What is Thermal Bridging? (or Why R-19 is never R-19)

by Erik North on July 2, 2011

Post image for What is Thermal Bridging? (or Why R-19 is never R-19)

 

Like wind washing, thermal bridging is something folks mention all the time during audits (meaning they NEVER ask about it). But what is thermal bridging and why do I keep bringing it up when my customers just want new windows? To understand thermal bridging, you need to understand your home’s wall assembly and the various materials used in its construction.

My House is Well Insulated, Right?

The exterior walls of your home are (hopefully) well insulated. The building shell of your home provides the structural barrier between your dream home’s interior and the rest of the world. It keeps out unwanted air infiltration (mostly), moisture (again, fingers crossed) and retard heats flow through the wall. If well insulated, your home ought to stay warmer during cold winter months and cooler in the summer.

R-value is the measure of how well your insulation resists heat flow across the material in question. R-values ought to allow you In theory (quite literally; material theoretic R-value is tested in lab conditions), R-values are supposed to be an objective comparison of the thermal resistance of different materials. Unfortunately this just isn’t so. Like I touched on in a previous post, we all wish there was a magic, universally applicable number like car gas mileage with which to compare various insulation.

Thermal bridging is one reason you can’t blindly judge insulation based on its R-value.

Standard wall assembly construction (at least in the Northeast) consists of drywall on 2″x6″ studs, 6″ fiberglass batts within the wall cavities, plywood or OSB cladding, Tyvek building wrap as a water control plane and your siding. The R-value for any cross-section of the wall can be found by adding the material R-value of each layer.

A Boring Wikipedia-style Definition for Thermal Bridging

Thermal bridging is when a more conductive (or poorly insulating) material allows an easier pathway for heat transfer across a thermal barrier. The most common form is probably within the eyesight of every reader of this article: wall cavity studs. Suppose your walls have 6″ fiberglass batts. I’m sorry. We’re working to update the building code.

Every 16 inches on center in that wall is a 2″x6″ or 2″x4″ stud board. The fiberglass has an R-value of around 3.5 per inch and the stud is around .75 per inch. The wood studs are allowing heat to transfer through 3 to 4 times faster than the surrounding insulated wall cavities.

Here’s the thermal image from above. It’s from my house where you can see the wall studs and corner hurricane brace through the plaster.

Thermal Bridging of wall studs through building cavity

D'oh! Thermal Briiiiiiidgiiiing!!!!

While the advertised R-value would be R-19 for a 6″ fiberglass batt, the building assembly’s effective R-value is about 3 lower. Bummer.

Cures for The Thermal Bridging Blues

The question then becomes…What to do? Simply enough, you need to eliminate or reduce the thermal bridging. There are many approaches. In new construction, you can build the wall assemblies from SIPs (structural insulated panels) or use advanced framing techniques which reduce the overall number of wall studs. A newer approach involves applying strips of insulation over the wood studs to provide a thermal break.

In retrofit scenarios, sheets of polyisocyanurate can be applied to the exterior siding as a thermal break. If your home has aluminum framed windows, the addition of insulated shades can help reduce that source of thermal bridging. The same holds true for metal framed doors where the addition of a storm door can slow the heat loss.

Thermal bridging is one of the hidden sources of heat loss home energy audits often uncover. Keep it in mind when thinking about the heat loss (and the heating bill) in your home.

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{ 1 comment… read it below or add one }

susan mohl powers January 1, 2012 at 1:39 pm

I should have a thermal imaging photo on my site. We have a current reference on Sailshades I personally made 30 years ago still in use. The house has sold twice.
A Glasgow professor testing a Sailshade last winter reports 54% reduction in heat loss.
We offer 10% referral commissions. Warmest holiday greetings. susan mohl powers

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