Does Roof Insulation Really Work?

In a word, yes!  Placing insulating material in your attic between the roof and the ceiling of your living space will definitely reduce both heat loss in the winter and heat gain in the summer.  However, not all insulating materials and methods of installation are created equal, and some produce better results than others.  Let’s look at some issues that impact how well roof insulation works.

First, we have to introduce R-values and U-values, which are measures of the efficiency of insulating materials.  R-values and U-values are what you need to understand, but their calculations can be mathematically complex and hard to grasp.  To put it simply, the R-value of a type of insulation tells you how well it keeps heat inside your home while its inverse, the U-value, tells you how much heat passes through the material

With R-values, the higher the number the better.  Higher R-values mean the insulating material affords more resistance to the transfer of heat through the material.  With U-values, the lower the number the better.  Lower U-values means the material lets less heat pass through.

Many homeowners focus exclusively on the R-value of roofing and attic insulation and forget about the more important System R-Value, which is the measure of efficiency of the attic as a whole, not just of the insulating material.  To explain that concept, think of your existing attic.  You have the roof rafters on which the roof sheathing is placed and ceiling rafters under which the ceiling material is installed.  Most attic insulation is placed in the space between the rafters, leaving the rafters uninsulated.  Wood is a notoriously poor insulating material so while the insulation might be doing a great job keeping the heat in your home, some of it is escaping through the rafters, lowering the System R-value.

Some experts claim that up to 50% of the heat loss or gain within a home is due to air and energy movement and transfer factors – like conduction, convection, radiation, and air infiltration -- not affected by the R-value.  The best form of insulation to deal with increasing System R-value is spray foam.  However, it is also the most expensive and the hardest to install.  Installing spray foam insulation is strictly a job for professional installers.

Spray foam insulation covers the rafters and fills all spaces where air can penetrate.  It eliminates air infiltration and energy loss through radiation, conduction, and convection.  There are only three downsides to using spray foam insulation for roof or attic applications:  cost, mess, and permeability.

Spray foam professionally installed can cost 30% more than other insulating methods so you have to carefully determine whether the savings in energy costs justify the installation costs.  In older, poorly sealed homes, the answer is almost always yes, but it will take several years to realize some savings.  Spraying foam insulation is a delicate process and can be very messy in the hands of an inexperienced installer.  If you’re going to go this way, it is absolutely essential you interview several installers and check their references thoroughly.  Finally, there are two forms of foam:  open cell and closed cell.  If you’re planning to spray foam directly to the rafters, you need to use open cell, which allows water to pass through in the event of a roof leak.

Less expensive insulation methods include blown in cellulose and fiberglass batts.  A homeowner can rent equipment to blow in cellulose insulation to fill the space between the rafters.  Fiberglass batts come in sizes that fit tightly in most rafter spaces.  Both methods fail to insulate the rafters and the air pockets between the insulating material and the rafters.  Although costly, it is possible to add foam board sheet insulation directly over the rafters, which will improve the overall System R-value.