By David Braddy LEED GA
This is a question I am asked on a regular basis, but it is a question that is not as easy to answer as it sounds. Do you want cellulose or fiberglass, batt or blown? What is the difference between the two?
In this area it seems that fiberglass is the insulation of choice, but does that mean it is better? It is probably the most readily available, it comes in batts so it is easy to handle and install in wall cavities. It also creates much less of a mess and is less labor intensive than many other forms of insulation. It can also be loose blown into attic spaces and wall cavities.
But when it comes to choosing the type to use homeowners and builders have two different issues to consider:
1. How well does it perform, in other words it R-value
2. What is its resistance to air and moisture movement
Are you looking for thermal barrier or an air barrier? What’s the difference?
Let’s start with the difference between an air and thermal barrier. Thermal barriers deal with keeping the heat in and cold out of the building envelope in the winter and the opposite in the summer, the higher the R-value the higher the thermal resistance, while a higher R-value will keep more conditioned or heated air in, it will not necessarily keep the air from moving through a wall, this is the job of an air barrier.
Some types of insulation can do both.
Fiberglass for instance, while the most popular insulation is not an effective air barrier. Air can filter through fiberglass, and if air can move through it so can moisture, which creates another set of problems if the wall cavity doesn’t have the proper vapor barrier (and what is proper in one part of the country may not be proper in another), but that’s another issue that deals with vapor drive and mold growth, which is another topic that I will discuss another time, so back to insulation.
What is the best for air infiltration and thermal resistance cellulose or fiberglass?
Neither is actually the best. The best is closed cell spray foam, it is a true air and moisture barrier at 2 inches thickness, it has an R-Value of 6.8 per inch, and it turns solid when sprayed so it adds strength to the structure. It has no off gassing of VOC’s as some fiberglass batts do (although formaldehyde free batts are readily available, just ask for them) and since it totally seals a wall cavity and air and moisture cannot pass through, it virtually eliminates mold problems. So why doesn’t everyone just use closed cell spray foam? Unfortunately it is the most cost and labor intensive, while it will pay for itself in the long run in several ways; it has the most upfront cost.
Next would be open cell spray foam, which has an R-Value of approximately 3.9 per inch, it is not solid and does allow some air infiltration, but since it is sprayed you still get a very good seal & coverage. While not quite as costly as closed cell, it is still much higher than cellulose or fiberglass.
Following very closely with an R-Value of approximately 3.4 to 3.8 per inch is loose fill cellulose which is blown dry (or wet, but I don’t recommend unless you allow ample time for drying) into a wall cavity behind a special fabric or loose in the attic. Cellulose is a much better air barrier than fiberglass and the loose blown is comparable in price to fiberglass. So cellulose has a slightly higher R-Value per inch and is a better air barrier, but blowing the walls is more expensive than fiberglass batts and very messy. I would actually choose cellulose over open cell spray foam, because the performance is close while the cost is usually not.
Now comes fiberglass at an R-Value of approximately 2.9 to 3.8 per inch of thickness, and it is the worst air barrier. Why is it still the most popular? If your walls are properly constructed with the proper vapor barriers in place or with a good sealant package, you can still enjoy a good performing energy package and still use fiberglass batts, which are very economical.
Keep in mind without a properly constructed, energy efficient wall to begin with you can lose up to 15% of the R-Value in a wall assembly due to thermal bridging of framing components, regardless of the type of insulation you use, so call a professional to consult with if you are unsure of the proper methods. The last thing you want is a drafty wall full of mold.
Believe or not there are more insulation options but these are the most common.