Fiberglass Batts and its Real Effective R-Value

Mar 30, 2012 @ 15:34

Over the last 50 years the majority of houses being erected were built with fiberglass batts in the ceilings and walls.  This is unfortunate because fiberglass batts have a low effective R-value.  Effective R-value is a term that describes how well the insulation works once installed in real world situations.  Under the best and ideal testing situations, a 6 inch fiberglass batt will have an R-value of 19.  However once installed into a house this fiberglass batt will have an effective R-value of around 15.  Over time as people move and walk on the batt it looses more effective R-value.

The other problem with batts is that they are a certain size to fit rafter and stud spaces.  As most home owners know, houses are not always built exactly to spec and even being off by a tenth of an inch between stud spaces can cause an air gap between the insulation and stud.  There are also wires, pipes and cross braces that the insulation will have to be cut to go around.  This is unfortunate because the heat will literally go around the insulation and through these gaps because it is the path of least resistance.  For insulation to work correctly there cannot be any weak points for the heat to escape through.

Using a spray in or blown in product gives you a product with a higher effective R-value.  In that same 6 inch space, Rockwool’s R-value will be 24 and its effective r-value will also be 24.  This is because its blown in at a pre-settled density.  Instead of having to touch the insulation when it is installed which lowers its effective R-value, the insulation blower separates and blows the insulation into areas in its most effective form.  Also being that it is blown in, it will conform and be packed nicely around all pipes, wires and stud spaces so there aren’t any air gaps.

With more and more towns adopting the stretch code (building codes that force homes to be more energy efficient) in Massachusetts, fiberglass batts will be used less and less because they just aren’t as effective.