One of the big revolutions in building technology has been the significant increase in insulation. And I’m not just talking going from R-13 walls to R-21. No, what I mean are the super-insulated buildings you see in both LEED and Passive projects (often with walls as high as R-40!)
The revelation, of course, is that energy is an increasingly expensive commodity, and that insulation is one of the cheapest, easiest to use building materials available. Simple changes to the design and framing can have a dramatic impact on energy use…to a point where homes can be heated by the equivalent of a hair dryer! Yes, that’s right – imagine heating your home with just a few hundred watts…
How much is enough (or is there such a thing as too much)?
Of course we all have a budget, so determining the “right” amount of insulation is critical. At the Live/Work project, we were recently faced with this dilemma. The plans called for 2 1/2″ thick insulation under the slab perimeter, and 1″ in the field. Since this idea of multiple thicknesses for insulation seemed somewhat silly (it was easier to just insulate the same thickness across the field) we ordered 2″ for everywhere under the slab.
However, after giving this some more thought and research, we considered adding more than one layer of foam. The question was: how much makes sense?
Do the Math
To help decide how much foam we needed, we performed a series of calculations based on the loss of heat per hour (BTU) and assumed a number of heating days per year. Here’s how we started:
- Assume 50 degrees outside, 70 degrees inside (20 degree differential equals “Delta T”)
- 1000 sq ft of surface
- Formula to calculate BTU loss: 1/R-Value x Delta T x Area.
Now, we were planning on using 2″ board across the surface – this would provide us with an insulation value of R-10. Pretty much standard building code these days. However, passive houses use as much as 16″ of foam (R-80), so we needed to dig a little deeper in our analysis.
- 2″ Pink Board (R-10): 1/10 x 20° x 1,000 sq ft = 2,000 BTU/hr – This was our baseline
- 4″ Pink Board (R-20): 1/20 x 20° x 1,000 sq ft = 1,000 BTU/hr
Ok, so I’m not sure what a 1000 BTUs looks like, but loosing 1000 of them per hour sounds significant. But what about the cost?
- 2″ Pink Board = $1,097 per 1000 sq. ft.
- Subtract the cost of gravel that will no longer be required under the slab ($216)
- Total additional cost for 2″ board = $877
Next, we needed to calculate the cost of a BTU. This is a little tricky, since the gas company measures in “gas units” and “therms.” So, to avoid getting even more geeky than what we’ve discussed so far, it boils down to this:
- 1 Therm = 100,000 BTUs.
- In Portland, using NW Natural, at current rates, the cost of 1000 BTUs per hour is roughly .015 cents (based on NW Natural’s recent rate reduction – it had been closer to .017 cents per 1000)
- If you’re using electricity, based on the rates in our NE neighborhood, your cost is roughly .027 cents per hour.
The next calculation gets even more subjective and is based on the number of heating days per season (in other words, out of 365 days per year, how many days require you to use the home’s heating system).
- Based on a 150-day season, running the system 24 hours a day, our annual heat loss cost is $54 (gas) at 1000 BTUs per hour.
- At that cost, and factoring in the $877 for extra foam, it would take us 16 years to obtain payback on our additional 2″ of foam.
In the end, we decided to split the difference and go with 3″ of insulation, achieved by two staggered sheets of 1″ and 2″ foam. This decision was made for two reasons. First, going to 4 full inches of foam was a bit of a budget buster. With the 16 year payback, it was a tough sell, especially considering that this math was based on several factors:
- We’re assuming 24-hour occupancy of the lower floor. Since the lower area will be used primarily as an art studio, its use will be much more limited in duration.
- We’re assuming a 70 degree constant temperature. In reality, a working studio space doesn’t need to be nearly this warm, as the act of working and moving around the space diminishes the need for higher temperatures.
One of the other reasons we decided to go with the 1″+2″ foam combo was that it would allow us the benefit of staggered seams. In other words, by overlapping the sheets of foam, we’d achieve a benefit of greater vapor resistance than if we had just used a single layer of 2″ or even 4″ foam.
There’s a lively discussion of this topic over at the Green Building Advisor. Just as we discovered, there is a tipping point for insulation where the cost of installation starts to outweigh the benefits. Other resources include (of all things) a great page from Fairbanks Natural Gas which shows how to compare the cost of natural gas to other heating sources. Of course, if you are factoring in CO2 emissions, there’s more to consider than just dollars and sense.
And, to get some perspective, here’s a photo of what 16″ of foam looks like (Photo credit: GreenBuildingAdvisor.com)