Venting rafter bay with skylight
semar
| Posted in GBA Pro Help on
Scope:
Building living space in attic – sloped cathedral ceiling to 8′ high level, then flat
using Velux windows and fixed skylights in roof assembly.
How do I vent the rafter bay when the rafters are cross-blocked at the skylight framing?
E.g., rafters @ 16″ oc – skylight rough opening 46 1/2 x 46 1/2″.
“Rafters” are 16-inch I-beams.
Venting: soffits consist of 1×4 clear cedar spaced 3/8″ apart. Installed perpendicular to house wall.
Black landscape cloth (perforated) continuously fixed underneath lookouts as bug-screen.
Ridge vent total length of roof, End gable louvers in “attic” above 8′ ceiling level.
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Replies
Drill allowable crossing holes in webs or not. Just having the vent gap is what has been done in the past and worked. You want airtight drywall and no penetrations in the drywall such as recessed lights. Also no humidifiers.
Maybe you can adapt one of the details at post #46 at this blog
https://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs
thanks,
2" hole/s just under the topcord through the osb stiffener ok?
16" joist will be filled with 2 layers of R-20 Roxul, underside gets strapped with 1 1/2" HD rigid foam.
No lightfixtures in ceiling, 5' ponywalls will give areas to install wiring and maybe wall sconces.
Heating: plan to use infrared heating system by Redwell.com -no ductwork, no chimney, no motors and/or fans, maintenance free, silent and comfortable (they also claim less than gas)
Rainer,
My own opinion: you can't vent rafter bays with skylights. People who recommend that you drill holes in your rafter bays are delusional. You won't get significant air movement; all you will get are weaker rafters. (Most engineers don't like to see all those holes and notches.)
If your cathedral ceiling has skylights, you need to build an unvented roof assembly -- in my opinion.
More information here: How to Build an Insulated Cathedral Ceiling.