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Insulation
Before discussing insulation, it would be good to consider the purpose of using insulation. The purpose of insulation it to impede the flow of energy from one place to another.
The whole point of insulation is to slow (ideally stop, but in reality, slow) the flow of energy. Thermal energy (heat) is usually the form of energy that one considers when house insulation is discussed.
Thermal energy naturally flows from warmer areas to cooler areas. In the summer, hot outside air tries to flow to the (hopefully) cooler inside of a house. There are several ways for the thermal energy to move to cooler areas.
One way is infiltration, that is leaks and cracks in walls, around electrical outlets, around windows, etc. This also includes opening and closing doors. If a wall cavity has both an exterior electrical box and an interior electrical box, there is a potential for infiltration. The building code requires foam around window and door jambs to address this issue there.
In order to minimize heat flow through exterior walls, insulation is required. Air is a good insulator with one condition: it has to be in small areas. If the area is too big, such as inside a wall, there is room for convection. Air on the hot wall rises and air on the cool wall settles. The cool air then goes up the hot wall and is heated while the hot air goes down the cool wall and warms it.
Fiberglass insulation does nothing more than trap small cells of air. If air cannot flow, then heat is not transferred from the hot wall to the cold wall by air. Fiberglass is glass, however and glass is a fair conductor of heat. Since the glass is spun into small pieces, the amount of glass between the hot and cold walls is very small, so little heat gets transferred this way.
If fiberglass is packed into an area, it looses its effectiveness because there is more glass and less trapped air. Too much insulation is, surprisingly, not a good thing for this reason.
Another problem with fiberglass insulation is that of voids. If the outside walls have anything in them such as tie down rods, pipes, wires, electrical outlets or such things, it is very difficult to place the insulation behind and around the obstacles. Remember any large pockets of air constitute thermal leaks and reduce the effectiveness of the insulation.
If fiberglass insulation is perfectly installed and there are no items in the wall to displace insulation or cause voids, it has an "R" value of about 3.75 per inch of thickness. The higher the "R" value, the better thermal impedence. "R" stands for resistance to heat flow.
Open cell foam insulation being sprayed on the roof sheathing.
A typical 2 x 4 wall with perfect fiberglass insulation has an R rating of 13. A 2 x 6 wall has an R rating of 19 – almost a 50% improvement over a 2 x 4 wall. That is just one of the reasons we use 2 x 6 walls.
As noted, perfectly installed fiberglass insulation is very difficult to achieve. For this reason spray in-foam insulation is preferred for the greatest efficiency. There are two basic types of spray in foam insulation: open cell and closed cell. First, we will consider open cell foam.
Open cell spray in foam insulation is a good alternative to fiberglass. However, an immediate downside of spray in foam is the cost. It is about three times the cost of fiberglass insulation. The insulation rating is about the same as (perfectly installed) fiberglass insulation.
The question then should be, "Why do people use spray in foam insulation if it costs more and it insulates the same?" There are several answers. The most important is that perfectly installed fiberglass insulation is rare, so the "R-13" fiberglass is not really R-13. Spray in foam, however, will flow behind and around wires, tie down rods, electrical boxes and other items in the wall.
Any cracks or paths of infiltration which would not be addressed with fiberglass are sealed with spray in foam.
Typically, we use open cell foam in the roof and walls of homes.
Note that we use open cell foam in the roof and not the ceiling. This keeps the attic space closer to the interior temperature of the home and lessens humidity. If air conditioners or other equipment is in the attic, they are much more efficient due to the moderate temperature and humidity.
"Blue jean" sound (blue) insulation and spray in foam (white) insulation.
One draw back to using any spray in foam in the roof is it masks the location of any leaks. If a roof leaks water will usually be stopped by the foam and will run down to the bottom before coming out. This makes leaks more difficult to find but it does lessen the risk of interior water damage.
The second type of spray-in-foam insulation to consider is closed cell. Closed cell foam is much more rigid than open cell and it has an R value that is twice that of open cell foam. However, it also costs twice as much than open cell per 'R'.
One significant benefit to closed cell foam is it is impervious to water. For this reason we try to always use closed cell foam under first floors of homes we build on a crawl space. Close cell foam also acts as a glue by structurally bonding plywood sheathing to the studs or rafters and increasing the building's integrity.
Although the initial cost of spray in foam is more, when you consider the price of a home, it is not too much more. With energy costs constantly rising, it does not take long for spray in foam to pay for itself.
Normally, a discussion of insulation would end here. However, we at Willis Sinclair are also concerned with noise pollution and dampening. As a standard, we include sound insulation around all bedrooms and bathrooms.
Fiberglass insulation is used for sound insulation normally, although we sometimes use insulation made of old blue jeans material. This material provides more sound insulation than fiberglass insulation, but it also costs more.
Sound insulation does not provide significant energy savings, but it does contribute to a quiet house which many people appreciate.
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