Turning a barn into a house: Roof quandary
Hi GBA community!
I am in the midst of converting a not too old timber framed barn into a house for a friend/client. It’s going well, and we have a good plan for the thermal enclosure, in my opinion. There’s a small matter of the roof, though.
There’s an existing screw-down metal roof (maybe 12 years old) over purlins (rough 1x, 16″ o.c.). There is no roofing paper. We are removing half the roof – the North facing slope – to include a dormer, but are hoping for some guidance with the other half – the South facing slope – which we are hoping to not remove if we can help it. The rafters are rough 2×10, 2′ o.c..
Our roof venting strategy is going to be fitting 2″ rigid foam into the bays with 1″ or 1&1/2″ rips to hold the foam down off the purlins (opinions on depth here?), sealed to the rafters with either can foam or sealant (e.g. Big Stretch), and then dense pack cellulose to fill the rest of the bays, followed by a so-called intelligent vapor retarder (e.g. MemBrain), sheetrock, etc. There will be a 9′ ceiling so the dense packed vaulted ceiling will give way to a loose filled small attic space with louvre vents on either side of the gables.
Is there some clever or safe way to either a) not include roofing paper with this strategy? or b) do something else that will mimic the papers function? or c) quit being lazy and remove the roof panels and do it the right way? Is there a d) that I’m not considering?
The future homeowner intends to have a PV array installed on the roof, if that changes the equation any. Also this barn-to-house is in Central VT, building climate zone 6.
I appreciate everyone’s time and attention to this.
Thank you!
Dave
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Replies
David,
The right way to proceed is to remove the metal roofing. Fortunately, this is much easier than it sounds -- especially since a 12-year-old roof is almost certainly installed with hex-head screws, not nails. Removing and reinstalling the roofing is easy.
The problem with proceeding with your plan is that metal roofing gets condensation on the underside of the roofing in certain weather conditions. (The classic example is a warm, humid day in early spring, following a cold night -- especially if the roofing is covered with a thin layer of snow.)
If your plan were perfectly executed, the rigid foam might catch the condensation drips and convey the condensation to the soffit. But the rigid foam won't be continuous, so you could end up with wet insulation.
Note that a layer of roofing underlayment is a code requirement, and there is a good reason that is so.
So the right way to proceed is:
1. Temporarily remove the roofing.
2. Install a layer of plywood or OSB sheathing above the existing roof sheathing.
3. Install a layer of roofing underlayment.
4. Re-install the metal roofing.
5. Create a ventilation channel underneath the skip sheathing as originally planned.
What Martin said on the building science. I offer an alternative option 50-year roofing system:
1. Install plywood roof sheathing
2. Install rigid foam on top of the roof sheathing.
3. Install WRB
4. Install 1x4 purlins
5. Install metal roof.
6. Fill 2x10 cavities with dense packed cellulose or open cell foam.
Thank you both for your input! I told the friend/client what the posted question garnered in terms of replies, and he is now resigned to removing (temporarily) the roof metal and doing it the correct way.
Martin, is installing plywood sheathing strictly necessary? Would infilling the skip sheathing with additional boards suffice? If not, why not? The roof deck wouldn't be sheathed over entirely with 1x boards but nearly so.
Armando, we had discussed the method your describe (I call it an "Operation Knit Hat"), but the previous desire to not remove the roof panels negated looking at it more seriously. We have gone too far down other roads in the project to do that at this point, unfortunately. I appreciate your suggesting it!
David,
If you want to create a solid deck by installing new boards between the existing skip sheathing, you can. Board sheathing works. Just remember to include roofing underlayment between the board sheathing and the metal roofing.