Welcome back to Fine Homebuilding House Kansas where today we are standing on our air barrier. We are excited about this detail because it’s going to solve a lot of problems down the road. The vented over-roof, or back-vented roof, has become a key feature of the build because it’s helping us solve some problems such as the cantilevered exposed rafter tails that make up our overhangs. The rafter tails are continuous to our interior and we’re accepting that thermal bridge sacrifice to keep the tongue-and-groove aesthetic on the back of the overhang.
Choosing insulation for a vaulted ceiling inside
In order to have the vaulted ceiling inside and avoid using spray foam, we like working with Rockwool Comfortbatt. On past projects in our market, we’ve seen improper and incomplete spray-foam applications with voids, shrinkage, and delimitation that could create pathways for moisture-laden air to reach the back of roof where it could condense into liquid water that would stain the ceiling and potentially create far worse problems like mold or rot.
In comparison, the vapor-open Rockwool allows drying if moisture-laden air ever did enter the system. That coupled with the high density of the product for ease of cutting and slightly compressed installation mean we will achieve contact on all six sides of the cavity to eliminate any convective looping. Since these vaulted ceilings share the structural member with the roof and we aren’t venting that cavity we want to be certain that we allow for the maximum drying potential should any water vapor enter the system from the interior.
Adding another layer of decking
In order to have a vaulted ceiling, we really need a way to vent the roof above it. So on top of our air barrier, we install furring strips on top of every rafter and add an entire second layer of decking to separate high temperatures in the attic from the back side of that first layer of asphalt roof.
We have to account for the difference in the thickness from the 1/2-in. ZIP System roof sheathing to the 3/4-in. tongue-and-groove so that our roof will plane out. In order to do that, we used AdvanTech 1-1/8-in. subfloor rips over the tongue-and-groove soffit and then transitioned to 1-1/2-in. furring strips over the ZIP roof decking. (We added Huber’s peel and stick underlayment over the tongue-and-groove to protect the wood, which is why we used 1-1/8-in. furring rather than 1-1/4 in.).
One of the reasons for building an over-roof is that it will provide a longer life for our shingles. In the Midwest we have to deal with hail and other strong weather conditions; you can’t out-guess natural disasters, but this roof will be well-ventilated and should last our homeowner a really long time. The same way a rainscreen extends the life of the paint job on your siding, this back-vented situation does a big service to your house.
Prevent bugs during ventilation
For our subfascia and fascia, we ran the tongue-and- groove an inch back to create a vent space. We then have a furring strip on top of that 3/4-in. material, as well as a bug screen in the ventilated intake shoot to keep out any insects in this space.
This space will allow the air to flow free all the way up to the ridge, which will help reduce the temperature of this upper roof deck.
If this were a typical hot roof, this roof deck compared to the interior temperature on a hot summer day could be a 50°F temperature difference: 70°F degrees inside and 120°F on the roof in the sun. That’s a big difference to make up in that cavity. So by cooling this space, the cavity may only see 90°, which means we’re only dealing with a 20° delta T at that interior ceiling.
Strategies for the back of the roof
On the back side of the roof we can see the exposed framing of our screened porch; this is not covered with ZIP System sheathing because it’s going to have the same material as our soffit and we want it to look from underneath like finished wood. One of the benefits of our over-roof strategy is that we don’t have to resolve for the roofing nails coming through that material because we’ve introduced that gap for our ventilation.
Because the underside of the porch ceiling is that same 1×6 tongue-and-groove as the rafter tails, the under-roof plane is actually 3/4 in. thick whereas the main house under-roof deck is 1/2-in. thick. We make up that difference, once again, with a 1-1/8-in. furring strip on top of peel and stick underlayment that protects the wood until we get a chance to cover it with roofing.
Article source: www.finehomebuilding.com