Insulation and R-value go together like hard work and sweat. R-value is, of course, the resistance to heat flow. We’ve been taught to think more is better, which is true to a point. I’m changing the insulation in the wall of my home from R-7, originally installed in 1952, to R-15, and I’m excited to see how much more comfortable my home becomes. There is an argument that at a certain level, more insulation will cost dollars to save pennies. At what point do we reach diminishing returns?

Insulating products have different R-values per inch, closed cell spray foam has an R-value of around R-6.5-7 per inch. Rockwool’s Comfortbatt is around R-4.2 per inch and XPS foam insulation is closer to R-5 per inch. No matter which insulation we choose to use, the resistance to heat flow will be the same as long as the installed R-values match the different insulation types. R-20 equals R-20 regardless of if we choose to use closed cell spray foam or dense packed cellulose. But here is the question, at what point is enough R-value enough?

Is stopping 75% of all conductive heat flow enough? (That’s R-4 by the way.) How about 98% of all heat flow? (Rounding has the 98% reduction between R-41 and R-66.) So, how did I calculate those values? We do it by converting R-value to U-factor. U-factor is the rate of heat transfer and is calculated by using the following equation: 1/R-value=U-factor. So, let’s take that R-4 insulation and turn it into the U-factor value. 1/4=0.25, so R-4 is equal to U-0.25. This means that 25% of the available energy is passing through the R-4 insulation, or in other words, we have stopped 75% of the heat flow.

Now, let’s move up to R-13, a typical 2×4 wall assembly using standard fiberglass batts as the insulation. 1/13=0.077. Only 7.7% of the available energy is making its way through the fiberglass insulation, we have stopped 92.3% of the energy flow. R-20 stops 95%, R-30 stops 96.7% and R-40 stops 97.5%. We’ve doubled the insulation level from R-20 to R-40 with only a small increase in heat flow reduction.

To be clear, this only counts towards conductive heat flow at the center of the insulation, convective and radiation heat movement is not taken into account. We are also not including other framing and building components, just the insulation. There is a very good argument to used R-20 cavity insulation with continuous exterior insulation. Another argument is these small reductions spread out over the life of the home do add up, it’s just the monetary reasons are spread out over a long period of time.

When is enough, enough? The answer to that question is the same as so many other building science questions, it depends…

Another great topic this week! As always it depends, and the waters get muddy when we switch from nominal to effective. But the law of returns remains!

Cheers, looking forward to the next one.

Aron Jones

Thanks Aron!

Great article thank you. I’m new to this forum and have been enjoying learning about Building Science. I’m am getting tired of “it depends” (It’s not just you, Randy). Is the science and building materials too new to say that A + B in zone Y…do X? I am wondering and wondering.

Hi David,

I get your frustration. I try to limit my blogs and advice to cold climate construction, but that includes climate zones 5 through 8, and with there being so many ways assemblies can be built, it’s hard to give exact advice that covers all the situations for a build. The last thing I want someone to do is implement something I would do in my area in Kansas City, the outcome might not be good.

That said, I do try to answer specific questions I receive either on this blog or by email. I warn the person asking the question that I am not on site and have to base my answers on how I understand the question they are asking, but I try my best to be specific in the answer. If you have a question, I’m more than happy to give you my thoughts.

Randy

The diminishing returns of insulation thickness is a legend referenced by many builders and architects which is actually incorrect. For general public it causes objections to “too much insulation” which is hard for high-performance designers and builders to counteract.

The U value is not a factor but a rate (or speed) at which the heat transfers through a material so expressing it in percentage terms of “available energy” is scientifically wrong and leads to incorrect conclusions.

An R-1 wall doesn’t have a 100% heat flow but rather 1 watt of heat flow per area of a wall. Comparing an R-20 or R-40 wall to an arbitrary R-1 or 1 watt of heat flow is just that — arbitrary. A 6 inch concrete slab at R-0.6 doesn’t make it pass 1/0.6 or “166% of heat flow”, right?

Famously, a 50% drop in stock market requires a 100% increase afterwards to get to the original value. The reference point is what matters and creates the perceived difference.

So the first layer of R-10 insulation reduces the heat flow by 90% (from what it was without it) and adding another R-10 on top of that will reduce the remaining 10% (of the original heat flow) by another 90% or to 1% of the original rate. So the second layer of insulation wasn’t less effective — it reduced the heat flow

by the exact same ratebut not the same absolute amount. This is what creates the confusion.So how much is enough? After adding enough for basic things like thermal comfort and condensation risk, it boils down to the cost of energy and its sustainability over the lifetime of the building. The passive house limits of 15kWh/m2 of energy per year or 10W/m2 of heating/cooling load are great guidelines based on the maximum amount of heat that can be transported with the ventilation air. It is also low enough to require paying attention to thermal bridges and air tightness. That amount of energy is easily available from photovoltaics during the summer and can be cheaply extracted from the ground via heat pumps during the winter.