Written by: Joe Pasma, Premier SIPs Technical Manager, P.E.
What’s in an R-Value? A good understanding of R-Value and air leakage in a home or building is crucial for any one that pays a utility bill. Besides home owners, even professionals like builders and architects often have not thought through R-value versus air leakage and they too, need to be educated on the topic.
So what’s an R-Value? Technically, it’s a measure of a material or an assembly’s ability to resist heat flow. The higher the R-Value the lower the heat flow though the material or assembly. This is why the new energy codes are requiring a higher R-Value. The theory is that by mandating higher R-Values for walls and roofs, less heat will be transferred through the walls and roofs thereby reducing energy consumption and saving the country from over consumption. (This is bad for the utility companies, and those on Wall Street that invest in the utility companies.)
Well that is all fine and dandy if that was the only way we lose heat from a home or building, but it’s not. We also lose heat when warm air finds its way to the cold outside through openings (air leakage). Take an open door for instance. How cold does the room get when the kids forget to close the door behind them on the way out to play. Well here in Minnesota, especially today with the high temp still below 0o F, it doesn’t take long at all.
The same thing happens in our homes and buildings if care is not taken during construction to seal up all of the holes, protrusions, joints, openings, seams, cracks and all the other things inherent with typical stick construction. It’s like the contractor left the door open permanently. In fact Oak Ridge National Laboratory, ORNL, has a study that shows sticks are 15 times leakier than SIPs.
A way to look at the combined effect of R-Value and air leakage (al beit over simplified) would be to just think in terms of R-Value alone. So for this over simplified model, let’s say that since SIPs are 15 times tighter than sticks, we’ll assign an R-Value of 15 to the SIPs and attribute this to the lack of air leakage.
Now let’s compare a stick framed wall and a SIP wall. Here in Minnesota, the code required minimum R-Value for residential wall construction is an R 19. The stick framed walls are built to that minimum. So R 19 for the sticks. A 6” SIP wall has R-Value on the order of R 21 or thereabouts. Now when we add in the effect of air leakage, sticks stay at R 19 and a 6” SIP wall goes up to R 36. R 21 + R 15. (Not really, but remember this is an over simplified model for most of us that just want to pay less for on our utility bill.) That makes the SIPs almost twice as resistant to heat flow in this engineer’s mind.
The point of this simple model is to show that it’s not so much the R-Value alone that makes something more energy-efficient, but the combined effect of insulation with proper air sealing.
As a testament to these effects I need only look as far as my monthly utility bill. Our local utility tracks our gas and electric consumption on a monthly basis and sends us reports bimonthly to show us how, we as individual users, compare to our neighbors. I live in a home that used wall and roof SIPs for an addition and the original batt insulation, in the stick framed cavity walls was replaced by rigid insulation and the cracks were sealed up. Our home consistently ranks in the top 5% energy-efficient homes in our square footage range. This means our utility bill of $150/month is less than one-third of what my neighbors are paying each month. You add it up. It pays to build with SIPs.
The thing to remember about R-Value is that R-Value is only part of the story when you are looking at the energy efficiency of a home or building. R-Value combined with the lack of air leakage is the real ticket and SIPs give you that combination now and they do it economically.
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