Fiberglass Insulation - Selection, Installation And Performance
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Blanket Facing (vapor barrier)
The facing (usually Kraft paper or vinyl) is a vapor barrier that should be installed on the inside of the house. Installing the vapor barrier on the outside will trap moisture inside the wall as it tries to escape the house. On a cold day, the temperature at some point inside the wall may drop to the dew point allowing the moisture to turn to liquid. This condensing water can cause damage to the wood structure of the house and may also be a mold problem so make sure the vapor barrier is on the inside of the house. Batts with a special flame-resistant facing are available in various widths for basement walls where the insulation will be left exposed. Blankets can also come Poly-encapsulated (surrounded in thin plastic). This makes the installation cleaner and more comfortable with less itch and dust.
The vapor barrier is also used to staple the insulation to the wood. If the insulation is not attached to the wood in a vertical installation, it can sag over time and end up in a pile at the bottom of the wall. If the blanket are installed as floor insulation, then use pieces of wire to hold the insulation in place and once again place the vapor barrier up.
Non-faced insulation is used as a second layer of insulation. You should only have one vapor barrier so if you need more insulation to fill the void or you are putting a second layer in an attic, you will want non-faced batts or rolls. You can install non-faced batts perpendicular to the rafters and get a more air restrictive seal.
Insulation Properties
This style of insulation can be made of fiberglass, mineral (rock or slag) wool. plastic fibers or natural fibers. Fiberglass is the most common. Fiberglass can either long fiber or short fiber. The long fiber is much better since it does release as many irritating fibers into the air. Fiberglass does not support mold or Mildew and does not supply a food source for insects or rodents, although it should be stored in dry place to avoid starting mold in the compacted state..
Standard fiberglass blankets and batts have a thermal resistance or R-values between R-2.9 and R-3.8 per inch of thickness. High-performance (medium-density and high-density) fiberglass blankets and batts have R-values between R-3.7 and R-4.3 per inch of thickness. See the table below for an overview of these characteristics.
Here are the typical fiberglass blanket properties
Thickness (inches) R-Value Cost (cents/sq. ft)
3 1/2 .........................11............................12-16
3 5/8..........................13............................15-20
3 1/2 (high density)....15............................34-40
6 to 6-1.4...................19.............................27-34
5 1/4 (high density)....21............................33-39
8 to 8-1/2..................25............................37-45
8 (high density)..........30............................45-49
9 1/2 (standard)........30............................39-43
12............................38.............................55-60
Blanket Installation
Blanket insulation can be installed during construction or renovation when the area between the studs, rafters or joist is open. Once the sheetrock is installed, it is not a cost effective option. Blanket insulation can be cut with a standard razor knife. If place the blanket on a piece of wood, you compress the insulation (with the vapor barrier up) and get a very clean cut. If the stud width is less than standard, use a straight edge to cut the width to about 1 inch more than the gap between the wood. This will give you enough vapor barrier to staple both sides of the blanket.
The insulation should be installed after all the electrical work is complete. When installing blankets around wires, you should flip over the insulation (vapor barrier down) and use a knife to cut half way through the insulation without cutting the vapor barrier, This will allow the insulation to set against the outside of house even where wires are installed. If have found that a serrated kitchen knife works best for partial cuts and trimming blankets to fit around obsticles. A serrated knife can also be used to cut out for outlets.
Once all the blankets are cut and installed, you can tear off small parts of the blanket to fill any other areas.
Warnings
If possible, buy Formaldehyde-free insulation. The out gassing of Formaldehyde can cause problems for sensitive, pregnant or young people and it is not good for anyone.
The installation of insulation can be hazardous so make sure you follow the safety guidelines and wear a respirator when working.
The actual r-value is very dependent on the quality of the installation. For best result, work with a professional.
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