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Minnesota shed roof on a narrow lot: Attic venting problems

user-7029162 | Posted in Plans Review on

Our architect designed home is under construction. Its a 27′ wide (on a 40′ wide lot) x 49′ long simple shed roof. The original plan was to have soffit vents on the low side eave and the high side eave and passively vent the roof. The only thing not shown in the images linked below is that we planned to spray foam the low heal to get R55 (not possible with cellulose alone). This low heal was a result of the unfavorable code related to shed roof height:
http://www.dudebrowtf.com/images/ProjectHouse/atticvent1.jpg
http://www.dudebrowtf.com/images/ProjectHouse/atticvent3.jpg

Because of the close proximity to the neighboring house the code also requires a solid piece of drywall to covers the proposed soffit vents (for fire issues). This info came late in the game and ruins the original venting plan. The builder is understandably trying to avoid any vents through the metal roof. The other issue our builder is worried about is sucking humid combustion air into the low soffit from the other neighboring house. Picture in link: http://www.dudebrowtf.com/images/ProjectHouse/atticvent2.JPG

The architect is recommending a 1/32 HP 12″ fan mounted high on one gable and then venting the other gable’s soffit and venting across.

The builder is worried about the architect’s plan and wants to spray foam the whole roof at an additional $9k+ cost.

Looking for some recommendations. Thanks for any help!!

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Replies

  1. User avater GBA Editor
    Martin Holladay | | #1

    User-7029etc.,
    First of all, can you tell us your name?

    Because of the low heel, you're going to need some closed-cell spray foam no matter what you do.

    My vote: install the minimum amount of closed-cell spray foam needed on the underside of the roof sheathing, and add mineral wool or fiberglass batts to make up the rest of the needed insulation. The only tricky part is figuring out how you can keep the batts in contact with the cured spray foam. Your builder might be able to use steel wire to hold the batts in place.

    We don't know your climate zone; it's either Zone 6 or Zone 7, depending on what part of Minnesota you're in.

    If you're doing a flash-and-batt roof in Zone 6, 51% of the total roof R-value needs to come from the spray foam layer. In Zone 7, you would need 61% of the total roof R-value to come from the spray foam layer.

    If you are aiming for R-55, that means that you need R-28 of spray foam in Zone 6 (4.5 inches), or R-34 of spray foam in Zone 7 (5.5 inches). Once the spray foam has been installed, the rest of the needed insulation (mineral wool batts, fiberglass batts, or cellulose) can be installed on the interior side of the cured spray foam, in direct contact with the spray foam.

    If flash-and-batt is too complicated, you may end up using closed-cell spray foam against the underside of the roof sheathing and then finishing off the job with open-cell spray foam on the interior side of the closed-cell spray foam.

    For more information, see these articles:

    Combining Exterior Rigid Foam With Fluffy Insulation

    Flash-and-Batt Insulation

  2. user-7029162 | | #2

    Thanks for taking the time to reply Martin. I'm in Minneapolis (climate zone 6 I believe). I'll run your suggestions past my builder.

    Thanks,
    Chad DeBaker

  3. user-7029162 | | #3

    In a last ditch effort to save the cost of the spray foam, what about using 2 gable vents on the high side without a fan at all?

  4. User avater GBA Editor
    Martin Holladay | | #4

    Chad,
    Since this roof has a slope of 2:12, it meets the definition of a low-slope roof. This type of roof is hard to vent with soffit vents and a ridge vent (because the stack effect is almost absent). Ideally, if you expect this type of roof to function as a vented roof, you need at least 6 inches of air space between the top of the insulation layer and the underside of the roof sheathing -- and you don't have that.

    In other words, this needs to be detailed as an unvented assembly.

    For more information on this type of roof, see Insulating Low-Slope Residential Roofs.

  5. Malcolm Taylor | | #5

    Martin,

    This is a bit of an unusual case. Unlike most shed roofs, the second section shows an attic which appears to end up with a height of around five feet at the peak. Even though the low slope wouldn't encourage any air movement, seeing as it is only 27 feet wide, don't you think oversized wall vents low on both ends and on the high side might provide enough ventilation to dissipate any moisture that accumulates? That would mean only having to spray foam the area where the cellulose is in direct contact with the roof sheathing.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Malcolm,
    You're probably right. I don't have enough experience to be sure, though -- and I tend to be conservative when it comes to giving advice on roof moisture issues.

  7. Malcolm Taylor | | #7

    ...and complicating things is that the builder isn't comfortable with that approach.

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