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Insulation: Fiberglass

Updated on September 24, 2011

Just about everyone knows that if you want your home or building to be more efficient, you ad more insulation right? Not really. Adding insulation may increase your building's R-value, but it may not save you any money at all. Fiberglass insulation is one of those materials that probably won't save you a dime on your energy costs. I will explain this later, but first let us talk about the science behind fiberglass insulation.

Fiberglass insulation is NOT an insulator. The tiny air pockets fiberglass insulation creates is the insulator. Confused? I was at first also. The way fiberglass is supposed to work is the area that the fiberglass insulation occupies is meant to trap the cold or warm air (depending on the climate and the season) from penetrating into your home. There are a few problems with this theory however:

1) Problem #1 If you have a fiberglass batt and you turn it on it's side, it is layered like a cake. Each one of these layers has these air pockets. Well, if you have air pockets stacked upon air pockets, isn't that just a really thick filter? Yes. It is going to slow the transfer of energy down, but as it does, the material itself loses R-value and efficiency making it significantly less effective while allowing air movement through the cavity it occupies. Loss fill fiberglass is that much more ineffective because of the larger air pockets allowing more air to travel through.

2) Problem #2The ideal situation thermal dynamically would be if the warm air met with the cold air in the middle of the insulation material so virtually no moisture is created. If you recall from high school when warm air meets cold air condensation forms. The reason is because there is too large of a temperature difference so the warm and cold air are generally meeting on the drywall or the plastic between the drywall and the insulation. When this occurs, the moisture condensates on the plastic vapor barrier, drips down the wall, and rots away whatever the water sits on creating mold and rot. Please understand that this scenario does not ALWAYS happen. It is much more common in homes that are sealed too tight and not properly ventilated and also in buildings that are framed with steel or aluminum studs.

3) Problem #3 Last time I checked, fiberglass is made primarily of spun glass fibers. Glass is a good conductor of energy. In other words, it absorbs heat and cold well. So if the temperature outside is 30 degrees, the temperature of the fiberglass insulation on the outside is going to absorb that cold and transfer it across the fibers in contact with the outside fibers making the air pockets cold and dropping the R-value of the insulation. If the air is hot, the same concept applies.

The fact is is that fiberglass insulation is effected too many ways to be truely an efficient insulation material. The reason that air sealing homes has become such a topic in recent years is because of fiberglass insulation applications and the problems that have been identified with it. So why is fiberglass the most widely used and advertised insulation material on the market? For one reason. It is easy to produce, easy to install, and its cheap. You can insulate to code with fiberglass insulation an entire home for as little as $2,000. If you are willing to use an inferior material or just do not care about anything other than price, this is definitely the material for you.

So here is a list of places where you absolutely should not use fiberglass insulation:

Box Sills Moisture and mold are a big issue with fiberglass insulation in this area. The proper insulation material for this area is either 2 layers of 2 inch polyurethane foam board insulation sealed with can foam insulation, or closed cell spray polyurethane foam.

Rafter Ceilings This is another area where moisture is a big issue and unless it is ventilated very well, there are usually issues. Proper materials for this application are closed cell polyurethane spray foam or dense pack cellulose insulation. Neither one of these materials requires ventilation.

Dormers There is usually inadequate space to even meet code requirements, but either way it is not recommended to use fiberglass for dormers as there tend to be issues where the roof meets the wall framing. There are several spray foam insulation applications that are acceptable along with dense pack cellulose insulation.

Cantilevered Floors These are floors that have finished space above them, do not have support posts, and require insulation in the floor area. There is too much air movement in these areas that tend to bring the cold or hot air right into the main house ceiling majorly decreasing the overall efficiency of the house. This is common in bi-levels and quad levels. The proper insulation material for this application is closed cell polyurethane spray foam. There really isn't anything else that works.

Please keep in mind that I am not saying you should never insulate with fiberglass. It is, in fact, better than nothing. If you are not going to be using a structure for more than the summer months and winterize the structure when not using it, then it doesn't make a lot of sense to spend the extra money on insulation as you will not recover the costs very quickly. Also garages, 3 seasons rooms, unheated porches, and sound insulation between floors and bedroom walls may not be worth the extra costs.

So is adding fiberglass insulation for efficiency worth the extra money? Probably not. The concept of the material is too complicated and is affected by too many things to be truly effective. I will cover other insulation materials in future articles. If you are curious about the efficiency of your home or building from a thermal perspective, call a professional thermographer and you will be able to see exactly where you need to improve your efficiency. This service varies from $100-$300.

For more information on insulation materials and performance, visit Oakridge National Laboratories to view side by side testing data on multiple materials.

For information on saving on heating costs, click here.


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    • Energy Guild profile image

      Energy Guild 7 years ago from Ripon, WI

      In general, fiberglass insulation is not a good choice if you are looking for optimal energy efficiency. Closed cell spray foam combined with fiberglass batts in walls can be good if applied correctly. Cellulose insulation is the cheapest and most energy efficient way to insulate your house as it can be loose blown in the attic and dense packed in the walls in order to accomplish significant air sealing and improve energy efficiency in this manner as well.

      Keep in mind that fiberglass insulation, properly installed, meets insulation code requirements, but that is about it. I do not consider fiberglass insulation energy efficient.

    • profile image

      lori  7 years ago

      is firbreglass energy- efficent ?