Looking for some input from the GTAM reno brain-trust.
I posted up my "loft" renovation a couple of pages back. My question is regarding "strapping" out the rafters to insulate.
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The rafters are 2X4 (measured 3.5" deep), 16 inch spacing and while it would be considered undersized by today's standards they have been doing the job since 1941.... I want to add R24 Rockwool so I need to strap them out to fit the 5.5 inch insulation plus the air gap above it (so min 6.5 inches). Adding some strength would still be a bonus...
My first thought was to add a second 2X4 glued and screwed edge to edge making a "virtual" 2X8. The issue is as expected 82 years on the existing 2X4s look straightish to the eye but they are bowed down and adding a 2X4 like this will add a lot of stress.
My next thought is to either buy 2X2s or to rip 2X4s down and stack them to achieve the above but it should be more forgiving and better conform to the existing rafters. I end up with the existing 2X4 and two layers of 2X2s.
You have a couple of options, but before insulating I would evaluate the roof. 2x4 rafters are OK, I have redone a handful of houses with them. few things I always do:
Easy and quick:
Rip 1/4" OSB strips 7.5 to 8" wide, nail them to the same side of the each rafter. This gives you a 16" cavity with 2.5" air gap and 5.5" for your R20 batts. It will also add a bit of rigidity and 1/4" is thick enough to take staples for your vapor barrier.
Upside - quick, easy and cheap. Down side is you can't panel or drywall.
Better:
1) Level the ridge, straighten any bowed rafters. 2x4s rafters tend to bow more in the center if the structure. This can happen if the walls are slightly tilting out ad from additional snow loads that happen in the middle of a roof. I'd bet you're an a little lower in the middle than at the gable ends. If the walls are tilting outward, pull them back in place using a heavy ratchet strap in the middle of the building. This will level the ridge somewhat.
If the centermost rafters are bowed, you can 'bend' the rafter back by loosening it's collar tie then wedging a jackpost post between the floor and the midpoint of the rafter (don't jack the ridge boad). Tap the 2x4 wedge at the bottom until it pushes the bow out of the rafter. While under tension, reattatch collar tie.
I always reinforce 2x4 rafters with ply or OSB triangles that run 2' down from the ridge board. This also gives horizontal edge with a little space above so you can install gable or ridge vents (I prefer gable vents in old buildings as you don't need to walk the roof -- might be tough for you with blocked gables).
Add collar ties to each rafter, it looks like you're every second -- the untied rafters will sag more than the tied rafters.
Best:
Add full width collar ties to every set of rafters, then blow in or lay bats flay in the 'attic' space. You can use your existing vents, you just need a handful of styrofoam baffles to get air from the soffit to the ridge. If you want to panel the ceiling, tie the collars to the rafters 3' back from the walls with 2x4, ply or OSB triangles.
Want the Cathedral look? Furr out the rafters using 2x6 metal studs screwed to the bottom of the rafters. Then you go the other way with steel battens. You will need batts designed for steel studs, they are a wider to fill the stud cavity. They will only act as spacers, but they are light and easy work -- you can hang T&G or drywall off this.
