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Would metal roofing (say Imperial Rib type profile) on decking and horizontal purlins work for venting a roof assembly with sealed fibrous insulation-filled rafter bays?

The main question is whether the air space provided by the metal ribs would provide adequate air flow. If not, how about vertical 2X 'purlins', forming site-built insulation baffles that would be insect screened at both ends?

Asked by Michael Roland
Posted Thu, 08/07/2014 - 13:43

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2 Answers

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Michael,
First of all, let's talk about code requirements. According to section R806.3 of the 2006 International Residential Code (IRC), when air-permeable insulation is installed between rafters, “A minimum of a 1-inch space shall be provided between the insulation and the roof sheathing and at the location of the vent.”

In spite of this poorly worded requirement, some builders have successfully argued in favor of installing ventilation above the roof sheathing rather than underneath the roof sheathing. With most types of roofing, ventilation channels above the roof sheathing require a second layer of roof sheathing on the top side of the ventilation channels. However, with metal roofing, it's possible to install 2x4 or 1x4 purlins (installed rake to rake) above vent channels created by installing 2x4 or 5/4 x 4s from eave to ridge.

The way I read the code, your idea (depending on channels created by the profile of the metal roofing) won't work.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 08/07/2014 - 15:21

2.
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Martin,
The insufficiency of the roof profile only for venting was my read as well - I was looking for your concurrence. But, I'm hoping to avoid installing two sets of purlins. So if the metal roofing mfg. is OK with installation on vertical (eave to ridge) 2x4s, 12"-16" o.c. (placed over 16" o.c. rafters or on and between 24" o.c. rafters), I'll go with that. Thanks.

Answered by Michael Roland
Posted Fri, 08/08/2014 - 11:50

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