Insulation and Thermal Bridging

Happy New Year!  I'm back with a big old house science topic: thermal bridging.  With all the changes we made last year, heat loss became a big issue, and even just a quick drive around the neighborhood showed me that it isn't just me with this problem... Big gains have been made in the past century, and I hope to be able to integrate some of that here.

​Working on old houses, and particularly planning around my own, the structural differences between old and new are consistently apparent in how systems are integrated.  This is especially true with how houses are insulated and sealed for cold -and hot- months.  Managing insulation becomes just as much about managing moisture as it does about managing temperature.  

Thermal bridging is one of the biggest energy losses in homes, and there are a few schools-of-thought for how to manage it!  No matter what, understanding a little bit about what is happening can help make the best choices for each situation.

Heat Loss and Gain

According to the International Conference of Civil Engineering and Building Services (IOC), ​30% of heat loss in a building is through thermal bridging across materials.  What exactly does this mean?  Different materials prevent or allow the passage of heat through it.  Without getting too deep into the physics of thermodynamics, heat transfers into cold.  A thermal bridge is a material that allows heat to pass through it easily.  

There are a few places in a home that really contribute to this loss of heat, and in turn, increase energy costs.  The biggest contributor to heat loss, is through the roof.  While heat transfers to cold, it also rises.  Since heat rises, thermal bridging through the roof is the single biggest factor that contributes to heat loss through winter.  Likewise, since roofing material is the most exposed to the sun, solar heat gain occurs through the same thermal bridges in the summer. Heat loss then becomes significant when "thermal bridges contribute to a lower thermal resistance of the envelope" (IOC).

Thermal bridging can be problematic both with heat loss and gain, but it also causes issues with moisture.  A glass of ice water sweats in the summer due to condensation across the thermal bridge: the temperature on the outside of the glass drops below the dew point while the surrounding air is above it.  Condensation forms on thermal bridges when one side is above the dew point and another is below because heat is able to quickly transfer through the thermal bridge.  The same thing happens with houses, however condensation on wood framing can lead to moisture problems.

Thermal Bridge Signs

How can a homeowner tell if thermal bridging is a problem?  There are a few ways, both inside and out.  Inside, it's mostly about feeling the places were cold air seems to rush inside. Usually, the major culprits that are felt inside are poorly sealed windows and doors, exposed rim joists between a basement and finished space, and floors over crawlspaces and unconditioned basements and crawlspaces.

Outdoors, the easiest way to find thermal bridging and insulation problems are on the roof deck.  Inadequately insulated roof systems are particularly prone to ice dams: shelves of ice on roof eaves caused by melted snow that runs down off a conditioned attic space and onto a frozen eave.  These can cause leaks as moisture backs up under shingles when it cannot flow off the roof.  Typical strategies to help this are roof-raking and heat coils on roofs to prevent ice build up and provide a place for water to flow, usually into a gutter.

Beyond ice dams, thermal bridging can be seen easily where roof cavities have been insulated between roof rafters, but the wood rafters have been left exposed to the roof deck and to drywall.  Heat can move fairly easily through wood fiber, so without insulating around roof rafters, the roof can have a "striped" appearance where snow has melted over the rafters but not over the insulation.

What Next?

If thermal bridging is an issue in an old house, solutions aren't impossible to find!  In the past, filling cavities with closed-cell spray foam has been a common way to seal and insulate these cavities.  Unfortunately, new evidence shows this can trap humidity in the attic space.  So, it's both a very expensive option, and not necessarily the best for an old structure designed to breathe.

Our own house has a lot of problems with heat loss.  One area where we have made a lot of progress is around our basement.  By using a combination of rigid foam to provide a thermal break and rockwool insulation to allow moisture to exit the components of the structure, we've been able to keep our heat in without creating concerns for moisture.  I can feel the difference in our main floor with the insulation in the rim joists, and the basement bedrooms are among the warmest in the house all winter.

What makes Zip System so cool in new construction is the thermal break and sheathing are all combined in one unit!  So, we're finding ways to bring similar systems in here.  With all the science around how to manage thermal bridging, I'll write about insulating under roofs next!