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Community and Q&A

Roof insulation/venting in heavy snow load climates

baking_fool | Posted in General Questions on

I put the cart before the horse, and selected the house depicted below to build in the Adirondacks, NY. After working out the design details inside the house, I moved to the outside. I questioned the building designer about the low profile 3/12 pitch roof and how it is vented on the high edge, and the reply was “this home uses water impenetrable closed cell urethane insulation, sprayed in to fill the cavity from below prior to drywalling, and it therefore unvented. This insulation (SPF) has an R-value of about 7 per inch, allowing for 2×8 rafters.” This sounded great, but the devil on my shoulder said I better look further. FYI, the larger roof is 30′ x 30′. This is post and beam construction, 6″ thick walls.

I then checked the snow load – it is about 70 to 80 psf. Then I did some searching online, and found a recommendation that at high snow loads, you must have a vented roof to prevent ice damming. The rationale appeared to be “at higher snow loads, thicker coverage of snow insulates the top of the roof, allowing above-freezing temperatures and melting, and thus is more susceptible to ice dams.”

So a conundrum has presented itself – SPF has wonderful insulating properties, but in severe cold climates has this limitation. Looking further, a “over-roof” venting system is recommended for high snow load areas. Using hard foam with channels, on top of the deck.

However, with the house/roof in question, this would return me to the problem of venting the high edge of a slab roof. Furthermore, for low pitch (3/12 and lower), a metal roof or something sprayed on (tar, etc) seems to be in order.

I appear to be trying to fit a square peg in a round hole. I can increase the pitch of this slab roof to 4/12 to allow more surface choices and better venting if the over-roof option is added, but it would still have the issue of how to watertight vent the top edge and possibly still be too susceptible to ice damming/leaks in this climate.

Turning to alternatives – if I switch to a home with traditional peaks in the roof, could a 4/12 pitch work in this climate? It seems like venting/ice dams is the biggest issue, but having sufficient pitch to avoid leakage problems is also a concern. I want to avoid 12/12, which of course is standard in the area, because I have lived with a 12/12 with a 20′ high ceiling peak for years and am tired of it. I am thinking of a hybrid, using the SPF underneath and the vented “over roof” as described above. It seems this would lend itself more to a 4/12 with a peak, versus a 3/12 slab.

Any constructive advice would be appreciated. There is the question of venting the top edge of a slab roof, venting an SPF sprayed roof at snow loads of 50 psf or higher, and whether the “over-roof” may be a solution at 4/12 in this climate, assuming a proper watertight vent is used. Thanks!

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  1. davidmeiland | | #1

    Seems likely to me that you can do an unvented 3:12 in your climate, the question will be how much foam and what R-value you're going to install. Too little and you'll have problems... and the foam contractors may want to install too little in order to keep their bids competitive. There are at least a couple of ways to vent as well.

  2. davidmeiland | | #2

    What you are calling a vented "over roof" would be fairly easy to do, just added labor and materials to do it... two roof decks with furring in between. Custom sheet metal drip edges can be made for top and bottom, and will look perfectly normal if you use a metal roof. You could also possibly vent side to side by simply installing purlins across the roof deck and then metal roofing directly over that.

    I don't live in heavy snow country. We get 3" here and everyone freaks out and they close the schools. I should defer to others who deal with this more, but I know that SIPS roofs are often used in snow country and I'm sure there's a way to make yours work at the pitch you want.

  3. baking_fool | | #3

    The assumption is the roof of the 3/12 will be filled completely to eliminate air. With 2x8's that is roughly 7 inches, or about R-49. Roof deck on top, drywall on the bottom. I have seen the advice ( in multiple places now) that unvented roofs in heavy snow load areas are not recommended,. However, I have also just seen a comment that at R-50, the roof may stay cool enough to prevent damming. So maybe the 3/12 with the SPF would work, if the right installer put it in. While you say "venting can be done in multiple ways" I am curious if there is a watertight way to vent the top edges of the two roofs pictured. Will a standard ridge vent work on a 14 degree angle? I guess it is possible. I suppose I may need the advice of one or more experienced roofers in the area.

  4. baking_fool | | #4

    Thanks for your replies. I don't know anything about metal roofs, and you've already mentioned techniques I was unaware of and may work. We'll see if anyone else chimes in. Yes, I was thinking that if I could count on the "over-roof" as an option, that would mean I wouldn't have to toss the house design out wholesale.

  5. davidmeiland | | #5

    I'm a little surprised by the 2x8 rafters given the snow loads.

  6. wjrobinson | | #6

    Don, you definitely can go spray foam unvented with closed cell. If you are at higher altitude and will be getting several feet of snow, then you should be designing for 90# snow load. Where I live is low altitude southern ADKs and we use 80#.

    Put on the metal roof right and you will be fine.

    The foam roof the scares me is a rigid foam built up roof. They leak warm air and so I like to vent above them to stop moisture damage.

    We have unvented spray foam roofs built and they work very well.

  7. GBA Editor
    Martin Holladay | | #7

    My recommendations:

    1. Check with an engineer to make sure that 2x8 rafters can handle the snow load.

    2. Ice dams are unlikely to cause problems on an unvented roof as long as you have metal roofing. If you want to install asphalt shingles, then I think vent channels above the roof sheathing are a wise investment. Sure, venting costs a little more, but the investment is worth it.

    3. A competent roofer or builder can easily install vents at the top of a shed roof.

    4. If you decide to choose a different approach to insulation -- one that includes a layer of rigid foam above the roof sheathing -- you'll end up with a system that addresses thermal bridging through your rafters, which your current approach doesn't do. For more information on all your options, see How to Build an Insulated Cathedral Ceiling.

  8. wjrobinson | | #8

    Martin and David, did you look at his home in the attached picture? It has purlin beams every 4' or so. 2x8s will do just dandy.

    NY has building codes and the home will have to be built with stamped plans. Where I build the code folks do check spans and loads and for the stamp.


  9. wjrobinson | | #9

    I would never do spray foam in the rafters and then rigid foam above the sheathing. Never.

    A spray foam roof like posted here will do a fantastic job of insulating not just because of it's U value but also because of the air seal will be nearly 100%. Air sealing saves more energy than anything a builder can do.

    If one truly desires to have a layer of continuous insulation with spray foam rafters then the best way might be to add a layer of 2x4 purlins above the rafters and then sheath the roof plane thus adding spray foam above the rafters that is adjoining. This way there is no sandwiching the sheathing,

    I would also recommend using plywood instead of OSB and why not even use pressure treated plywood so to ward off any future rotted sheathing in one or twenty years when the roof finally fails which is also a good reason to go with a metal roof,

    Lastly being a modern design the metal roof might go well with the design.


  10. baking_fool | | #10

    AJ Builder - yes, I did note that I may need 90 psf once I speak with a local code person. Please note that the pictured building is a stock photo, and the rafters can be made deeper if needed. I have seen an actual picture of this design, with 2x8s, built in Wisconsin. However, 2x10's could require 2" more of expensive foam, unless something else is used to fill the remaining 2". This home design has glulam beams on 5'4" centers - those are the ends you see. They can go end to end with no post in between, if the buyer is willing to pay for deeper beams. The stock design has a post about mid-span of each beam. It's done by Lindal in Washington. Lindal would do the calculations, and provide stamped plans (at a cost, of course).

    You guys have covered all the bases here, and I thank you. I do have one question for Martin, regarding the thermal bridging that you mention. If I used metal roofing with an unvented roof, installed in a competent manner, I believe your opinion is that I would be alright with respect to a watertight roof w/o ice dam issues. If so, what would be the reason to add the cost to eliminate bridging - efficiency only? I imagine it would be also be good insurance just in case - perhaps this is an option you are tossing out that the roof builder or engineers may recommend for my actual application? Thanks again!

  11. GBA Editor
    Martin Holladay | | #11

    If you use closed-cell spray foam between the rafters, you will end up with an airtight roof assembly. You will also end up with a high R-value, although you will still have thermal bridging through the 2x8 rafters.

    Whether this thermal bridging causes problems depends on a few questions, including (1) how much you care about thermal performance -- for example, thermal performance matters more to someone pursuing Passivhaus certification than it does to most homeowners; and (2) whether ice dams will cause problems.

    An ice dam on an asphalt shingle roof can lead to a ceiling leak. An ice dam on a metal roof rarely does.

    I agree with AJ that if you address thermal bridging by installing rigid foam above the roof sheathing, you shouldn't use closed-cell spray foam under your roof sheathing. It makes more sense to choose a vapor-permeable insulation between your rafters once you decide to go with rigid foam above your roof sheathing.

  12. baking_fool | | #12

    More great information. Thanks again guys. My main question was if this house is even viable in that climate, and the answer appears to be "yes". I am hunting for land tomorrow, so there was urgency to the timing. I appreciate the rapid responses.

    I assumed that once I go down this road and get more deeply involved, more information as you are providing will emerge - especially once contractors and engineers are consulted.

    I'll admit I was a bit confused about the "over-roof" venting incorporating rigid foam or not. It appears you would use purlins. I guess you would eliminate 95%+/- of the thermal bridging spoken of, depending on spacing.

    This site is fantastic. Thanks so much for your help and willingness to assist others in understanding. I once had an HVAC question, and posted it on a trade blog site. They pretty much told me to pound sand because I wasn't in the industry. I'm glad I didn't find that here.

  13. onslow | | #13

    Mr. Moore,
    Here in southwest Colorado, I observed long icicles hanging from a newly constructed home that I knew to be very carefully detailed and meticulously insulated . The driving force was not heat loss from inside the building envelope, but instead sun driven melting and subsequent refreezing. Other homes that are known to be well insulated display the same issue and independently of the roof's orientation. When the sun hits the top of the ridge or other windblown bare spots that appear after a snow fall, the solar gain in metal and asphalt shingle alike is sufficient to melt the snow, which then trickles down until it freezes at the bottom edge or points between.

    I thought the frequent use of heater cables along eaves was due to poor design, but they are there to save gutters and to avoid dangerous ice falls. The region I am in is very sunny and subject to 30-40 degree temperature swings each day all year so the Adirondacks local conditions may suppress this effect most of the winter. However, don't be too surprised to see icicles at the bottom edges if the sun comes out long enough, especially with a metal roof.

    As to venting the roof having any effect, I recall reading articles from Building Science and this site as well that tend to support the concept of non-vented structures. One article involved a fan driven vent scheme for an "over roof" gap that actually showed very poor air exchange under the roof deck. Additionally, during transitional weather periods in spring and fall it might be more likely that you could be actually warming the underside of the roofing surface which could promote melting. There are many articles on the site discussing venting and my perception is they favor non-vented for low pitched roofs if not all roofs.

    Florida studies have demonstrated very little difference in vented vs unvented roofing material temperatures and a different article made a compelling case that venting attics was less effective than believed based on the contention that the main source of heat load was the radiative heat from the roof deck not the heated air in the attic. Imagine taking refuge under a sheet metal lean-to in the blazing sun. Completely free ventilation, yet being cooked like a rotisserie chicken. Moisture issues aside, air exchange alone was not altering total heat flow enough to improve the delta T of attic temperature to living space.

    I mention this, as a vented channel under the roofing material is like a mini attic. The radiative heat from the roofing surface being introduced to this channel would necessitate a very high air flow simply to reduce the air temperature. The radiative condition is unaffected. If you do elect to use metal roofing, I have noted one installer (southern, humid climate) using 1x sleepers on top of the ice/water barrier to avoid direct contact with the material due to the metal's potential temperatures. The sleepers are laid out with a sloping angle (like dashed lines) to allow condensate that forms on the metal's underside to drain off the roof, not for air flow. I think the Adirondacks can get quite humid, but I don't know if it gets really hot like Houston.

    If you are looking for input in general, I would think carefully about the middle low roof connecting the two main areas. If you are subject to as much snow as AJ and others indicate, then wind flow patterns may deposit much more in this location. The flashing details at the walls would be one concern, where the snow will fall or need to be pulled down to is another.

    Edit: An issue I must be completely misunderstanding is the question of the roof sheathing being able to "breath". I had thought that with unvented roof assemblies with closed cell spray foam on the bottom side of sheathing, one was simply forced to make sure that the sheathing was dry on the day you sealed it off forever. The unvented metal and asphalt roof profiles numbered 4-05055 and 4-05051 do not show any venting space. I would think this confirms what I noted above, but I am certainly willing to listen if wrong. A current discussion of this is going on.


  14. baking_fool | | #14

    Roger - I am planning to propose to the architect function that the connecting hallway roof be pitched. I am concerned about that roof. FYI - it is about 10' between buildings x 15' deep including overhangs.

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