Roof Rafter Bridging

Brush up on thermal bridging 101 here.

Roof rafter bridging.

For this size job you would want a large foam canister and professional spray applicator. Thermal bridging is the movement of heat through a material that s more conductive than the air around it. It can consist of lateral members spanning in between trusses t or i bracing on individual members sheathing or metal strapping. The weight of the piano is centered over a single joist but the bridging transfers some of the weight to the adjacent joists.

Steel framing typically reduces the in cavity r value by as much as 50 while wood framing reduces in cavity r value by a bit less than 10. Plus you get the highest r value per inch for any type of insulation. With steel framing at about r 04 per inch it s thermal bridging on a whole different scale. Place a long piece of framing lumber alongside the rafter so that the narrow edge is flush with the underside of the roof deck and the bottom reaches the ceiling joists.

The main purpose for this type of bracing is to reduce the bucking length of the individual truss members and to assist in erection spacing of the trusses. In either case seal the foam edges well to the rafters with spray foam. Most codes allow foil faced foam to be left exposed. Drive a nail into either end to hold the board in place.

For example with bridging installed between joists a piano leg sitting over one 2 10 is actually supported by several. Structures such as sheds or barns can be frames with rafters every 24 again make sure to consult local codes. Failure to mitigate can account for overall heat loss of up to 30 so it s not a thermodynamics lesson to be taken lightly. The uninsulated roof framed with wood r value around 1 1 per inch means rafter lines covered with snow.

If your project had a rafter length of 8 or less a safe and strong method is to use 2x8 framed 16 on center.