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Ventilated vs warm flat roof design in cold climate- last minute design decision

JonPiana | Posted in General Questions on
Hello All,

I am needing to make a last second decision regarding the design of a flat living/green roof in Vermont. seems that there are two camps out there for flat roofs in cold climates:
Ventilated cold roof or warm roof with insulation boards atop sheathing which serves to raise the dew point above the structural members of the roof. 
Our original design actually called for a hybrid, with a vapor membrane beneath the rafters, then 5.5” of Roxul batts in the rafters (below the sheathing) and then roxul insulation boards atop the sheathing. However, a call was made on site without our consent sent to install 2×4 strapping for ventilation on our two flat roof bump outs contray to the design. Our understanding is that flat roofs in general don’t vent as thoroughly  as pitched roofs with with the presence of gable vents. 
Now we need to decide wether we go with the momentum of the strapping and install a capped vertical vent for enhanced ventilation or go with our original hybrid warm roof design and try to seal the 2×4 strapping vent (possibly using the membrane that we will be using for the green roof, that will be attached atop the insulation board) 
The fact that it will be green roof may also play into this decision? 
We would be most grateful for any insight you could offer to help us make this decision. And actually, it’s an urgent matter, because of where we are with hempcreting the interface between the flat roofs and main frame of the house (where we have already installed a layer of  GRACE ice and water shield for added protection on the seam which is attached about 1.5’ up the studs on the main part of the house and 1.5’ along the  strapping of the flat roof an in L shape). 
To complicate matters even more, one of the bump outs has already been sheathed with advantech on top of the 2×4 strapping and does not yet have the GRACE membrane, where the other bump out Has just been strapped and not sheathed but Has the GRACE  membrane already installed. 
I’m not opposed to undoing what has already been done if highly recommended. But I would prefer to work whith what we have going if both ventilated ad warm roof designs are worthy for flat roofs in cold climates. Any thoughts? 
Jon Piana
Barnard, VT

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  1. Expert Member


    You can ventilate flat-roofs, but under very specific conditions. You need a very deep air-space over the insulation, and you need high roof lanterns or doghouses as vents. Otherwise there is no mechanism to move the moisture that accumulates in the vent space to the outside.

    I'd stop and carefully examine the implications of adding extra insulation on top of the sheathing in terms of how it affects the areas where the sloped-roofs interface with the flat area. Long before I started buildng these roofs I'd want sections that illustrated how they would work - especially with a "green" roof. This isn't something you get two cracks at, and the consequences of failure are many times more serious than other roof types.

    Edit: I may have misunderstood your description as to there you are using the Grace I&WS, but it isn't an appropriate membrane for use on flat roofs.

  2. JonPiana | | #2

    Hi Malcolm,

    Thanks for your response. I tried reading the linked article but it cut off half way through and there is the “start free trial” tab. I thought I was already signed up?

    Anyway. How much airspace space above the insulation do you recommend? Could you explain or point me in the direction of incorporating high roof lanterns or dog houses in flat roofs?

    One of my flat roof bump outs connects to a gable end and the other flat roof bump out connects to the side of a Monopitch that is sloping away from flat roof so there are no intersections of flat roofs and sloped roofs.

    We are not using the Grace shield as a membrane for flat roofs but rather a redundant damp proof layer where flat roof interfaces with side of Maine house. We will also be using some sort of exterior flashing at this interface as well.

    I hear you on having this detail crystal clear before building.

    Any other guidance would be most appreciated. Thank you.


    1. Expert Member
      MALCOLM TAYLOR | | #3

      From the link:

      "Venting the attic under a low-slope roof is possible but difficult
      So what’s wrong with insulating a flat roof the traditional way? Nothing, really — as long as the job is done correctly: that is, with an airtight ceiling and adequate attic ventilation.

      The problem is that most of these roofs aren’t built correctly. The ceilings leak air, and the attic ventilation is inadequate. There’s just enough ventilation to pull warm, moist interior air through ceiling cracks; once the moist air is in the tiny attic, the moisture accumulates in the cold roof sheathing. The result is rot and mold.

      Here’s some advice from Joe Lstiburek, a principal at the Building Science Corporation: “If you have an airtight ceiling, and you have an air gap of at least 6 inches between the top of the insulation and the roof deck, and if you have perimeter air coming in at vents at the soffit or fascia above the insulation, and if you also have ventilation openings near the center of the roof through some kind of cupola or doghouse — not just a whirlybird turbine vent — there is nothing wrong with your roof assembly,” Lstiburek told me recently. “You can build a 2 foot by 2 foot doghouse that sticks up a few feet, and put in some rectangular vents. If the ceiling is airtight, then the makeup air comes from the outside. That’s the least expensive way to do things.”

      Lstiburek continued, “The problem with this type of roof is that it is rarely executed correctly. Usually, architects don’t want to provide any ventilation around the perimeter. Or the architect won’t provide a deep enough truss to get enough insulation. If you just have a few whirlybird vents and a leaky ceiling, the whirlybirds will suck moisture-laden air out of the building and the roof will rot.”

      Bruce Harley, the technical director for residential energy services at the Conservation Services Group, shares Lstiburek’s contempt for turbine vents. “I dislike turbine vents,” Harley told me. “I’d prefer a big mushroom vent or two over a turbine vent.”

      The right way to vent a low-slope roof
      If you want to build a low-slope roof that is insulated with fluffy insulation, here are the details you need to include:

      Specify very deep roof trusses. The trusses should be deep enough for 12 to 16 inches of insulation (depending on your climate), plus room for an air gap of at least 6 inches between the top of the insulation and the roof sheathing. Even better: frame the roof separately from the ceiling, so that there is an attic that is deep enough for human access.
      Provide vents at the perimeter of the shallow attic. These can be soffit vents, fascia vents, or wall-mounted vents, as long as the vents allow exterior air to connect with the air gap between the top of the insulation and the underside of the roof sheathing.
      Provide one or more vented cupolas (“doghouses”) in the center of the roof. Most building codes require 1 square foot of net free ventilation area for every 300 square feet of attic floor area; half the ventilation area should be located at the perimeter of the building, and half of the area should be located at the cupolas near the center of the roof.
      Perform air sealing work at the ceiling before the insulation is installed. Pay close attention to electrical penetrations, plumbing vent penetrations, the top plates of partition walls, and access hatches. The ceiling should be airtight as you can make it.
      Bruce Harley emphasizes the importance of air sealing. He said, “Besides the standard bypasses — the partition walls and plumbing penetrations — remember that these older masonry buildings often have furring strips at the perimeter walls, and the cavities created by the furring strips may reach into the attic and need to be air sealed.”

      What if you don’t want to depend on roof venting?
      Let’s face it — it’s hard to vent a flat roof. That’s why most commercial low-slope roofs, including the roof on your local WalMart, are unvented.

      In many ways, it’s easier to build an unvented low-slope roof than a vented low-slope roof. If you go this route, there are several possible ways to proceed:

      You can install a thick layer of rigid foam insulation (6 inches or more) above the roof sheathing.
      You can install a more moderate layer of rigid foam insulation (2 to 4 inches) above the roof sheathing, supplemented by a layer of air-permeable insulation below (and in direct contact with) the roof sheathing.
      You can install a layer of closed-cell spray polyurethane foam roofing on top of the roof sheathing, supplemented by layer of air-permeable insulation under the roof sheathing. (For more information on spray-foam roofing, see Spraying Polyurethane Foam Over an Existing Roof and Roofing With Foam.)
      You can install a thick layer of closed-cell spray polyurethane foam on the underside of the roof sheathing.
      You can install a more moderate thickness of closed-cell spray polyurethane foam on the underside of the roof sheathing, supplemented by a layer of air-permeable insulation below that.
      Of course, the total R-value of your roof insulation must at least meet minimum code requirements. Moreover, if you install a combination of foam insulation and air-permeable insulation, you need to be sure that the foam insulation is thick enough to keep the roof sheathing (or the lower surface of the foam insulation) above the dew point during the winter. The minimum R-values for the rigid foam insulation needed for this type of roof assembly are shown in the table below.

      These roof assemblies dry inward
      The insulation methods described above — those used for unvented low-slope roofs — are similar to the methods used to create an unvented cathedral ceiling. To read about the methods in greater detail, see How to Build an Insulated Cathedral Ceiling.

      While vented roof assemblies are designed to dry to the exterior, unvented roof assemblies are designed to dry to the interior. That’s why an unvented roof assembly should never include interior polyethylene. (If a building inspector insists that you install some type of interior “vapor barrier,” you can always install a smart vapor retarder like MemBrain to satisfy your inspector.)

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