Underdeck venting around skylights/dormers

[davidforhim]

Above and below skylights it is not possible to run a rafter vent from behind a kneewall to the attic space above. This also occurs where a dormer roof joins into a higher roof in a perpendicular fashion. There are rafter spans that do not exit at the bottom since they end over the living space.

What is the best approach for venting/insulating in this case?
Should I use insulation that will fill the space? Or should I just go ahead and leave a gap even though no air can really flow through the space?

David

[Wgoodrich]

Example is a set of rafters headered off to create a skylight. Now you have a dead end rafter space. I think this is what you are discribing. When you have a dead end rafter space right below where it dead ends you drill holes through to the adjoining rafter space or notch out before you install your decking so that the air may flow between rafter spaces allowing for the air to flow up that dead end rafter space then over to the next rafter space as an outlet. Then teat that dead end with vented notches same as any other rafter space allowing the insulation to be installed not compressed and leaving a 1" air gap between insulation and bottom of decking.

Remember you may notch on the compression side of a spanning lumber but not the tension side. Remember too that ceiling joist tension side is the bottom of the joist as well as tension side of the rafter is bottom of the joist. You may notch the top of a rafter, floor joist or ceiling joist but not the bottom. Exception is a cantilevered span. Then the tension is on top being the reversed due to it flying as a cantilevered span.

HOpe this helps

Wg