Roof structure for passive solar home
I’m working on the roof structure details for our passive solar home. We plan to use the “Perfect Wall” concepts from Building Science Corp. for our frame walls (advanced framing, exterior rigid foam) and would like to make that continuous with our roof structure. However, I need help putting the layers together. Here are some details:
-Standing seam steel exterior
-Rigid foam exterior insulation (polyiso or EPS)
-Dense pack cellulose fill between framing members
-T&G decking for interior finish material
-8′-spaced glulam beams (not necessarily structural – looking for ideas on how to best integrate the beams)
The roof is a single shed (4/12 w/high side facing South). We want to keep the ceiling open in the house, rising in height from North to South. We are working with and architect, but I’m trying to gain a better understanding of what options we have for building the roof using the components described above. I appreciate your feedback.
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Replies
You need an architect who understands the building science sufficiently that s/he can offer framing, air sealing and insulating options that make sense for such a house.
If your architect is not giving you the options you'd like, then you need to find someone else. Or you need to let go of your insistence on one approach to a tight envelope (and one that I have criticized as problematic in many ways). The "perfect wall" approach is viable only if every building element and detail is perfect at construction and remains perfect for the life of the building. A more forgiving building envelope leaves much more room for imperfection.
Michael,
You need to decide whether to install your steel roofing on one layer of strapping or two. If you use one layer of strapping, the strapping will be parallel to the eaves and will not allow continuous venting from the eaves to the ridge. If you use two layers of strapping -- first a layer that is parallel to the rakes, and then a second layer that is parallel to the eaves -- you'll get your ventilation channel.
Either approach will work, but the "cold roof" crowd sleeps better at night with the ventilation channel.
Don't install T&G boards as your ceiling material without first installing an interior air barrier. I recommend taped drywall under the T&G boards.
I just built a similar gabled roof, but used the (structural) beams spaced at 5'-0" o.c., 2 x 6 decking, Typar weather resistant barrier and 1'-6" open web trusses filled with cellulose. Continuous Accuvent channels provided venting from soffit to roof. Plywood sheathing was covered with asphalt shingle roofing. This system would eliminate the need for rigid insulation and provides a thermal break. Good luck with your project!
Jon,
Would you be willing to share a drawing of those roof details with me (schonlau at gmail dot com)? I'm still unclear as to how the beams and trusses fit together.
Jon,
That sounds like a great roof system. How did you blow the cellulose - through the roof sheathing?
Just to be clear on terms and concepts, an open-web truss significantly reduces thermal bridging compared to solid wood framing, but does not create a thermal break, like a continuous layer of rigid foam would (not that I'm advocating foam). There is an important distinction.
Robert is correct in that the open web trusses do not create a thermal break, but the web members of the roof trusses do reduce thermal bridging.
Because the building is timber framed with a wood deck, all air sealing was done from above and insulating done from the peak of the roof through holes cut in both sheathing and vent channels.
Michael, a wall section was emailed to you as requested.
By the way -- is there a way to post a jpg of a drawing to this forum?
Jon,
Not easily. You can always post a jpeg to Flickr.com and post a link here.