Thursday, November 4, 2010

Fiberglass Batts

Fiberglass batts are indubitably the cheapest way of insulating your wall cavities. However, with the push of building scientists on the importance of air infiltration, green builders are quickly moving away from this type of insulation.

Why? For insulation to be effective, all six sides of the insulation must be flush with the wall cavity, otherwise air can circulate between the gaps which yields to heat loss via convection. This essentially reduces the R-value of the insulation product.

With fiberglass batts, gaps between the insulation and the cavity are introduced right from day one -- during its installation. A professionally installed R20 fiberglass batt behaves more like R18 (8% loss - Oak Ridge National Laboratory) because it is impossible to get all 6 sides of the batt flush with the wall cavity.

By professionally installed I assume the batts are properly cut to fit the cavity's width and height, cut to go around obstacles like plumbing and wires, and the batt is not compressed (compression also reduces R-value). Unfortunately, actual professional installations are not that... professional. The quality of installation described above is quite time consuming and most contractors just don't have that kind of time.

Furthermore, the performance of fiberglass batts also greatly suffers from wind and moisture. On a rainy day with 30 km/h winds, an R20 fiberglass batt (R18 installed) behaves more like R15... but you paid for R20! Cost-effectiveness now requires a hard second look. Especially in Newfoundland where you can expect windy and rainy fairly frequently.

So what are the alternatives? There are other insulation types that also come in batts... but that is not really solving the air movement problem.

Closed-cell polyurethane sprayfoam is a great substitute. It completely fills the wall cavity, it meets the national building code for moisture permeance, exceeds the national building code as an air barrier, it has 60% higher R-value per inch than fiberglass batts, and its installation is fairly quick without compromising performance... Sadly, it costs 2 to 3 times more than fiberglass batts.

More recently, another good alternative to fiberglass batts is fiberglass itself -- blown-in fiberglass. It completely fills the wall cavity, it can be recycled insitu, and its installation is quick without affecting its thermal performance. It costs around 1.5 times more than fiberglass batts. In Newfoundland it is currently sold by the name of Spyder Insulation by Johns Manville through Twin City Contracting.

Remember to use the cost per diminished R-value per inch of fiberglass batts when comparing to seemingly more expensive insulation options.

Wet-spray cellulose and blown-in mineral wool could also be decent substitutes. They have an R-value per inch and cost similar to fiberglass. There are some installation considerations with wet-spray cellulose (drying time and density) so beware of hasty contractors.

*Note that I didn't consider thermal bridging through the framing (framing normally accounts for 20-30% of a wall without windows, which means only 70%-80% of the wall can be filled with insulation)


Green Building Advisor - Installing Fiberglass Right:

Sealection 500 by Spray Foam Energy Solutions - Promo Video:

Spyder Insulation by Johns Manville - Promo Video:

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