What 'R' You Paying For?
There is a lot of misunderstanding of what R-value really means for consumers. We are constantly told that the higher the R-value the better the insulating power, but what does it really mean? An R-21 is much better than an R-19, right? An R-44 costs twice what an R-22 does so it must be twice as good? Some things are not as simple as you would hope that they would be.
Below are some charts that should help answer the question about what R-value actually translates to as far as energy savings and energy efficiency. When we get to the R-19 values we see that we have stopped 95% of the heat loss thru our envelope and by doubling our R-value to an R-38 we only gain 2% more efficiency while doubling our insulation cost.
Why does building code require such high R-values for our attics?
Fibrous insulations such as fiberglass are susceptible to air movement thru the material and when you have a large temperature difference between one side of the insulation compared to the other you end up with convection air currents. What are convection air currents? We all know that warm air rises and cold air sinks. When you have a ceiling of a house that is at 70 deg. and an attic at -20 deg. the cold air can sink thru the fiberglass insulation until it touches the warm sheetrock ceiling where it warms up and rises thru the fiberglass taking your heat with it. This process is even more accelerated by such things as recessed light fixtures which are mini-chimneys. By increasing the thickness of the fiberglass insulation they can slow down this air movement but they cannot eliminate it due to the light loose nature of the material. Building code is not looking so much for R-value as they are looking for thickness. Oak Ridge National Labs did some testing on fiberglass insulation and convection currents (the test can be found on our General Information page) and found that fiberglass insulation loses 50% of its R-value as the temperature difference increases.
Spray foam insulation, due to its airtight cellular structure, does not suffer from convection currents like fiberglass insulation does. It can maintain its R-value much longer over a larger temperature difference than other insulations. So why don’t we use only spray foam in attics? Cost and return on investment. From the charts, we can see that an R-14 to an R-19 of foam stops 94-95 % of our heat loss. This is the best value for the money and by putting a blow in insulation on top of the foam we can comply with building code requirements. It does not make sense to double the price of the insulation for only a 2% better return!
Why don’t we do a layer of foam with some fiberglass over it in walls?
In a short answer, “dew point”. Dew point is the temperature at which water vapor in the air will condense into liquid water. If we spray a thin layer of foam onto our exterior sheathing and cover it with a fiberglass batt we will not be able to keep the surface of the foam above the dew point. Any warm moist air that travels thru the fiberglass insulation will be able to condense onto the surface of the foam and will accumulate in the wall cavity unable to escape out due to the air seal that the foam provides. So why don’t we reverse the assembly and put the spray foam on the inside? Spray foam cannot be applied over a fiberglass batt in a wall cavity because it is not a good substrate for the foam to adhere to and over time the foam will separate from the side of the stud allowing warm moist air to condense onto the cold sheathing. That is why you should only put spray foam insulation into wall cavities. By having the dew point somewhere inside of the air-tight foam, warm moist air cannot get to a surface to condense.When considering whether to use spray foam or fiberglass insulation in a home, one needs to consider what kind of performance one is getting for the money spent. Will the heating and cooling bills be so high as to suddenly make the home a financial hardship? Will water condensation in the walls make air quality inside the home unbearable? Spray foam insulation removes these hurdles and improves the comfort and value of your home.