Thermal Bridging... And How We're Fixing It

Previously, I wrote about thermal bridging and why it matters in houses.  A thermal bridge is any material that allows heat to pass through it easily.  A thermal break is the opposite: it is a material that does not allow heat to pass easily, and it is used in a place to disrupt the passage of heat.  By adding thermal breaks in strategic places heat loss can be minimized, and the energy used to condition homes can be reduced. 

We have done a bit of work on this already by insulating our basement: by adding rigid foam along our rim joists and on the foundation walls, we have reduced the ability for heat to transfer between two thermal bridges: concrete and wood.  But we still have a lot of work to do, particularly in our attic.  When our electrical was redone, we didn't take the time to properly reinsulate the attic floor, so our R-value in some places is less than R-10 with poor sealing.  It's like we're leaving a small skylight open all winter.  

Old House Roof Insulation

Our next big area to tackle is heat loss through our roof.  After reviewing a lot of research, talking with architects, local preservationists, and not missing out in online discussions on "old house Instagram" (a seriously opinionated, yet well informed group), we've landed on doing a reverse of the system we used in our basement.  There is also a lot of information on what products are available in the This Old House website. 

​We have decided against using spray foam in the majority of places in our old house for a few reasons.  The biggest is that it does too good a job of sealing against wood surfaces where it is applied.  In most applications, it is applied directly to structure.  Like any unvented application, this can trap condensation and moisture against the wood and cause rot over time.  Since we have a metal roof and no need to replace in the next century, that wasn't worth the risk for us.  There is also debate over how well it maintains its R-value over time as the foam deflates.  We decided to stick with what we know to be reparable and replaceable.  

Rim joist cavity with missing insulation from HVAC install.

We won't hit the expected standard of R-49 insulation factor in our attic space, but we should get close between the attic and main floor.  By ensuring good air sealing and a thermal break, we'll be able to make up for the lower R-value. Our attic is built with 2x4 rafters set at 16" on center.  Not uncommon for our area: but the height already tops out at about 6'.  We aren't going to be able to gain much "countable" square footage. Although, our city allows us to finish the space, but not as a bedroom which requires 7' of head room for emergencies.  

What we will be able to do is add baffles that allow airflow across the roof deck.  This will allow any heat and moisture to escape through the eave and ridge vents.  Then we'll add the highest level of rockwool we can to the joist cavities.  To make sure we have a thermal break, and interrupt heat loss through the rafters, we're adding 2" rigid foam with taped seams to the underside of the rafters.  Under that, we'll do thin, horizontal purlins to attach the wallboard.  If we're really ambitious, we'll add more rigid foam in between those.  

Ridge venting system prior to installation of ridge cap.

Like we did between the conditioned basement space and the main floor, we're also looking into beefing up the insulation between the attic and the main level.  With our bungalow, we only have the main floor, basement, and attic.  Different considerations are needed for houses with multiple floors, but the best thing to do is find ways to separate levels and keep heat on each floor or keep conditioned air moving.  

Conventionally, insulation between the main floor and the attic is blown in cellulose.  This is a great option that allows all the systems to breathe (and even didn't give us issues with our knob and tube!), but since it runs between joists, it doesn't provide the thermal break that applying insulation over joists as well can offer.  We're looking at a combination of systems that will better seal the attic floor in the eaves, and give us a bit of conditioned workspace in the future.  

Ceiling hole cut for vent fan. Cellulose and fiberboard insulation are visible in the hole.

When we had to address our wiring last year, the cellulose between the main floor and the attic was highly disturbed, which greatly reduced its ability to insulate our attic floor and main level ceiling.  Without it, our energy use has changed dramatically. While I don't have a side-by-side comparison because of our HVAC upgrade, I'll be able to compare the data between this winter and next winter.  To be honest, the energy usage this winter has been much higher than I anticipated, but I'm also sure we can make the needed adjustments to help that.