What is the minimum R-value for continuous insulation to impact thermal bridging?
JeffHewitt
| Posted in Energy Efficiency and Durability on
Some rigid insulation products are R-3.6. Will this actually stop thermal bridging? If so, why not just add furring strips to each stud and then cover with osb?
JJ
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Replies
There is no product or R-value that will "...stop thermal bridging..."- it's a meaingless concept, because it's all relative. A 2x4 is about R4.2- doubling that would be "reasonable", but R8.4 is still quite a bit more thermally conductive than an R13 batt in the cavity between them.
But 2x4/R13 + R4.2 continuous insulation won't even meet code minimum performance in US climate zones 3 & higher.
Essentially roughly doubling the framing fraction's R-value whatever that might be, (a 2x6 is about R6.6) would be a good start, and would generally be considered a "thermal break", but there is no such thing as stopping thermal bridging- nothing done to the other layers reduces the thermal conductivity of the materials.
A more reasonable question would be "why not use strips of rigid foam insulation between the studs and exterior OSB?". You could and it would reduce thermal bridging, but it would reduce racking strength of the wall and it wouldn't do anything to reduce cold sheathing. So better to put such strips on the interior side.
JJ,
As Dana pointed out, insulation reduces the rate of heat flow. It doesn't stop it. So you can't stop heat flow with insulation. All you can do is slow it down.
If you are designing a wall and you are concerned about thermal bridging through the studs, it would be helpful to know your climate zone or your geographical location.
For more information on this issue, see How to Design a Wall.