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Community and Q&A

Ice damming — Low-slope roof — Resolvable or hopeless?

crosis38 | Posted in GBA Pro Help on


I own an older home in Ontario Canada that may have been built between the 1920s to 1950s. It originally had knob & tube wiring.

Each winter I have to deal with ice damming along 2 sides of the roof, no where else. The areas in question are a Low-Slope roof (3/12 pitch, east & west side of the ridge) and a Cathedral style roof (8/12 pitch — inside the home there is a sloping ceiling that is covered by drywall). See a photo of the home:

My goal is to stop the ice damming.


While standing on the roof, the measurements are as follows:
** See a bird’s eye photo:

* Low Slope (West) – Pitch: 3/12 || Dimensions: 12′(from eave to ridge) x 34′(from North to South)
* Low Slope (East) = Pitch: 3/12 || Dimensions: 12′(from eave to ridge) x 20′(from North to South)
* Cathedral = Pitch: 8/12 || Dimensions: 10′ (from eave to ridge) x 14′ (from North to South)


The attic is beneath the Low Slope (East) & Low Slope (West) regions. The Cathedral, of course, has cavities which lead up into this attic opening. Access into the attic is only threw a tiny hatch on the upper floor in the home. Within the attic I’d say there’s about 36″ from the ridge to the attic floor and so one can only crawl threw it. At the moment the attic is filled with old cellulose (came with the house) and new cellulose (i had a crew add more overtop). In total it is perhaps 12″ thick but I can’t confirm if it is evenly distributed throughout the attic space.

The home has your typical Home Depot perforated 4-panel soffit vents along its exterior perimeter. However, I’m fairly confident most, if not all, soffts are obstructed by wood and/or cellulose; air intake is almost nil within the attic. There are two Maximum Ventilation (model #301) exhaust vents at the top of the roof. There is a bricked chimney running threw the attic up to the roof, along with 2 bathroom vents (non-insulated flexible hoses) and a vent stack. Though i can’t confirm i’m sure there are some open stud cavities hiding in the darkness of the attic. Lastly, the roof decking is barn-board NOT OSB or plywood.


I’m experiencing some serious ice damming along the Low Slope East and West regions and the Cathedral. The photo above, showcasing some ice damming, doesnt do the problem justice as it was early in the winter at the time. Within the attic you can see several black colored wood — rafters or the barn-board sheathing (i suspect moisture has been collecting up here for years).

I’ve asked several local roofing companies on how to resolve this issue. On top of opening all soffit vents and adding the generic pink baffles between every rafter, they’ve suggested some of the following:

* Dont use shingles, go with metal roofing
* Cold Roofing = a new decking overtop the existing but with an air gap in between decks
* Remove all existing attic insulation, air seal the attic floor, then blow in new insulation
* Remove the existing low slope and cathedral roof and replace it with a steeper pitch new roof


At the moment i’m leaning toward having the following performed:

  1. Remove the roof’s barn board sheathing to expose the attic
  2. Remove all the existing attic insulation to expose the attic floor
  3. Air seal all the visibly obvious problem areas (chimney, vent stack, bathroom flex hoses, open stud cavities, etc)
  4. Open all soffits between every rafter and add standard pink baffles between every rafter
  5. Blow in new cellulose onto the attic floor to a height of 18″ to 20″, no more than that leaving at least 16″ air gap from the ridge to the top of the new cellulose for cold / warm air exchange.
  6. With the old barn board sheathing already removed lay down new OSB sheathing for the entire decking.
  7. Install a ridge vent running from North to South (34′ run).
  8. On the new sheathing place quality ice and water shield from eave to ridge protecting the entire deck.
  9. Install new shingles.



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  1. GBA Editor
    Martin Holladay | | #1

    No ice damming problem is hopeless. This is solvable. But the solution won't necessarily be cheap.

    If you haven't yet read them, I urge you to read these two articles:

    Prevent Ice Dams With Air Sealing and Insulation

    Ice Dam Basics

    If (a) you have enough room in your attic for adequate insulation -- especially at the eaves -- and (b) you can hire an experienced crew of workers who understand air sealing, then your suggested approach will work. In many older homes, however, this approach won't work, because space at the eaves is limited, and because skilled workers are hard to find.

    A foolproof approach is to install new thick rigid foam above the roof sheathing. This approach (a) is expensive, (b) requires you to insulate the gable walls in your attic, and (c) requires you to re-work your exterior trim details at the rakes and eaves. The approach will solve your ice dam problems.

    In a retrofit situation like yours, you might want to consider installing nailbase. Nailbase is available in a variety of thicknesses from several manufacturers. For example, ABT Foam sells 9-inch-thick nailbase panels rated at R-33.

    For more information, see How to Install Rigid Foam On Top of Roof Sheathing.

  2. crosis38 | | #2

    The rigid foam / nailbase looks very interesting. Though i would imagine one would still want to address the blocked soffits by opening them and adding baffles but the attic is only accessable by first removing the sheathing / barn boards. For the record the troubled region of my roof measures nearly 800 sq ft (cathedral + low slope west/east).

    What about Cold Roofing where the roof company installs a new deck over my existing deck but leaves a small air pocket between. They would cut a hole in the lower deck at the eave to allow soffit air from way below to pass up into the underside of the new deck. They would still need to do some custom aluminum fascia work to hide the 1"+ gap at eaves and rake.

    You can see what i mean by the company's own video:

    To save money i can skip MY THOUGHTS approach and just do Cold Roofing.

    Your thoughts?

  3. crosis38 | | #3

    You've given me some wonderful information. I'm starting to review it now. I neglected to mention, the pricing for MY THOUGHTS comes out to be $25,000 CAD == about $20,000 USD. This includes new shingles and new OSB for the entire home. I'm going to hunt around online for quick pricing of your foolproof solution. I'd only apply your rigid foam solution only to the cathedral and low slope regions not the entire house.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "The rigid foam / nailbase looks very interesting. Though I would imagine one would still want to address the blocked soffits by opening them and adding baffles."

    A. If you install rigid foam on the exterior side of your roof sheathing, you don't want any ventilation under the roof sheathing. In most cases, you would need to carefully seal the soffit vent openings and the ridge vent if you take this approach.

    Some builders prefer to include a ventilation channel above the rigid foam; in fact, some nailbase panels include this type of ventilation channel. But you wouldn't want to have any ventilation under the original sheathing.

  5. crosis38 | | #5

    Ahhh, no ventilation in the existing attic. Wow i wasnt expecting that. Can you please explain why? Does it have to do with condensation?

    Ignoring the exterior rigid foam approach and looking back at ventilating my 3/12 attic, my understanding is if there is a lack of intake and exhaust condensation will occur because the sheathing is being cooled on the exterior side but warmed on the attic side. With proper ventilation a stack effect will occur and drive this warm air out before it had time to condense on the underside of the sheathing. If that's the case why is this a bad thing for exterior rigid foam? It seems to help reduce the heat problem.

    BTW your articles are absolutely fabulous and detailed. I read them both and am still wrapping my mind around all the concepts.

  6. GBA Editor
    Martin Holladay | | #6

    If you want to have a vented attic, you can. Vented attics are insulated at the floor level.

    If you decide to install a thick layer of rigid foam on the exterior side of your roof sheathing, that means that you have decided to convert your vented unconditioned attic into an unvented conditioned attic. There are many reasons you may want to do this; one reason is to reduce the chance of ice dams. For more information on this option, see Creating a Conditioned Attic.

    If you go this route, you certainly don't want to invite any cold winter air into your house. The point of the insulation is to separate the warm cozy interior from the cold exterior.

    Roof sheathing is only ventilated when the roof sheathing is on the exterior side of the insulation. Once you bring the roof sheathing into your home's thermal envelope, you want to keep everything warm and cozy (to limit your energy bills).

  7. crosis38 | | #7

    Thank you Martin. Your help is greatly appreciated. One more thing. I'm going to explore getting myattic well ventilated, air sealed, and insulated. Who would you contact about air sealing the attic? One of your articles had comments which asked the same thing but i cant seem to find the link again. I've been calling roofers but i get the sense air sealing isnt critical to them. Normally i'd do the air sealing myself but for one to do anything in my attic, the sheathing must first be removed and once completed the sheathing installed + shingles. To have this job done smooth without delay i'm thinking one company is needed to do it all.

    I'm new to all this.

  8. GBA Editor
    Martin Holladay | | #8

    The type of contractor you are looking for (a contractor capable of blower-door-directed air sealing work) is called a home performance contractor or a weatherization contractor.

    If you are having trouble finding such a contractor, you could contact a local home energy rater (a RESNET-certified or BPI-certified rater -- visit the RESNET web site and the BPI web site to locate a rater near you) and ask the rater to recommend a home performance contractor in your area.

    Here is a link to an article that discusses the work: Air Sealing an Attic.

  9. AlanB4 | | #9

    I have found one certified BPI rater in Canada
    and no RESNET certified in Canada

  10. user-2310254 | | #10

    Try CRESNET ( They have a memorandum of understanding with RESNET.

  11. GBA Editor
    Martin Holladay | | #11

    CRESNET actually has 26 members -- admittedly, fewer than I would have guessed. See the list in the image below.

    For more information, visit the CRESNET website.


  12. GBA Editor
    Martin Holladay | | #12

    Here is a link to the BPI Canada website.

    I was unable to find a list of members or certified contractors. But I noticed that one of the steering committee members is a GBA blogger: Greg Labbé of Blue Green Consulting Group.


  13. crosis38 | | #13

    Im now leaning toward sealing any open soffit (there should be next to none) then adding exterior rigid board to create an unvented conditioned attic. Neat idea, Martin.

    How would you handle my Cathedral roof? From the photo above you can see it has the 8/12 pitch but on the inside of the home the ceiling is sloped and covered with drywall. We did have a crew dense pack the space between but i dont know how well it spread out. Cleared warm air is still getting thru creating ice dams. Would you spray foam the entire cathedral cavity from eave to where it ends near the ridge then add a suitable amount oif rigid board overtop the exterior?

  14. GBA Editor
    Martin Holladay | | #14

    Q. "How would you handle my Cathedral roof? ... Warm air is still getting through, creating ice dams."

    A. Installing rigid foam on the exterior side of the roof sheathing -- the thicker, the better -- is always the best approach, as I advised you in Comment #1. The question of whether or not you need to beef up the insulation on the interior side of the roof sheathing depends on the thickness of the rigid foam you're willing to install.

    Depending on where you are in Ontario, you're probably in Climate Zone 6 or 7.

    In Zone 6, the minimum R-value for the rigid foam is R-25. In Zone 7, it's R-30. (This is explained in my article on the topic, How to Install Rigid Foam On Top of Roof Sheathing.)

    Of course, thicker rigid foam is also fine. If you install R-49 of rigid foam, that's enough to meet U.S. building codes (and probably Canadian building codes) even if you have no insulation at all between your rafters.

  15. crosis38 | | #15

    Thanks for helping me out on this, Martin.

    I had a recommended insulation company come out to take a look. He recommended I use closed cell spray foam for both the Low Slope regions and the Cathedral. He stated to make both regions Hot Roofs, no venting. The plan, he said, was to remove the sheathing and spay foam the "attic floor" between each joist in order to seal and insulate the attic floor with no more spray foam than the height of each joist. All intake and exhaust vents would be closed off. As for the cathedral they would also remove the sheathing and spray foam the entire cavity from eave to ridge. No venting. I have no interest in making my attic space into a livable region. I'm all for the cheapest approach to eliminate ice damming.

    Though there is spray foam along the floor I am concerned the Low Slope attic will very gradually get warmer and warmer due to no intake / exhaust venting until the unprotected sheathing underside gets hot enough to melt snow. Apologies for learning process but is there some advantage to NOT having ventilation in this scenario? The insulation guy said spray foam below the sheathing vs directly on the attic floor are identical.

    Thanks. I'm now reading your link on Conditioned Attics.

  16. GBA Editor
    Martin Holladay | | #16

    Installing spray foam insulation on an attic floor does not create a conditioned attic. The attic is outside of the home's thermal envelope, and it should be vented. There is no reason for the spray foam contractor to seal the soffit vents and the ridge vent if the contractor is installing spray foam on the attic floor.

    In most cases, the best insulation to insulate an attic floor is cellulose, not spray foam, although spray foam is often useful for air sealing.

    When it comes to insulating a cathedral ceiling (sloped roof assembly), you can either create a vented assembly or an unvented assembly. If the contractor is removing the roof sheathing and insulating the rafter cavities from above, it's possible to leave a ventilation channel above the spray foam. In that case, you would leave the soffit vents and the ridge vents open.

    For information on installing insulation on an attic floor, see these two articles:

    How to Insulate an Attic Floor

    Borrowing a Cellulose Blower From a Big Box Store

    For information on installing insulation in a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.

  17. crosis38 | | #17

    I spoke to another insulation specialist. His suggestion for my low slope 3/12 and cathedral is the following:

    1. Low Slope - 3/12: Suck out all the old insulation. Spray foam the attic floor including the tight eaves. Spray up to 5" even covering the floor joists. He would block off all the soffit insulation but there will be intake ventilation. More on this later.

    2. Cathedral - If there is any insulation in here remove it. Spray foam the entire cavity while standing on the roof. But they will leave a small air cavity on top. The soffits will remain open so it can allow air to run thru this small air gap above the spray foam up to the main attic space.

    For inintake ventilation in the main attic (this is where the Low Slope roof is) he really likes the products of Maximum Ventilation.. For intake i am currently using these Air Intake VMax AT1- 1213-DF units (2 along the low slope east and 3 along the low slope west) placed about 4' away from the eave up toward the ridge. They serve as attic intakes. As for exhaust vents i am currently using these VMax Model 301 units (2 near the ridge but in the Low Slope East region). These serve as exhaust vents. I was originally attracted to these exhaust units since they stand higher than a ridge vent when we get snow that consistently buries our roof. For the intakes they were used to help compensate for the lack of soffit intake / tight eaves for the 3/12 roof.

    I am considering installing ridge vents but the insulation specialist claims these Maximum Ventilation units (exhaust and intake) are far superior solutions and even the intakes would serve to draw in air allowing them to fully spray foam the corners tight maximizing the R-value there.

    Your thoughts on these Maximum Ventilation units vs Ridge Vents + Baffles for the 3/12 roofs

    Oh and for the record he said cellulose in place of 5" spray foam overtop the attic floor in my low slope case wouldnt be as good. I asked if adding cellulose on top of the 5" sprayfoam would help that much more. He insisted it would not help at all; waste of money. Of course this coming from a guy who'd be making a cool $5000+ on spray foaming my attic floors.

    I've attached images of the VMax Model 301 Exhaust and the VMax AT1 intakes that are currently on my roof which the insulation specialist approves of me re-using for my after the spray foam.

  18. GBA Editor
    Martin Holladay | | #18

    If you are using spray foam to insulate your low-slope roofs, you don't need any ventilation -- either intake vents or exhaust vents.

    If you install 5 inches of spray foam, you will end up with either R-18 insulation (if the contractor uses open-cell spray foam) or R-32 insulation (if the contractor uses closed-cell spray foam). In either case, the total R-value is less than code-minimum requirements in the U.S., even for milder climates than the one where you live. (In the U.S., codes require a minimum of R-49 insulation in Virginia and Kentucky, or anywhere farther north.) If you are worried about ice dams, you don't want to skimp on R-value.

    For more information on ways to insulate a low-slope roof, see this article: Insulating Low-Slope Residential Roofs.

  19. crosis38 | | #19

    Thank you Martin for all the info. I'm still reading through all your provided links and still trying to understand it all. In the meantime, you are saying i DO NOT need to ventilate my low slope attic if closed cell spray foam is applied to the attic floor.

    I dont understand. In your previous message above you stated:

    "Installing spray foam insulation on an attic floor does not create a conditioned attic. The attic is outside of the home's thermal envelope, and it should be vented. There is no reason for the spray foam contractor to seal the soffit vents and the ridge vent if the contractor is installing spray foam on the attic floor."

    For the record i do not care for a conditioned space, only that the low slope roof region doesnt continue to ice dam. With 5" of closed cell spray foam (R32) i am still short, In Ontario it appears the goal is R50. Would you then say cellulose MUST be added on top? And to be clear intake and exhaust is not necessary at all within this attic space? This attic will not be used by anyone.

    UPDATE: Oh i think i understand. Are you saying if i can meet the min requirement of R50 then ventilation is not necessary but if not the venting is necessary? Even if i do meet the requirement and the fact that heat will always make it threw all insulation wouldnt an unvented attic slowly get warmer and warmer; it will take a while but eventually it will become a pressure cooker and so venting is still a necessity?

    Thanks for the clarification, Martin.

  20. user-2310254 | | #20

    Jake. You have two types of roof assemblies, correct? One is a cathedral ceiling and the other is a conventional vented attic.

    For the cathedral ceiling, one contractor suggested removing the roofing and roof sheathing and spraying the rafter cavities, correct? If so and the contractor used closed cell foam, you would have R-32 of insulation based on the depth of the rafter cavities, which Martin notes would still falls short of what is recommended in the U.S. You really want more insulation, and I believe you have been offered some options that would work in your climate.

    In your conventional attic space, Martin suggested keeping the ventilation and placing a thick layer of cellulose on the floor over the existing insulation. And while it makes sense from a cost standpoint to keep the space ventilated, you would want to do as much air sealing as possible at the interface between the attic and the part of the house you live in.

    Keep in mind that spray foam contractors want to sell spray foam. Earlier Martin suggested bringing in a CRESNET rater to check out your home and recommend a strategy. Personally, I think that is the way to go. Otherwise, you may end up wasting funds and possibly making your situation worse, not better.

  21. GBA Editor
    Martin Holladay | | #21

    I apparently misunderstood. I thought the low-slope roof assembly was a tight space between the ceiling and the roof sheathing -- too shallow to stand up in.

    If it is a stand-up attic, then you are right. It's best if the attic can be vented.

    Vent openings that penetrate the roof are not ideal, but are sometimes unavoidable. It might be possible to include soffit vents or wall vents if this is a stand-up attic.

  22. AlanB4 | | #22

    That insulation specialist is making me do multiple facepalms, he sounds like one of those stereotypical foam solves everything, R value myth types.

    For a standard attic the advice is simple, air seal (blower door directed is best) then put R50 of cellulose on the attic floor. Make sure soffit and ridge venting is adequate, measure amount you have in sq inches and size of roof and post here to get numbers.

    For cathedral (meaning no conventional attic, low slope or high slope) Adequate foam on top of the sheathing is recommended, and fill the rafters with open cell foam, drywall, airseal and paint.
    Foam thickness on top of sheathing depends on rafter thickness and climate zone, your location in Ontario would be helpful. Post those here to get numbers.

    This should solve the problems at better cost then some of the crazy and wrong solutions you have been given.

    If i am incorrect someone please correct me.

  23. crosis38 | | #23

    For the record I am in London, Ontario CANADA. Sorry for the slight confusion. Yes i have 2 troubled roof regions:

    Cathedral -- i'm not so sure that is its official name but it has a 8/12 slope. From the inside of the home you can see the ceiling stairwell (running upstairs) sloping upward and the upstairs room has a complete sloping closet; quick measurement shows there is no kneewall hiding behind the closet wall; the closet wall slopes from exterior wall to the ceiling / attic floor. This cathedral space is not accessible from within the home unless I want to tear down existing drywall to get to the sheathing. I figure to be more efficient have the roofing crew remove the sheathing and have the insulation guys do it from the roof downward.

    Low Slope -- this is a 3/12 roof and the main attic area of the home. Im guessing there is about 3 ft of space at the ridge to the floor and of course gets impossibly smaller at the eaves. It has about 12 in (uneven) of insulation. New cellulose was pray overtop about 1.5 yrs ago to elevate it this high. It is NOT a walk in region.

  24. GBA Editor
    Martin Holladay | | #24

    If it's not an attic you can stand up in, then you don't have to vent the air space above the insulation, as long as you install spray-foam insulation on the attic floor. Remember, you want to aim for at least R-49 insulation.

  25. AlanB4 | | #25

    I do not understand, if he spray foams the floor and does not vent does he not have an unvented and unconditioned zone with vapour barrier on the top and bottom (assuming closed cell foam)?
    With open cell would he not then have a vapour trap?

  26. GBA Editor
    Martin Holladay | | #26

    As long as the spray foam work is not performed in the rain, the assembly will be fine. The spray foam stops humid air from the interior from exfiltrating through the ceiling, so interior moisture won't be entering from below. The roofing keeps out the rain.

    If it's possible to install vents that connect to the air space above the spray foam -- especially soffit vents or wall vents -- the vents would be a good idea. But I don't like vents that penetrate the roofing. They invite leaks.

  27. AlanB4 | | #27

    So if this assembly will be fine with no vents why does it differ from an attic you can walk in where venting is recommended?
    I agree with no roofing penetration vents, i'm having a problem i have to deal with now, a royal headache, metal roof without proper flashing and not knowing what flashing to use or how to install it (they used silicone caulking in lieu of flashing, its leaking) :(

  28. GBA Editor
    Martin Holladay | | #28

    Low-slope roof assemblies insulated with spray polyurethane foam are often unvented. The fact that there are a few inches of air above the cured spray foam doesn't necessarily make these roof assemblies risky.

    Venting the air space provides an opportunity for excess moisture to dissipate, obviously, so if Jake can include a soffit vent or a wall vent, he should do so.

  29. AlanB4 | | #29

    Are you referring to closed cell spray foam done to R49 in this case?

  30. crosis38 | | #30

    Thanks so much for all the assistance! I've managed to read through all the provided articles. My head is still dizzy with data but I'm better off.

    Martin, i want to back up for a second and ask you about my "cathedral" roof. I'm not so sure i'm using the correct name. The roof is probably more like a one and a half storey roof. Again you can see the slope on the outside, and from inside you can see a sloping ceiling only at the stairway and within the upper floor closet. That is all; no other sloping can be seen but the stairway + closet does cover the full area of the outside slope. From the inside this sloping ceiling is all covered by drywall. Is this still defined as a "cathedral" roof / ceiling?

    Secondly, using rigid board on the exterior of my roof to obtain the overall R50 insulation. My worry about this approach is how it will look. I would only be applying it to the low pitch roof and the cathedral roof, no where else. Im concerned the existing, relatively new, gutters will have a, worst case, 10" gap between the shingles and the top of the gutter. The Dave 1 region will not have any rigid foam and so there would not be any gap there. It seems unsightly. Do you have any pics or experience on how it all will look with this gap or mismatch between roofs with rigid foam and no rigid foam?

  31. GBA Editor
    Martin Holladay | | #31

    Yes, the sloped insulated ceiling above your stairs is a cathedral ceiling (or cathedral roof), as long as there is roofing above the ceiling joists (rafters). If there is an attic above the ceiling, then it's not a cathedral ceiling.

    If you decide to install R-50 of rigid foam above your existing roof sheathing as part of a re-roofing job, the rigid foam would ordinarily be installed on your entire roof, not just certain sections of your roof. If you decide to install the R-50 rigid foam on only certain sections, then of course you have to consider aesthetics.

    If you install that much rigid foam, you will need new trim details at your soffits and fascia, and you will have to remove and reinstall your gutters. Below is an illustration from Building Science Corporation showing one way to rebuild your fascia and soffit to disguise the thick exterior foam.

    If you are uncertain of the aesthetics of this insulation project, it's time to hire an architect to help you.


  32. crosis38 | | #32

    Placing rigid foam on the exterior of the sheathing would certainly solve my issues but what seems like an astronomical cost and extra changes (eg. readjusting all the gutters and downspouts) is horrible.

    The home's age is between 1920s to 1940s.

    Regarding the low pitch 3/12 roof...

    I cant say for certain the exact height of the attic but with the 3/12 pitch and 12 ft run on either side of the ridge i can guess there is a 3 ft height at the ridge (from the bottom of the sheathing to the top of the attic joist). I dont know what size joist they used back then for these homes but I will guess 2x6 and the rafters appear to be 2x6 as well. If that is the case then the height at the eaves, from bottom of the sheathing to the bottom of the joist, is about 5" or less. There is no raised heel truss. See attached photos. (photos taken months back when leaking was discovered in upper floor bathroom ceiling)

    To resolve my ice damming issue in this tight attic space i'm thinking the following:

    1. Remove the roof sheathing to expose the entire attic. Remove all existing insulation to expose the entire attic floor

    2. Patch any large holes (eg. around chimney).

    3. Open all soffits and insert Accuvents between every rafter. Insert insulation dam deep in the eave to protect the soffit.

    4. Close cell spray foam the entire attic floor and especially right in the eaves. The spray foam will serve to air seal entire floor. Spray as much foam to cover the entire floor joist to stop / reduce thermal bridging. The height will be about 6" of spray foam since it seems spray foam always must be between joists/studs and not too far beyond them. The eaves are tighter but the foam will ride up the back of the baffles to appear leveled with 6". At the eaves, with the baffle in place, at best the spray foam will be 2"-3" in height. I would have to move 2 ft away from the eaves to finally have a sheathing to bottom of joist clearance of 6"

    Sadly the 6" of spray foam does not achieve the R50 i need for my Climate 6 - London, Ontario region, so

    5. Either add more height to the attic joists by adding 2x6 framing and then spraying more foam to achieve the 10" minimum height i require OR blow in at least 7" of loose fill cellulose overtop the 6" spray foam.

    6. Add 2x Maximum Ventilation Exhaust Vents (model 301) equally spaced out on the top of my roof -- i figure since it stands higher than a ridge vent then it has less chance of being plugged by snow allowing for decent ventilation during our rough winters. Im open to ridge vents but with this being a 3/12 roof and our heavy snow region im concerned about it functioning.

    What are your thoughts, Martin, on my approach to stop the ice damming? Again, the exterior foam is my #1 choice but cost and the added complications are the deal breaker and so must resort to the next solution which im hoping does also solve the problem.

  33. GBA Editor
    Martin Holladay | | #33

    Your plan sounds sensible. The success of your plan will depend, of course, on the skill of your spray foam installer.

  34. crosis38 | | #34

    A few more questions:

    1. Must the close cell spray foam always be sprayed between joists and so building up my framing to 10" or so is important if i want to add more spray foam to the attic floor? Or could they just spray foam the existing 5" joist and keep spraying until the entire floor rises to 10" without any additional framing?

    2. You mentioned the skill of the spray foam installer. Dont they just have to spray the entire floor region, leave nothing untouched, creating a uniform white across my attic floor? As long as they reach the maximum height evenly across the floor then the job is done correctly. Idiot proof?

    3. Closed Cell Spray Foam SMELL -- i read an article (or comment) on GBA about the odor of the foam. I assume as long as the contractor actually does make the correct mix there should be no chance of odor creeping back into the house? Or is there always a little bit of odor no matter how skilled the insulation specialist is?

    4. Do you have any thoughts on ridge vents vs those specialty static vents such as Maximum Ventilation Model 301? Is the one form of exhaust ventilation better than the other during heavy snow winter time?


    I am guessing the area isnt very deep guaranteeing insulation will come up short here. Again if exterior rigid foam is not an option than the ONLY solution is remove the sheathing from the roof and closed cell spray foam all the cathedral rafters however leave a small air channel (1" or less?) from eave to where it opens up into the main attic. I'm going to guess there is at most 2x6 rafters here and so at best 4" to 5" of insulation and we still have thermal bridging happening but nothing more can be done. is this correct?


  35. crosis38 | | #35

    Bear with me on this. I'm trying to explore all my options, even ones that dont directly address my root issue however do solve the ice damming problem.


    After reading your superb articles i am now aware the first line of defenses are air sealing and insulation. I've made mention of COLD ROOFING, above, which is simply creating an air gap overtop my existing sheathing for air to pass through then another layer of sheathing above that which the new shingles are placed. The air comes from the soffits by cutting a hole in the lower sheathing. Furthermore there is a hole at the ridge in both the lower and upper sheathing with a ridge vent in place.

    For now let's ignore the attic's moisture and poor air sealing. How effective is this form of cold roofing at keeping the underside of the shingles cold in winter? I mean, if i were to leave my attic as is (the ice dam champ that it is) but then add this cold roof overtop, wouldnt this air gap create a cold barrier between the warmer lower attic sheathing and beneath the shingles effectively stopping the ice damming?

    During winter up here if i were to open a window in the bedroom then return in a few mins/hour I can promise you the bedroom will be cold. Isnt this the same for the cold roof?


  36. GBA Editor
    Martin Holladay | | #36

    Q. "Must the closed-cell spray foam always be sprayed between joists and so building up my framing to 10" or so is important if i want to add more spray foam to the attic floor? Or could they just spray foam the existing 5" joist and keep spraying until the entire floor rises to 10" without any additional framing?"

    A. Closed-cell spray foam does does not require floor joists for support, so your spray foam contractor can "just keep spraying," as long as you don't need joists for fastening a subfloor. That said, most types of closed-cell spray foam can only be installed in shallow lifts of 2 inches. If you try to spray more all at once, the foam can overheat and your house can burn down. The foam has to cool between lifts. Finally, 10 inches of closed-cell spray foam would be extraordinarily expensive.

    Q. "You mentioned the skill of the spray foam installer. Don't they just have to spray the entire floor region, leave nothing untouched, creating a uniform white across my attic floor? As long as they reach the maximum height evenly across the floor then the job is done correctly. Idiot proof?"

    A. Installing spray foam is anything but idiot-proof. There are countless ways to screw up a job. Here are some links to relevant articles:

    When Spray Foam Goes Bad

    Air Leaks in Homes Insulated With Spray Foam

    Getting Spray Foam Right

    Spray Foam Insulation is Not a Cure-All

    Spray Foam Insulation Is Not a Magic Bullet

    Spray Foam in Cold Climates

    Spray Foam Jobs With Lingering Odor Problems

    Concerns When Using Spray Foam in Retrofits

    Air Leakage Through Spray Polyurethane Foam

    Q. "Closed-cell spray foam SMELL -- I read an article (or comment) on GBA about the odor of the foam. I assume as long as the contractor actually does make the correct mix there should be no chance of odor creeping back into the house? Or is there always a little bit of odor no matter how skilled the insulation specialist is?"

    A. Installed correctly, the cured foam should be odor-free.

    Q. "Do you have any thoughts on ridge vents vs those specialty static vents such as Maximum Ventilation Model 301? Is the one form of exhaust ventilation better than the other during heavy snow winter time?"

    A. Ridge vents usually work fine (unless your ridge is unusually short). The importance of attic venting is often exaggerated. For more information, see All About Attic Venting.

  37. GBA Editor
    Martin Holladay | | #37

    Q. "How effective is this form of cold roofing at keeping the underside of the shingles cold in winter?"

    A. Creating a ventilation channel under your roof sheathing can be part of a successful strategy to address ice dams. That said, plenty of unvented roofs are free of ice dams, and installing thick rigid foam above the roof sheathing (without a ventilation channel) is a proven method to address stubborn ice dams.

    A ventilation channel can lower the temperature of the roof sheathing -- but not as much as some people think. For more information on this type of ventilation, see All About Attic Venting.

    One more point: If you want to lower your energy bills and prevent heat from escaping through ceiling leaks, you really want to choose a solution that seals air leaks through your ceiling and adds R-value to your roof assembly.

  38. AlanB4 | | #38

    Do people get e-mails telling them their questions made a Q&A spotlight article?

  39. GBA Editor
    Martin Holladay | | #39

    Good question. I'm pretty sure that Scott Gibson notifies those who have been chosen -- but I'll double check.

  40. GBA Editor
    Scott Gibson | | #40

    I don't normally contact the author of the question chosen for a Spotlight. That's because the format of the column is to use what's already been posted. However, when something in the original question needs clarification, or more information or a better illustration is needed, Peter Yost or I would get in touch.

  41. AlanB4 | | #41

    Interesting, though i think you should because otherwise if they are not regular readers they would not know their question was reanalyzed, responded to by Peter Yost with a second or third opinion and the article and replies to it usually contain even more analysis and ideas which can help with new solutions to the problem.

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