure. I don't know how condensation could get in between the foam and drywall, but if it does it would be trapped. Am I stressing about nothing here? Sean

Dana1

Senior Member

Posts:6991

14 May 2010 11:32 AM
Posted By sgo70 on 14 May 2010 09:46 AM Okay Dana, Now I have cathedral ceilings with the 2lb sprayed on the inner sheathing of the roof deck (then 1.5" air gap and second sheathing layer, grace ice and water sheild, shingles), so the only way I can see condensation getting in is from the inside. So is a vapor retardent paint a good idea or the 6 mil poly, or nothing at all and I shouldn't have to worry???? It is a heating dominant climate for sure. I don't know how condensation could get in between the foam and drywall, but if it does it would be trapped. Am I stressing about nothing here? Sean So from top to bottom you have: Shingles Ice & Snow barrier (highly impermeable) Nailer deck >>>>1.5" of ventilation gap<<<< Roof sheathing 2lb foam (how thick?) Air(?) Fiber insulation? gypsum paint The ventilation gap allows both the nailer deck and inner sheathing ample drying capacity toward the exteriror (this should be done more often!) In cold climates if you're using fiber inside the foam, not just cc foam for insulation you may or may not need an interior vapor retarder, depending on how much of that R is fiber vs. foam. If it's all cc foam, you're good to go in any climate- a vapor retarder would impede its drying capacity with little effect on it's ability to reject moisture. If it's more than 2" of cc foam, an interior side vapor retarder would create a moisture trap, and should be avoided. In a combined foam/fiber stackup a semi-permeable vapor retarder might improve the moisture picture, but poly would create a serious moisture trap. Standard latex paints run ~ 2-3 perms (semi vapor retardent), which would be fine. Purpose-designed vapor retardent latex paints run ~0.5 perms or less, and should only be used if the analysis shows it improves overall performance. Modeling foam/fiber stackups with & without interior retarders of various types for in your specific climate using something like WUFI isn't a bad idea, but would not necessary if you're going all 2lb foam with a ventilation gap above the foam.

sgo70

Basic Member

Posts:125

14 May 2010 09:22 PM
Thanks Dana, I trust your information and that made me feel better. The foam is 6-7" thick giving me around R40 or so and I don't think I'll need to stack any other insulation in there. It's been around 70 F lately and it's still cold inside, good sign for me. I talked to the big three spray companies here and none of them would go any thicker unless I demanded it, they say anything else is a waste of money and they don't want to screw me. I imagine that comes directly from BASF and not just their good nature. Thanks again, Sean

jacksond

New Member

Posts:5

17 May 2010 01:11 PM
Posted By Dana1 on 13 May 2010 06:45 PM The roof doesn't have a vapor barrier (at least with most conventional roofing). Roofing felt is highly permeable to water vapor, and the shingles are not air-tight, which allows the roof deck to dry toward the exterior. Liquid water doesn't penetrate, but water vapor (gas) diffuses quite readily through the felt layer. And even if AIR can't get through the foam, and even if it's waterproof to liquid water, WATER VAPOR can (and does) get through, but far more so with half-pound foam than with the 2lb density stuff. If you have 8" of closed cell foam, don't sweat it, but half-pound/open-cell goods are likely what you're talking about at 8" of depth (~R30-ish, not R50-ish, is that right?). While an excellent air-barrier, half pound foam is fairly permeable to water vapor, whereas 2lb foam is semi-impermeable to water vapor and considered a vapor barrier for all practical terms at thicknesses greater than 2". You need about 3-4 FEET of half pound foam to equal the vapor retardency of 8" of 2lb foam (which is still more permeable than 0.006" thick polyethlene sheeting.) If you're not sure what you have, 8" of 2lb foam costs ~$8-10 per square foot, whereas 8" of half-pound runs ~$3 per square foot. Did it break the bank yet? Read this to see if you need any sort of interior vapor retarder with open cell foam in your climate zone. (Look for the Full depth ocSPF column on the charts on p.11) Dana - thanks so much for your responses. They are very helpful. Yes, open cell is what we're talking about. I believe it is R38 minimum so probably a bit more than 8 inches. So you're saying that air (as a gas) will not penetrate the foam but water vapor will? That seems odd. Aren't water vapor (H₂O) molecules bigger than nitrogen (N₂) molecules?

Dana1

Senior Member

Posts:6991

17 May 2010 02:25 PM
Hydrogen only has one electron shell, so even with the two naked H-nuclei (protons) attached to the hip of the O who is borrowing their electrons, the H2O molecule is "smaller" in all respects than a molecule of N2 gas (about half the size.) If hydrogen had more than one shell, the loss of the single electron in the outer shell to fill out the orbitals of the O wouldn't diminish it's apparent size, but with only one electron, it's apparent size drops to near-nuthin'- the H2O molecule has pretty much the same dimensional aspects of mono-atomic oxygen from a gas-particle-model point of view. But regardless of the whys, vapor transmission of water through solid or air-tight materials is a well known and studied phenomenon, with multiple ASTM tests for characterizing & quantifying vapor-permeability of construction materials under different conditions. sgo: When dealing with a premium-priced material like SPF you hit the cost-effectiveness limits at a lower R-value than with cheaper stuff. It's becoming a fairly standard practice to use some amount of SPF for creating an excellent air barrier/vapor retarder, and filling out the total R to a higher value with cheaper stuff. With the high quality air seal of SPF already in place many of the foam-industry criticisms of fiber insulations go away or are significantly abated (infiltration losses can't happen in air-tight structures, the R value loss of fiberglass layers with delta-T is cut in half if half the R is foam, since that reduces the delta across that fiberglass layer, etc.) Using fire-rated spray fiberglass as the code-required thermal barrier for the SPF is an idea with merit too.

BCarr

New Member

Posts:2

17 May 2010 02:47 PM
Does anybody use foam denser than the 2lb stuff, like say 4lb or 6lb? Or is 2lb pretty much the standard for "high" density foam in home construction?

Roberth

New Member

Posts:97

17 May 2010 09:27 PM
Posted By greentree on 24 Mar 2009 07:18 AM Why dont i ever hear of anybody spray foaming the ceiling and then spraying cellulose over it? It's always the roof deck. It depends on what you are trying to acomplish. Many homes have some or all of the HVAC in the attic. Any leaks in the HVAC system would basically be to outdoors or worse -worse drawing in hot attic air in the summer. What I see a lot of times is that the top of the ducts are not covered with insualtion other than the little bit around the duct work. I would think that the minimally insualted duct being in a 140 degree attic would have a vey active convectice loop when the a/c is not operating. Then there is the fact that the system is sitting in the hot/cold and would take longer to reach its peak effectiveness. If you insulate the attic at the roof you now have the ducts and air handler at the same temp as the indoors. Also since it is now open and easy to inspect it should be easier and more convenient to seal any duct leaks with the remaining duct leaks now inside and without the big penalty. Plus this area is now more usable storage. If your ducts are not in the attic I am of the opinion that using a combo of spray and cellulose will likely be more cost effective. What I have read is that open or closed foam max's out at about an r22-24. So if you did use 8 inches of closed spray at R6 per inch its effective R value would be R22-24. To me it makes more sense to do an inch and then pile up the cellulose. Since I have read about the open or closed cell reaching a max R24 it has made me wonder about the real value of an the R value rating system. It would be interesting to see performance data on different types of insulation systems. What would get you the best performance. Would the best performing wall be just a single type of insulation or mixing different types. Are there better and more cost effective systems say using combinations of air sealing, exterior foam board and cavity insualtion.

Dana1

Senior Member

Posts:6991

19 May 2010 06:27 PM
Roberth- methinks you must be misinterpreting something about SPF "maxing out" at some particular R value. Got any web references to those assertions? (8" of cc SPF will be well north R40, more like R50.) Since it's not dimensionally stable with temp, the thicknesses with which it can be applied in one go without cracking or separating from structural stuff is limited to lifts of ~4" (about R24-ish) in any 24 hour period but that's not to say it doesn't get applied in thicker layers, with higher R even against the manufacturers recommendations. It can be applied to almost any thickness in successive sprays separated in time, and allow it to cool sufficiently between lifts avoids those issues. It takes awhile to cool, being an insulating material & all... Half-pound foam is harder to do in multiple lifts, and you can typically only get about 6" (R22-ish) in a single pass. But that's not to say the cost-effectiveness of ccSPF won't max out around R22-24in most places at current fuel & foam prices. :-) BCarr: It's sprayed at higher density all the time in applications where more mechanical rigidity is required. 3lb stuff is common enough- I'm not sure where the theoretical limit is, but at some density it starts losing R value. Since it doesn't increase in insulating value with that higher density, ~2lb density ends up being an approximately optimum point of vapor-retardency, rigidity, sound abatement & R value etc. for many many construction applications. But it's kind of arbitrary- 1.5llb SPF is also coming into favor in some places.

jacksond

New Member

Posts:5

21 May 2010 07:31 AM
Posted By Dana1 on 17 May 2010 02:25 PM Hydrogen only has one electron shell, so even with the two naked H-nuclei (protons) attached to the hip of the O who is borrowing their electrons, the H2O molecule is "smaller" in all respects than a molecule of N2 gas (about half the size.) If hydrogen had more than one shell, the loss of the single electron in the outer shell to fill out the orbitals of the O wouldn't diminish it's apparent size, but with only one electron, it's apparent size drops to near-nuthin'- the H2O molecule has pretty much the same dimensional aspects of mono-atomic oxygen from a gas-particle-model point of view. But regardless of the whys, vapor transmission of water through solid or air-tight materials is a well known and studied phenomenon, with multiple ASTM tests for characterizing & quantifying vapor-permeability of construction materials under different conditions. sgo: When dealing with a premium-priced material like SPF you hit the cost-effectiveness limits at a lower R-value than with cheaper stuff. It's becoming a fairly standard practice to use some amount of SPF for creating an excellent air barrier/vapor retarder, and filling out the total R to a higher value with cheaper stuff. With the high quality air seal of SPF already in place many of the foam-industry criticisms of fiber insulations go away or are significantly abated (infiltration losses can't happen in air-tight structures, the R value loss of fiberglass layers with delta-T is cut in half if half the R is foam, since that reduces the delta across that fiberglass layer, etc.) Using fire-rated spray fiberglass as the code-required thermal barrier for the SPF is an idea with merit too. Wow - Dana, you really seem to know your stuff. OK, so apparently on our house, there is currently no plan to add a vapor retarder to the open-cell foam on the roof. This comes at the advice of the spray foam installers. When the builder asked whether we should add a small layer of closed-cell foam to act as a vapor barrier, they said it's not necessary (and they would have made extra money doing this, so they must really think it's not necessary). But now I'm not so sure. I've now asked them specifically about why they don't think it's necessary and am awaiting their answer. I'm very curious to see what they have to say. Is there any way I can know for sure whether the additional vapor barrier is necessary? Or does it simply make sense to do it anyway because it can't hurt and the consequences would be quite severe if it is necessary and we don't do it?

adkjacUpstateNY User is Offline

Basic Member

Posts:167

21 May 2010 09:47 AM
jacksond... just built a home in Lake George, NY.... spray open cell to roof deck 6" or so... and the spray company and manufacturer and the code officials... came up with a spec... they sprayed vapor barrier paint on the spray foam ... took them a few extra hours and not sure how much paint... all were happy.... and the house is performing super... way better than days of old... batt glass in 5.5" 16" OC construction... vented roofs leaking air that we pay to heat.... ice dams... I really like the unvented roof and feel that and sealing the rim joist works wonders... I also have always sealed windows and doors well and use subfloor glue under wall sill plates before tipping up walls.... aj this house has no ice dams... no icicles for the kids... (one draw back)

Rosalinda

Basic Member

Posts:353

24 May 2010 09:05 PM
I am overwhelmed by all this info. I have a Cathedral ceiling I need to insulate with a 2X10 rafter, so only 9.5 inches to work with. I am in zone 5, Fingerlakes NY, HDD 6600. The roof is 5/12, 30 yr architectural shingle with continuous ridge vent, roofing felt, ice and water shield (I assume 24 inches ) 50% soffit vents. The back 20 ft of the house has baffles and blown in R38 cellulose, it is the front 26 foot that needs insulation, and I would like to get as close as I can to R50. Do I go with air baffles and unfaced fiberglass batts HD 30 and then put 4 inches of XPS foam board on the bottom of the rafters, 1X3 and sheet rock for a vented attic? Go unvented and Spray 2 or 3 inches 2#CC foam on the roof deck then do the rest with OC foam and 1 inch of XPS, or no XPS then sheetrock? Any combination thereoff? I am looking for ideas. The simplest and most expensive would be to go with 7 inches of 2#cc but at around $839 an inch for my 798.6ft2 of roof area is it really worth the money? I am interested in anything that would work and not cause moisture problems, insulation failure etc. HELP????? -Rosalinda

Sum total of my experience - Designed, GCed and built my own home, hybrid - stick built & modular on FPSF. 2798 ft2 2 story, propane fired condensing HWH DIY designed and installed radiant heat in GF. $71.20/ft2 completely furnished and finished, 5Star plus eStar rated and NAHB Gold certified

adkjacUpstateNY User is Offline

Basic Member

Posts:167

24 May 2010 09:20 PM
Rosalinda... Dana should have some ideas for you. I feel that cathedral ceilings should never be internally vented. Open Cell spray foam would be nice. At this point in your build it would not be easy to go to R-50 at a reasonable cost. But... unvented open cell will work so well that you may just have to adopt me... and the price should be competitive now as there are 10 times the spray companies today as there were 2 years ago. Northeast Spray Foam did me right... and I have another name for you if you would like it. What you should know is the unvented aspect of doing spray foam to me will make your roof more incredibly better insulated than vented batt fiberglass. I personally am moving away from foam but sticking with not venting roofs at least not internally to an attic or cathedral joist space that is tied to inside air. aj

Dana1

Senior Member

Posts:6991

25 May 2010 04:29 PM
Posted By Rosalinda on 24 May 2010 09:05 PM I am overwhelmed by all this info. I have a Cathedral ceiling I need to insulate with a 2X10 rafter, so only 9.5 inches to work with. I am in zone 5, Fingerlakes NY, HDD 6600. The roof is 5/12, 30 yr architectural shingle with continuous ridge vent, roofing felt, ice and water shield (I assume 24 inches ) 50% soffit vents. The back 20 ft of the house has baffles and blown in R38 cellulose, it is the front 26 foot that needs insulation, and I would like to get as close as I can to R50. Do I go with air baffles and unfaced fiberglass batts HD 30 and then put 4 inches of XPS foam board on the bottom of the rafters, 1X3 and sheet rock for a vented attic? Go unvented and Spray 2 or 3 inches 2#CC foam on the roof deck then do the rest with OC foam and 1 inch of XPS, or no XPS then sheetrock? Any combination thereoff? I am looking for ideas. The simplest and most expensive would be to go with 7 inches of 2#cc but at around $839 an inch for my 798.6ft2 of roof area is it really worth the money? I am interested in anything that would work and not cause moisture problems, insulation failure etc. HELP????? -Rosalinda For an almost-R50 stackup with minimal fuss: shingles roof deck 4" of ccSPF (~R25) 5.25" of JM Spider blown-in-bag @ 2lb density (~R22) which also provides the ignition barrier for the foam. ... and you're done, at about R47-ish. Make that 5" of SPF and you'll be at R50, but lifts of ~4" are about the most they can do in one trip without running into problems with the installation of ccSPF. For modeled verification that it really works moisture-wise for your location, see this. It looks like you could also do 2" of ccSPF (R12), and 7.25" of 2lb Spider (R30) for ~R42 without moisture problems given that you have dark shingles. (see the tables on p. 11) If you were already in the process of re-shingling you might look at ~4" thick panelized iso to put outside the roof deck and some spray fiberglass or cellulose on the interior to acheive the total-R. If you go the interior SPF + noo-skool spray fiberglass route, when the shingles give up, adding up to an inch of XPS over the roof deck or semi-permeable glass faced iso will bring it over R50 and provide a thermal break for the rafters/trusses. (If you went with ventilated iso panels on the exterior you'd give the roof deck a lot more outward drying capacity as well Putting substantial amounts of XPS on the interior would be a mistake- it would lower the temp of the rafters and lock them into a vapor trap between the ccSPF and an impermeble thickness of XPS. Keep it under an inch if you go that route. A 1/2" layer of XPS is already under 1perm, and a full inch has about the same vapor permeability as kraft vapor retarders on batts- semi-impermeable- enough allow roof leaks & condensation to dry toward the interior but not very quickly. At 4", fuggedaboudit- it's a vapor trap.

Rosalinda

Basic Member

Posts:353

25 May 2010 06:33 PM
Thanks Dana, that is exactly what I needed to know. I can't open that PDF, tried to get it from their site yesterday to no avail. Could someone get it and email it to me as a message rather than a link to [email protected] ? I talked to a few contractors today and also got suggestions for 1.2# medium density OC for the full rafter depth and also using 3 inches of 2# CC and fill the rest with .5 OC. I must admit the blow in fiverglass or cellulose gives me the heebie jeebies (though I am reading about the spider system now) given the sealed roof, though I know it shouldn't. Should have ball park prices in a few days. If I live long enough to need to replace the roof, we will hopefully have lots of new amazing technologies available. It is bad enough to have to rip down a brand new ceiling, pull out all the new guts and rebuild it, I just can't manage having to rebuild the brand new roof. Unfortunately I just can't live with the wrong ceiling they gave me no matter how hard I tried to convince myself to do so. At least the company who sold me the home is making the ceiling right, so I will take the opportunity to do the insulation right, even if it costs much more than I was planning on spending. Hopefully it will save me a grand or so on the heating system, so I consider that a rebate. -Rosalinda

Sum total of my experience - Designed, GCed and built my own home, hybrid - stick built & modular on FPSF. 2798 ft2 2 story, propane fired condensing HWH DIY designed and installed radiant heat in GF. $71.20/ft2 completely furnished and finished, 5Star plus eStar rated and NAHB Gold certified