Which R-Value Cuts Heat Loss in Half?
I’m evaluating between two wall designs, and considering the rule that doubling the R value cuts heat loss in half. However, which R- value is that considering between the various R-Value? Effective R? whole Wall? I’ve been reading plenty about insulation and wall designs, but have not found the answer to this yet.
For example, these two walls;
2×6 with R-19 batt, plus 1” rigid (R-5) is R 24 or effective R-21
2×4 double wall with 2.5” gap and R-32, or effective R-28
The double wall has a 42% increase in R-value by design (R24 to 32), but only 33% increase in Effective R (R21 to 28). If I’m following the rule correctly, the reduction in heat loss would be either 21% or 16%.
To be honest, the reduction is getting pretty small for the investment in the double wall for my case in Eastern Canada, edge of Zone 5 to Zone 6. My modeling shows the walls with effective R 24 (R20 plus 1.5” ci) contribute 20% to the overall heat loss. As I crank up the wall R-value (all the way to R50), the overall design hourly heat loss doesn’t drop significantly.
Because walls are only 20% heat loss in the grand scheme, it isn’t making significant changes overall I believe. I saw almost the same overall gain by doubling the under slab insulation from 2.5” to 5” or attic insulation from R50 to 80. I believe it could be due to single level with more slab and ceiling surface area than two story (wall to ceiling/slab ratio).
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Depending how much glazing you have, increasing insulation could be insignificant overall. I suggest reading this Instagram post from Christine Williamson: https://www.instagram.com/p/CUPci0_tzsw
Very insightful.
> "doubling the R value cuts heat loss in half"
> "Which R-Value"
The answer to your question depends on whether you want to cut heat flow in half across specific components or the entire assembly. If you want to cut heat flow in half across the center-of-cavity wall section, use the corresponding R-value. If you want to cut heat flow in half across the 'whole wall', use whole wall.
If you want to cut heat flow in half across the entire assembly, you need to account for every component (as modeling attempts to do).
Note that the equation for heat flow is Q=U*A*dT
Halving U-factor (doubling R-Value) halves heat flow. The U-factor in this case should be the weighted total for the entire assembly if your concern is with heat flow across the entire assembly.
https://www.energyvanguard.com/blog/the-diminishing-returns-of-adding-more-insulation/
https://www.energyvanguard.com/blog/4-types-of-r-value/
I would add as a caveat that 'diminishing returns' is a technical point, not a financial or moral one. Sure there are technically more gains from decreasing window U-factors given the mechanics of heat flow, but doing that may be MORE financially burdensome. In other words, the financial calc differs from the purely technical one given the cost of upgrading various building components (which is why dumping obscene amounts of loose-fill insulation in the attic is so often recommended: it's a diminishing return on heat flow reduction, but it's easy and cheap compared to getting that imported triple glazed window).
When it comes to walls, builder familiarity/comfort (and hence cost of labor) needs to be considered in addition to pure material costs. Some builders may be more comfortable slapping up a double wall than dealing with exterior CI, especially if it's squishy rockwool.