Attic Insulation: Part 1

We've started the first phase of insulating our attic, and are on our way to having more energy efficiency, better climate control across our house, and storage we can use year-round.  While a lot of our attic may be used for a kids' space at some point, the real need it will fill is storage.  With all our basement work, we've run out of a lot of what we used to use for storing off-season items!  It'll be great to have that back.

There were a lot of considerations we made to plan the insulation upstairs, and I've been thinking about this for a really long time. It really started with preventing ice and eliminating thermal transfer through the roof: this is the main source of heat gain in summer and heat loss in winter, and we really hadn't made major improvements in that area.  I wrote about that issue this winter, and about our initial plans too.

​Vented Attic

One of the most important decisions in planning our attic was to decide if we were going to have a vented or unvented assembly. What that means for our attic is where the insulation is in relation to the underside of the roof (the roof deck).  Vented attics allow air and moisture to pass through a cavity along the roof and leave the attic, while an unvented assembly is completely sealed off from moisture and air penetration.

With a hundred-year-old house and a new metal roof that should last the next hundred years, we decided to use a vented roof.  Since air and moisture barriers need to be near-perfect to avoid problems on unvented roofs, I wasn't confident that it would be able to stay near perfect for the life of our roof.  Without the typical 20-year lifespan of an asphalt roof, issues with ours could easily be missed and become dangerous. Vented it is!

Baffles and Insulation

To vent our attic, we installed styrofoam baffles by stapling these along the roof deck.  Since they are very poorly sealed, and the the purlins and OSB leave plenty of gaps, air and moisture should be able to easily move along the underside of the roof deck to the ridge vent. 

The insulation we chose is Rockwool: since our attic has 2x4 construction, we could only get R-15* insulation in the joist cavities.  This is nowhere near the R-49 goal for our area, but it is a start.  Eventually, we'll add more insulation under the rockwool that will provide a thermal break and contribute to air sealing as well.  

*R-value refers to the resistance to heat transfer.  The higher the better.  U-value indicates insulating performance.  U is the inverse of R. U=1/R

Installation

The installation for these first two steps was... HOT.  Since we had nothing in previously, heat was passing through our roof into the attic very easily.  Making this process very uncomfortable, but also worth it.  The baffles were stapled into the purlins, and the Rockwool was tucked into the joist cavities.  It was a simple, but important part of getting this closer to finished.  It also made the attic much more comfortable for work.

The other important factor in making these changes was removing the collar ties at every fourth rafter bay.  Now, we have head room across the attic!  This is one of the things we had evaluated early on with a structural engineer.  These were not structural, and were approved for removal.  Do not make that decision without consulting the experts!  We got really lucky on this one, and with a lighter weight roof that sheds snow easily (even with the solar system), we know we're in good shape.