Myths. Fiberglass insulation is cheap compared to other insulation options, you can install it yourself, and it’s easy to work with except for the itching.
Reality. Some people think if they buy R-19 fiberglass insulation for their walls, they are actually getting an insulation value of R-19, this is not so. Although the manufacturer labels fiberglass insulation with an R-value like R-19, once installed in a typical 2×6 wall system, the actual thermal performance is only between R-10.5 and R-12. This means that you lose 33%—42% of what you paid for. Additional hidden costs with fiberglass insulation are, the increased time required for correct and careful installation (especially around irregular framing members) and the reduction in insulation value if air movement is present. One big flaw with fiberglass insulation is that it provides little resistance to air leakage. Any air leaks will pass right through and you’ll be cold and/or wasting money.
The R-factor alone is a terrible measurement. Unfortunately, R-Value has taken hold in our minds as a universal method for comparing insulation - the higher the R-Value, the better the insulation. It becomes a game of getting the highest R-value for the least amount of money. All R-Values are not created equal, because they measure only one of the factors that determine how insulation will perform in your home. To understand actual performance and longer term costs and comfort, we need to evaluate performance by taking into account a variety of other design factors like framing methods, air infiltration, thermal mass, and solar gain. Sound transmission is another criteria that should be considered when choosing an insulation.
So what can you do?
When building a new building, evaluate your insulation options in the design phase with a qualified professional. Alternative options are out of scope for this article but they do exist and some may actually be cheaper over time. The money spent with giving proper consideration in the design phase or with initial construction cost increases can save you in the long run. During the planning cycle computer programs can help model energy usage of your home and provide insight where you can save money and or decrease project costs.
Minimize thermal bridging. Minimum of 1 inch of continuous exterior rigid foam will reduce thermal bridging and air leakage. 1 inch rigid XPS foam will decrease heating costs on a typical 2×6 wall ~30% per year. On a 2×4 wall heating costs will be reduced by ~45%. On estimated heating cost of $1000/year, this puts the payback on new home construction between 5 and 8 years. Other ways to minimize thermal bridging are to insulate headers with rigid foam and, if possible, eliminate and minimize unnecessary framing members.
Slow down and pay attention to detail when installing. Common mistakes include; torn facing paper, sloppy cuts around electrical boxes, compressed insulation at wiring or pipes, incorrect cutting for odd-sized bays, fastening to inside of bays, gaps in the bay. Mistakes in installation, such as these can reduce effectiveness by 10 to 15%. This is another example of where small things matter. Careful planning and layout of wall studs prior to installation can potentially reduce the number of stud cavities and reduce odd sized cavities. This will reduce thermal bridging, as well as saving on lumber costs.
Seal and caulk. Stop air infiltration where possible! This should be a goal regardless of the insulation method selected but with fiberglass, air movement cuts deep into fiberglass insulation’s ability to keep you warm. Remember to seal and caulk the bottom plate of walls, between the rim joists and sill plates, and around electrical boxes. For electrical and fan boxes in the attic, seal the perimeter and cap with a box made of rigid foam. Tape seams on housewrap and exterior rigid insulation. Also use caulk, canned foam and gaskets around windows and doors. Seal penetrations in ductwork. It is not recommended to install ductwork outside of the conditioned space, such as attics but when it is done be sure to insulate such ductwork.
Holiday, Martin “Choose the Best Insulation” Energy-Smart Remodeling Fall 2012 The Tuanton Press pg 48-53
Stiburek, Joesph Builder’s Guide to Cold Climates Energy and Environmental Building Association 2004
Info-501 – Installation of Cavity Insulation by Building Science Corporation