I’m constructing a shop building for myself – 44′ x 30′ with a second floor/attic space. The main floor will be heated workshop space with a ceiling hung unit heater. I plan on using the attic space for a woodworking shop, unheated for now.
I am using Room In Attic trusses for the roof construction. The top cords are 7 1/4″ deep. The bottom cords are 11 1/4″ deep. Looking for advice on the best insulation strategy. I want to avoid spray foam due to cost and quality issues. I plan on using rock wool for its fire resistance and higher R-value compared to fiberglass. Should I?:
A). Insulate the ceiling space between the main floor and the attic space with 11 1/2″ of rock wool? Also insulate the 7 1/4″ top cords in the attic space with 5 1/2″ rock wool (leaving an 1 1/2″ ventilation channel above the rock wool to the bottom of the roof sheathing – I’d use Accuvents for this).
B). Fur out the 7 1/4″ top cords to get enough room for the 1 1/2″ ventilation channel and 12″ of rock wool? My plan would be to use metal studs, attached on edge, to the bottom edge of the truss cords. The “C” shape of the metal studs allows for easy attachment to the wooden truss cord. The rock wool will fill the space in the metal stud, helping to eliminate thermal bridging.
C). Your idea :).
Replies
https://www.finehomebuilding.com/membership/pdf/7319/021228066.pdf
Metal studs will conduct quite a bit of heat, and they still cost more than wood.
The article above shows several options. If you want to insulate the top, (which makes sense to me), expanding with wood 2x4 suspended with plywood gussets makes more sense to me than the metal studs.
I would consider using poly foam below, and either rock wool or fiberglass between the top elements. You can get fire rated foam, and leave exposed, if you would be careful about damage. Compare the foam board R value with the R value of a stud and insulation assembly.
You did not mention what, if any, covering for the inside surface.
for a shop, the utility of the space and desired R values of the insulation depends on what your use profile will be. If you will only use (and heat) the shop for a couple hours a month in the winter, it is hard to justify residential-level insulation levels from a cost basis. (just a point to consider)
On a related note, if your use will be periodic, insulating the ceiling of the lower level can make sense if rapid warm-ups and use in extreme winter conditions is a concern. (so you don't have to heat the whole structure)
Thanks Uncle Mike for your reply. I will cover everything with fire code drywall - 1/2" on the walls and 5/8" on the ceilings. Usage-wise this shop will be intermittent, some weeks everyday & some weeks not at all. The lower half will be for car restoration (old air cooled VW's) and car repair. The upper level with be woodworking (tablesaw, bench, joiner, drill press,bandsaw, etc.)
BTW, I did cost out metal studs vs. plywood gussets & 2x4's. With the crazy high lumber prices right now(thank you COVID) the difference in price is only a few pennys per truss. The labor savings tilt things in favor of the metal studs.