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Adding rigid foam above rafters but below ventilated roof deck

user-6848033 | Posted in Energy Efficiency and Durability on

Hi all, wondering if this plan will work:

Climate Zone 6,Nova Scotia Canada
Simple geometry of roof.
Trying to create a ventilated roof system.

Rip off rotten roof sheathing.
Lay 3inch EPS rigid foam over top of rafters that are every 24 inches. R11-12
Planning on screwing on 2 by 4s on the flat from eave to ridge right above the rafter, effectively sandwiching the EPS foam between the rafter and the 2 by 4s.
Screwing on 5/8 plywood as roof deck onto the 2 by 4s, creating a ventilation gap between eps and roof deck.
Adding Nova seal as Ice and water shield.
Metal roof on top of that with ridge vent at the top.

From underneath I would insulate between the 2×6 rafters with 5 and 1/2 inch roxul batts R22
and then a 2 inch roxul board R 8 across entire bottom surface of rafters. Put up poly vapour barrier below this and seal it.

How does this sound? Would give R 41-42 which is not R 49 but still almost 10 points higher than NS building code. Would properly ventilate the roof. I just havent found any information on sandwiching foam between top of rafter and 2 by 4.Is that crazy?

Thanks in advance.

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  1. Expert Member
    Dana Dorsett | | #1

    In zone 6 you're going to need a minimum of half the total R to be outside EPS/fiber insulation, lest the fiber insulation can collect moisture in winter. So with R13.5 (3" of Type II insulation, up rated for the temperature & stackup) the most you can reasonably safely put under the roof deck is R13.

    With your stackup it will probably even collect frost over the winter, then drip liquid moisture on the warmer days.

  2. user-6848033 | | #2

    But if their is a ventilation gap between the rigid foam and the plywood roof deck does the 51% rule you're citing still hold? If there is a vapour barrier beneath with R 41 insulation above, an air ventilation gap and then the roof deck, wouldn't that prevent the moisture build up? What am I misunderstanding here...

  3. Expert Member
    Dana Dorsett | | #3

    Yes, the 51% rule isn't necessary to protect the roof deck, but it's absolutely necessary to keep the fiber from getting wet over the winter, growing mold and dripping on your ceiling.

    The 3" of EPS is a class-II vapor retarder (meets Canadian code definition of "vapour barrier", so moisture drives from the interior aren't going to dry into the vent space. The average temperature at interface of foam/fiber and the dew point of your conditioned space air in winter (assume 4C) determines just how much moisture collects. If the average temp at that point in the stack up is above 4C, it'll stay pretty dry. If it averages much below that, it'll get pretty wet.

  4. user-6848033 | | #4

    Interesting. Ok so if the foam was thinner than 3 inches it wouldn't be a vapour barrier and the assembly would work? Say if I put 1.5 inch EPS foam board?

  5. Expert Member
    Dana Dorsett | | #5

    if the foam was 1" of unfaced Type-I EPS (runs about 4-5 perms) it would sort-of dry into the cavity, but don't count on it not collecting frost. There's a huge range of possible permeance between vapor barrier and vapor open, but you really need the layer outside the fiber layer to be VERY vapor open if it's going to dry into the cavity. (Even 5 perms is still the mid-range of Class-III vapor retardency.)

  6. user-6184358 | | #6

    The roof ply needs to be attached directly to the top of the rafters to be a structural diaphragm for the building. The roof sheathing provides the structure to the roof to transmit the lateral forces your building is subjected to around and down the walls that keep the building upright. So what you propose fails from a structural point of view.

  7. Expert Member

    Outside of high wind or seismic areas, is it really that important that a roof act as a diaphragm? Houses, especially those with metal or shake roofs, were almost always framed with skip sheathing or purlins without incurring any problems.

  8. user-6184358 | | #8

    Yes, it is needed to distribute the forces around the building to the resisting walls. Skip sheathing is in the IRC under roofing - not a a diaphragm. Skip sheathing it is still directly attached to the framing to provide lateral support to the rafters. I would suggest since you are proposing a unconventional design to get it verified by an Engineer or Architect. If you applied sheathing directly to the framing then stacked the foam as described on this site you would be better off, no sense reinventing the wheel.

  9. Expert Member

    I think you have me confused with the OP. I'm not proposing any unconventional assembly. It was a theoretical question. I build in a high seismic area where all roofs have to be sheathed.

  10. GBA Editor
    Martin Holladay | | #10

    You have two choices:

    1. Go forward with your plan to use EPS above the roof sheathing (not simply "above the rafters," but above the sheathing, because EPS needs to sit on roof sheathing to work), but adjust the ratio of EPS to fluffy insulation to meet the 51% rule. For more information on this approach, see Combining Exterior Rigid Foam With Fluffy Insulation.

    2. Go forward with you plan to create a ventilation channel above the roof sheathing, but omit the EPS, and put all of the insulation below the roof sheathing (using 100% fluffy insulation). For more information on this approach, see How to Build an Insulated Cathedral Ceiling.

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