Flat roof attic ventilation

I had a question on flat roof ventilation. As we do have space which is formed between the ceiling and the roof structure, do we also have to provide attic ventilation?

In general I have not come accross ventilation on flat roofs, but would like to know more about it. If it is done, how is it taken care of specially for water penetration.
Original Post
after reading that link its still unclear about ventilating flat roofs. The way Mikeelia has explained is if the insulation is on the op of the sheathing then the entire building can be treated as an envelope. But what if there is insulation between the rafters?
quote:
Originally posted by archana:
after reading that link its still unclear about ventilating flat roofs. The way Mikeelia has explained is if the insulation is on the op of the sheathing then the entire building can be treated as an envelope. But what if there is insulation between the rafters?


Correct. You don't ventilate the space you heat or cool to the exterior.

Mike
This is a topic that has popped up every now and then in the Phoenix area.

The situation involves low-sloped roofs with insulation installed directly below the roof deck/sheathing, and whether or not a ventilation space is required between the two.

The reason for providing the ventilation is to prevent water vapor from condensing on the cold roof deck or sheathing without a means of drying out. Otherwise, the constant moisture will corrode metal decks or decay wood sheathing.

The requirement for providing the ventilation resides in Section 1203. The problem in complying with this requirement is found in this statement from the section:

"The net free ventilating area shall not be less than 1/150 of the area of the space ventilated, with 50 percent of the required ventilating area provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents."

So, the big question is: where is the eave or cornice on a roof with a 1/4" per foot slope that is bounded on all edges by a parapet?

Obviously, you can't physically comply with the requirement. Therefore, most insulation is installed on the top side of the deck/sheathing, thereby making the space above the ceiling plane part of the conditioned space.

The IRC addresses conditions that allow low-slope roofs, but the IBC does not.

In environments like Phoenix, where the vapor drive is usually from the exterior to the interior, the roof covering prevents water vapor (humidity) from reaching the interior, and interior water vapor won't condense on the deck because the deck/sheathing is likely to be warmer during most time of the year. So, the ventilation requirement doesn't make much sense for low-slope roof conditions.

Sometimes building officials in my area will allow this type of construction. However, for best energy performance, it is best to place the insulation on the top side of the deck to provide a continuous insulation layer. If installed below the deck, the joists create thermal bridges that greatly reduce the overall effectiveness of the insulation.
quote:
Originally posted by archana:
I had a question on flat roof ventilation. As we do have space which is formed between the ceiling and the roof structure, do we also have to provide attic ventilation?

In general I have not come accross ventilation on flat roofs, but would like to know more about it. If it is done, how is it taken care of specially for water penetration.


I'm late to this discussion, but am running into this very issue now here in California. I've found the following website that has great information on the use of spray foam insulation:

http://www.airtightinsulation.com

As posted elsewhere here, the 2006 IRC allows for the SPF to go on directly to the underside of the roof sheathing, but the 2007 California Building Code is based on the UBC which requires venting. I have to appeal to the Building Official, using the materials I've found (in part on the website I cite above), to allow it.

I don't believe a flat roof will vent properly with openings only at the eaves even if additional vents were placed somewhere in the field of the roof. Hope the Building Official will see it that way also, given the arguments in the documents, and the allowance in the IRC.
Likes (1)
J. Swain
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