Insulation is installed in walls, floors, and ceilings to slow down the flow of heat between the interior and exterior of the house. “R-Value” is the term used to measure a material’s insulating value, the higher the “R”, the higher the resistance to heat, and therefore the better the insulation.
Each time the R-Value is doubled the heat loss is cut in half. Doubling R-1 to R-2 cuts the heat loss in half, and results in a huge savings because you are starting without much R-Value. This would be a single pane window compared to a double pane window and the savings is a full 50% of the loss through the glass. In an attic, doubling R-20 to R-40 also cuts the heat loss in half, but since you are starting from a smaller heat loss, the result is a much smaller gain because you are starting will less loss. You can readily see the diminishing returns for heaping on more and more insulation.
The amount of insulation that you will install is only one concern. The material that you install is an equally important decision. Fiberglass, both blown and batts, cellulose, mineral wool, and open cell 2-part foam all have approximately the same R-Value per inch – about R-3 to 3.5. The factors that would recommend one material over another are ease of installation, durability, cost per installed R-Value, and the ability to cut down on convective air movement within the cavity.
The quality of the installation is very important. Improperly insulating an attic, for example, can reduce its real R-Value by 1/3 or more. Batts do not sit down tightly on the drywall where they are pushed up by wires, fans, lights, framing members, or poor installation. In addition, with batts there is no insulation on the attic side, the top, of the wood framing on the attic floor, leaving these areas under insulated when compared to the batt. The overall R-Value in an R-30 batted attic may be lower than R-20. Blown insulation solves these problems by contacting the drywall, working in around the wires and fans, and by covering the joists with an additional layer of insulation. The R-30 blown attic will be much closer to R-30, plus it is cheaper to boot. We recommend blown cellulose in attics.
In overhangs, cantilevers, and areas like garage ceilings, we always recommend filling the cavity with insulation. In cases like this, we make sure that the insulation is physically touching the surface that we intend to insulate – that is, the floor sheathing above the garage, and the band and soffit in an overhang.
Any wall that is exposed to an attic on one side, such as knee walls or walls that back on an attic over a garage, must be given careful attention. If we only batt these walls we end up with 3 potential problems. First is that the batts may not be touching the back of the drywall, second is the attic side of the batt is exposed to air movement in the ventilated attic which reduces its R-value, and third and worst, in a few years some of the batts may fall leaving uninsulated areas. We recommend sheathing the attic side of all exposed walls to make sure the batts touch the drywall, to keep air movement away from the back of the batt, and to keep the batt from ever falling off.
When insulating, always remember the rule that the insulation must physically touch the location of the energy loss. This is the drywall of walls, kneewalls, and ceilings, and the floor sheathing of rooms over garages.