By 
Vented attics have a number of benefits, but also one well known and pesky downside: it is very hard to perfectly air seal the ceiling plane given the number of penetrations you have. This results in energy losses but also, often times, pulling less hygienic air from the attic into the home itself. LG Squared, a firm out of Atlanta offering architecture, interior design, construction, HVAC design and building science consulting, found an interesting way around this problem and wrote about their approach in a recent blog. It involves continuous mineral wool insulation on the exterior, loose fill stone wool in the attic and a unique application for zip sheathing.
So, where is the zip sheathing?
In the attic. They applied a continuous layer of zip sheathing on top of the attic joists. They then sealed the seams with tape. Basically, they treated the attic just like you would a wall with zip sheathing. There is no insulation in the ceiling cavity; that will be applied on top of the sheathing. This approach eliminates the need to air seal all the penetrations or worry about the drywall below the sheathing. Basically, you do the zip sheathing right and you don’t need to worry about anything below. An added benefit is that framers love having somewhere to stand as they build the roof. LG Squared combines this approach with a raised heel where the 2×8 roof rafters bear on the exterior wall, to achieve a continuous, uninterrupted layer of R-60 loose fill mineral wool insulation. They also installed 3” of continuous mineral wool insulation on exterior walls, which then meet up with the attic insulation to form a complete thermal boundary uninterrupted by thermal bridging or space limitations above the top plates. We spoke to Chris Laumer-Giddens, Principal of LG Squared, on the reasoning for this approach. “Materials in assemblies react to conditions. The more of the building we can protect from conditions the better it is for the durability of the building as well as the safety and comfort of the occupants. We keep our control layers, moisture, air, vapor and heat, on the exterior of the enclosure as much as possible. This also helps reduce the number of typical obstacles contractors face that can result in installation errors.”
What about the mechanicals? Or attic access?
Chris is a believer that the any gap in the thermal boundary of the home can compromise its performance. An attic access hatch can be done right, to prevent a failure in the boundary, but moving it outside of the boundary altogether eliminates the opportunity of failure during execution. So, they allow access to the attic through a hatch in the porch, and enter the attic from the side. This way the attic is accessible to investigate for roof leaks, if ever there are any, but the hatch is outside the thermal envelope. As for the mechanicals, they are in the conditioned basement, which is inside the thermal envelope, “where they belong” Chris states. The ductwork runs in the floor between the two levels of the home. In their blog, Chris provides a good analogy on why you don’t want your mechanicals in a ventilated attic “Asking your mechanical systems to operate in a ventilated attic, especially in winter, is much like asking your heart to operate outside of your body. You simply wouldn’t do that. It would have to work a lot harder to keep going, to keep the blood flowing and to keep you warm”.
Choice of Mineral Wool
We also spoke to Chris about why he chose to use mineral wool for the project. Turns out, he just prefers it generally, not only for this project “We have used all types of insulation and ultimately find that mineral wool combines the beneficial properties that an insulation product can offer. We keep it simple and do it well”. He especially likes the drying potential of mineral wool in exterior applications, among other reasons “when paired with zip sheathing or a comparable control later, like gyp sheathing and liquid flashing, we like its drying potential for exterior continuous applications.” While these are some of the top benefits, Chris also appreciates that “mineral wool doesn’t burn, isn’t a carcinogen and its R-value improves in colder temperatures, unlike Polyisocyanurate.” While some think mineral wool can be tricky to install and difficult to handle, his experience has been the opposite “Our installers find it is easy to work with” he says “we show our crews a few tricks to make sure there are no gaps and then they fly, with our eagle eye ever watching to ensure the perfect execution”
We encourage you to check out the full blog which has detailed designs of the home and its control layers, as well as images of the house during construction.