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Wednesday
Mar032010

The Trouble With R-Value

You've lived in your home for 10-years. You've marveled at the ice formations on your roof every winter. This winter things are different - the ice formations were so large that water leaked back into the wall structure causing sever water damage and mold growth. Couple that with ever rising gas bills and you've finally have had enough. You call an insulation contractor for an estimate. Before you know it you've entered into a contract to increase your attic insulation. The contractor installs 17-inches of lovely pink fiberglass insulation - they tout its R-value of 38 (or higher). You pay the bill - feeling snugger already. Fast forward a year - your energy usage is still rising - the ice formations are still majestic. You can't figure it out.

You are not alone. The insulation industry has been hyping R-value since the Pink Panther was a kitten. It sounds great but it is irrelevant. R-value is a measurement of a material's ability to resist heat flow. The test measures how long it takes 1-degree to travel through 1-square foot (1-inch thick) of material. The problem with the test is that it does not deal with convective heat loss. Convective heat loss is heat that travels in air currents, the major cause of heat loss in cold weather states like Michigan. This short sighted testing protocol would have us believe that 3.5-inches of fiberglass is as effective as 3.5-inches of dense cellulose or 3.5-inches of styrofoam. It doesn't take a rocket scientist to see that the test doesn't deal with performance in real world conditions. Furnace filters are made from fiberglass, nearly an inch thick. Does the filter stop heat from traveling through the filter? No, in fact it isn't even a very effective filter material - stopping less than 3% of the average particulate inthe air stream. Now try installing 1-inch of styrofoam in the furnace filter holder. Well, don't actually try it because you risk damaging the furnace blower and other components in the furnace. The point is that styrofoam does stop convective heat movement - it works that well as an insulation material too.

The fiberglass insulation industry would suggest that the problem is not with the insulation but with the air sealing of the house. They'll tell you that if the builder did a better job of air sealing the house during construction their insulation would be just fine. Sounds good in theory doesn't it? Let's look at reality.

Fiberglass is fluffy and has a lot of entrained air within the insulation. Cellulose is a little denser and has less air. Foam insulation also has air but each air bubble is encased in a cell of foam. The air cannot move throughout the insulation. So heat loss through foam is actually conduction and the R-value is real. A cavity filled with fiberglass allows air to move within the insulation creating convective loops. The air actually circulates within the insulation. Colder air within the cavity but near the outside wall is pulled through the insulation, reducing its effectiveness and performance. If the outdoor temperature is 10-degrees the effective R-value of fiberglass (or other loose, fluffy insulation) may be 75% less than advertised.

So, if you are going to add insulation you need to stop convection. Install the densest insulation that your budget will allow. We are all aware of the price of things, we are seldom aware of the cost. Fiberglass insulation is cheaper than high performance insulations but in the long run it costs more in wasted energy than you would have spent if you purchased a more expensive insulation material.

If you really want to save money invest wisely in insulation that performs in Michigan - fiberglass can't live up to its promises in a cold weather state.

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