Attic Truss, Cellulose & Venting
We are designing a small home (1100 sf footprint) hoping to use attic trusses and two sets of shed dormers to create a small office and art studio space upstairs. I prefer fitting these spaces into the roof, if possible, instead of adding a full 2nd story. The walls are 1′ thick Faswall. I live in North Idaho, and the winter air is often quite humid. I prefer cellulose over spray foams for environmental reasons, but would still like to have R50+ in the roof.
At this point in the design, the attic trusses have energy heels and non-structural nailers on the angled ceiling sections to make room for insulation. There will be a minimum of 18″ space for venting/insulation, more at the heels and over the flat top area. We intend to drywall the upstairs, being careful with sealing and minimizing intrusions – no recessed lighting.
I have two major concerns.
1) How to create the soffit-to-ridge venting channels. Will the moist winter air entering this vent cause any moisture problems in the cellulose? Can these vents be constructed to protect against this, without causing other condensation problems? Will this be complicated with snow standing on the roof and closing off the ridge vents?
2) Will it be possible to successfully install the cellulose using this type of truss system? How to deal with the large triangular area towards the heel? How to deal with the short vertical wall formed by the attic truss? The angled section of the roof?
Thanks!
Ron
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Replies
Ron,
You have two choices. You can either :
1. Create a ventilation channel under the roof sheathing -- I prefer to see site-built channels of stiff material like fiberboard or plywood rather than the polystyrene ProperVents -- and fill the remainder of the space with cellulose, as you plan; or
2. Install enough rigid foam insulation on top of the roof sheathing to keep the roof sheathing above the dew point, and then fill in the area under the sheathing with cellulose -- with no ventilation channels.
Ventilation channels work fine, even in snowy areas. If snow covers your ridge vent for a couple of months, your roof will still perform well.
The best way to handle the triangular area near the eaves is to continue the insulation along the roof plane, right to the eaves. Be sure that insulation covers the top plate of your perimeter wall.
A different perspective: There are environmental spray foams you can use like http://www.biobased.net/. For your application, I would be in favor of 4”of rigid insulation (R28) on top of the roof decking and 5 1/2” spray closed-cell foam insulation (R28+)under the roof deck; AND still use the 2x6 top cord on the attic truss without adding more lumber and it will save you more headroom because is thinner insulation. You will save the venting issues, winter moisture issues from outside, knee wall issues, seal & thermal bypass issues, etc. etc. It would make it so much easier for you and probably save labor $$$. Just a thought!!!
Ron,
Stick to your environmental values and use cellulose (it's all I ever use for superinsulated homes). A vent channel improves performance and durability of the entire roof assembly, but construct it of rigid material (as Martin suggests) from eaves to ridge. Then blow dense-pack cellulose into the trusses (your installer should have appropriate techniques for this). 16" of dense-pack can give you an R-60 roof, leaving 2" for ventilation.
Ron,
I forgot to add that the only ridge vents that perform properly under all wind conditions are those with an exterior wind baffle, such as Lomanco OR-4 or AirVent Shingle Vent II. Continous soffit strip vents are the other have of the equation.
Is there anything I can do during the design of the attic truss to help with the cellulose installation?
Ron,
As long as there is sufficient depth for ventilation and insulation in all segments of the truss, there should be no problem. I generally nail 1½" spacers to the sides of rafters flush with the top edge (in this case the upper chord of the trusses) and a rigid ¼" board material underneath for vent channel.
There are several ways to install dense-pack, but my preference in such a case would be to leave 4" spaces between courses of ½" ceiling drywall, backed by 6" strips of window screen or insulweb, and blow through X-slots cut in the screening. Then the 4" gaps are filled with 3/8" drywall and taped for a flush seam.