2x4 Wall and Roof: Insulation and Ridge Vent

[Danno]

Hi!

I'm building a 14 x 28 workshop w/ gambrel attic trusses. Everything is 2x4 except the bottom chord of the trusses, which are 2x6. Here's a link to another thread with a pic:

http://www.construction-resource.com/forum/ftopic4532.html

I've already nailed the pre-primed osb panel siding directly to the stud walls (2' oc) w/o any wrap. I plan on putting FG batts with either plastic, felt, or foil backing on the inside. Initially, I will insulate only the ground floor walls and ceiling.

Later I would insulate the attic roof but want to prepare for that now. So I have cut a 3" wide swath in the roof sheathing at the ridge. Ridge vents come in 20' lengths, so could I get by with leaving 4' on each end unvented?

And I need to have an air space between the underside of the roof and insulation. So I'd have soffit vents along both eaves the whole length of the building. Air would come in there and exit through the ridge vent.

Any suggestions on how to best do this with 2x4 framing for the roof? Walls?

Thanks!

[Sweep]

With only a 2x4 rafter space to fill I woulld consider foaming it or installing 2" of Styrofoam and forgetting the vents. The foam is costly but so are the vents and there will be little effective air movement in such a tight rafter space. Or the foam board could go on the outside fo the roof.

[Danno]

Thanks, Sweep.

Are you talking about styrofoam sheets that I could cut with a knife and place in between the rafters? If so, I could also put them up as an intact sheet spanning 4 rafters at a time, like I did to sheath the outside roof.

Another way would be to insulate the attic floor between the wall and the vertical web. Insulate along the vertical webs to the roof, then across to the other vertical web. This is what I've drawn in red in the pic. There's less roof to insulate this way. Maybe in this roof area I'd use the styrofoam underneath the rafters, leaving the 3.5" as vent space?

Cheers!

[VALENT]

you can definitely leave the outside 4' without the ridge vents if you decide to go with that.

[Danno]

Great. Thanks, V :-)

[VALENT]

by the way, around here you can buy the ridge vent in the 20 foot rolls or in 4 ft sections.

[Danno]

Oh! I only saw it at Lowes in 20' rolls. VentSure, I think. Thanks :-)