Comparing Continuous Insulation R-Values in Steel vs. Wood Framing
Originally published by: Building Enclosures — May 29, 2017
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The value of continuous insulation is a source of contention among some building design and construction professionals. Increasingly stringent energy code requirements are calling for minimal amounts of continuous insulation that some claim are prohibitively expensive, structurally complicated, or marginally impactful versus increased cavity insulation.
Professionals are generally aware of the potential for thermal bridging in construction assemblies—the phenomenon by which heat conducts more readily through structural/support members relative to the adjacent cavity insulation, thus reducing the effective performance of the overall thermal barrier. Yet, few professionals understand the effective impact structural framing has on the rated R-value of a cavity insulation product.
Seeing impressions of studs in a wall system during an infrared scan is one thing, but how much of a difference does thermal bridging actually make?
... Well, if your assembly utilizes steel studs, the answer is: A LOT.
The following table presents the effective insulation/framing layer R-values for both steel and wood stud wall framing.
Effective Insulation/Framing Layer R-Values
Table by Daniel Overbey.
One might discern a few observations from the table:
Structural framing will diminish the effectiveness of the thermal barrier. Whether steel or wood, there will be an impact. All building materials conduct heat at different rates. There is not a threshold beyond which a material becomes categorized as "insulation." However, both wood and steel framing will conduct heat more readily than cavity insulation products.
Steel framing conducts a great deal of heat. According to ASHRAE, a layer of R-19 batt insulation is reduced by a staggering 63 percent to an effective R-7.1 when 2x6 metal studs are spaced at 16 inches-on-center.
Wood framing also induces thermal bridging, but it is not as bad a metal studs. Although less conductive than steel, wood will still diminish the effective R-value of batt insulation somewhere between 14 - 18 percent.
When used as infill in a 2x6 metal framed wall (assuming studs at 16 inches-on-center), the effectiveness of rated R-19 batt insulation may be reduced by a staggering 63 percent.
Illustration by Daniel Overbey.
By comparison, when 2x6 wood studs are utilized, rated R-19 batt insulation may only be reduced by about 16 percent.
Illustration by Daniel Overbey.
Continuous Insulation Makes a Difference
An important conclusion to draw here is that continuous insulation is critically important - especially where metal framing is utilized. Moreover, the girts and other supports for the continuous insulation layer should be low-conductive or thermally-broken. That effective R-value of continuous insulation will be negatively impacted by its support system, but the overall performance advantages makes continuous insulation a clear improvement over cavity insulation options.