Why 'R value' is not accurate
Robert Koci
Videos Canadian Contractor ProductSteve Maxwell explains why 'R value' makes no sense any more
We have lived with R value as a method of understanding the energy efficiency of a product or wall or house for as long as I can remember (and that’s a disturbingly long time.) Our Tools Editor Steve Maxwell explains how new products are exposing the limitations of R value as a measure of efficiency.
Lesson learned – new questions
1) So what combination of insulating at lowest cost with highest benefit would meet minimum code? Would it be 1″ of spray foam and 4 1/2″ of batt in a standard stud wall?
2) Where should the spray foam be, on the outside next to the sheathing? Or inside next to the drywall, saving 6 mil poly vapor barrier?
Success,
DaveZ
Hi David,
The problem really stems from the fact that the code recognizes R values as the only legitimate measure of insulation performance. This differs significantly from physical reality and it creates a disconnect between real-world energy performance and the code requirements that seek to mandate minimum levels of performance.
So what does this all boil down to? For now, we’ve got to jump through the R-value-dominated code rules, flawed as they are. If you meet the numbers with fiber-based insulation products your buildings will use roughly twice as much energy compared with meeting those same numbers using foam-based products. Foam is also more expensive per unit of R value up front, and though this fact doesn’t mean much in terms of actual building performance, it can be hard to convince clients of that fact. You’ll need to hammer the R value issue out on a client-by-client basis, depending on the sophistication of the people you’re dealing with.
When cost constraints are the major issue, I believe the optimal compromise involves using fiber-based insulation in wall cavities in the usual way, with rigid foam products on the outside of exterior walls. This keeps wall cavities warmer, reducing the risk of internal condensation as well as the internal forces of convection through fiber-based insulations as documented by the people at the Oak Ridge research lab. You’re always much better off putting foam on the outside of a cavity wall than on the warm side. In fact, foam insulation on the warm side will actually increase the chances of internal wall condensation by lowering internal wall cavity temperatures.
Does this make things more clear?
Bye,
Steve
Great, concise explanation
What about roof insulation. Why do you need R50 in a roof and R27 for a wall. If you are using spray foam for a cathedral ceiling do you need to use R50 or can one use R27.
Thanks
John
In my opinion we use R-50 in vented attic spaces to compensate for the cooling effect (during the heating season) of air blowing through the fibrous insulation. The top portion of insulation effected by the wind wash effect allow us to maintain maybe R-40 worth of effective insulation. So your top 3″ of insulation are sacrificial to the effort?