Last time I brought up three crucial differences in the merits and faults of various insulations: air movement, water vapor, and installation. Air movement can lower your insulation’s effectiveness. It develops from at least three conditions:
One is simple leakage. If the wind is blowing, or even if there is no wind but there is a difference in temperature between inside and outside – insulation’s raison d’etre -- air will get into and move through any cracks or holes in a house's wall, floor, and ceiling assemblies. The threat of a windy day ruining your coziness is apparent, but the more insidious leakage will happen on a calm day. Remember elementary physics: warm air rises, and it expands. Your hard-earned warm air will find its way out through cracks in the closets or around the holes cut in your ceiling where light fixtures are installed. Nature abhors a vacuum, we’ve heard, so up comes cold air from under the house. On cooling days, of course, the process reverses. This is sometimes called the “stack effect”.
The second type of air movement is created by induced pressure differences. Running a range hood or bathroom vent tends to create negative pressure inside a house, because they exhaust air to the outside. Again, we have a vacuum effect, which means air will be sucked in through any holes or cracks in your home’s walls, floors, or ceilings.
The third variety of air movement is a convective loop within an assembly. Even within a well-sealed cavity between two wall studs, warmer air will rise, pushing cooler air down in a constant cycle.
Since insulating materials gain their effectiveness from the tiny pockets of still air within them, if the air in these pockets moves, the insulation loses effectiveness. Imagine blowing on a spoon of hot oatmeal, versus just waiting to eat it. This is sometimes called “wind-washing”.
Your first reaction should be “isn’t that what insulation does, stop cold/hot air from getting in?” Well, kind of. Many insulation materials don’t stop the flow of air, they only slow it. Knowing that moving air is far more penetrative than water, would you fish in a bass boat made of cellulose or fiberglass insulation?
Spray foam insulation would make a perfectly serviceable boat, and is therefore superior to other types of insulation on the issue of air movement. Being quite solid after installation, the foam keeps its insulating air pockets trapped and is therefore impervious to the loss of effective R-value caused by wind-washing. Plus, it penetrates and seals shut any cracks or holes in a wall assembly.
But, even though fiberglass and cellulose are susceptible to air movement, I haven’t been convinced that this makes spray foam worth the amount it costs.
Fiberglass and cellulose don't have to suffer from wind-washing. Carefully planned and installed floor, wall, and ceiling assemblies can resist the first and second types of air movement. An airtight drywall installation, caulking cracks and holes before insulating, and having an air barrier on the exterior of the wall assembly should greatly reduce, if not eliminate air movement through fiberglass or cellulose insulation. The third type of air movement mentioned above I suspect is too minor an effect to be a deciding factor in the choice between insulations, under ordinary circumstances. Moreover, I have yet to see a trustworthy report on what percent of its advertised R-value a fiberglass batt will lose to wind-washing. Does it retain 85% of its effectiveness? Only 10%? Personally, on a blustery January night, I’ll choose a shelter with fiberglass insulation over one without any.
On the issue of air movement, I am at the moment inclined to favor cellulose or fiberglass because I fear the cost of spray foam isn’t worth its superior resistance to air movement.
But that’s only one of the three major issues.
