While I was teaching the last couple of days, several issues came up and one of them was fiberglass insulation. I tell my students at the beginning of the semester that I hate fiberglass insulation and very rarely use it, but that’s not really fair or true. Used in the right context, fiberglass insulation can be just fine. However, I find all too often that fiberglass is used in the wrong way. It really is not great as wall or attic insulation and it’s often found in basement ceilings where it’s installed up side down. So I thought it would be good to discuss when and where to use fiberglass insulation, and why it doesn’t work in all locations.First, fiberglass insulation works by trapping the air in between the fiberglass fibers. So fiberglass insulation is really only effective when there is absolutely no air movement where it is installed. Air movement through the insulation removes those trapped pockets of air and essentially makes it a filter. And no matter how tight you build a building, you are still going to have air leakage in some areas. That’s why; when you pull it out of the box sill in your basement it looks black. That’s just the air infiltration from the box sill being filtered through your insulation and making it useless, since it is no longer trapping air pockets within its web of fibers.The box sill or band joist, is often one of the leakiest locations in a home, and therefore one of the worst places to install fiberglass insulation. Fiberglass insulation rarely works well in the wall cavity because your siding breathes and tongue and grove wall surfaces are not airtight. Wall cavities can also be open to the box sill below. If you follow the principle that hot air rises, then that air is always going to be traveling up through your wall cavities, taking warm air with it, and cooling off the sheetrock on the inside. It also performs poorly in the attic due to wind washing. Wind washing is the effect that happens when the air enters your attic through your soffit venting and blows through the insulation. Contractors install proper vents to try to direct the air above the insulation. But I have been in many homes that have improperly sized or installed proper vents, or none at all. Not installing the proper vents and insulation dam causes the wind to be pulled through the fiberglass insulation, again releasing the trapped air molecules in the fiberglass and making the insulation less effective.If you have a heating system, plumbing, or laundry in your basement then the insulation does not belong in your basement ceiling. People argue with me all the time that they do that just to make the floors warmer; well that’s not a good enough reason. You’ll be thanking me when you don’t have frozen pipes and the excess heat from your boiler can rise to the floors above. If you have any of the things I mentioned in your basement then the thermal boundary of your space is the wall. If you have rubble stone or granite the best wall insulation is spray foam. If you have smooth concrete then the best insulation is rigid insulation. If you live in Maine the rigid insulation needs to be Thermax insulation approved by the Maine State Fire Marshal’s office for use without covering. Otherwise, you have to cover your rigid insulation with a 15 minute thermal barrier – which is 1/2” Sheetrock or ¾” OSB. You are also required to cover your spray foam insulation with a thermal barrier that any spray foam installer can spray on as part of the insulation process.But I digress, we were talking about fiberglass, and why it seems to always be installed in the wrong place or the wrong way. The Kraft paper side of the fiberglass always needs to be to the warm side of the structure. So in Maine, it needs to face to the inside. Fiberglass is only as good as it is installed. The Kraft paper should be face stapled to the studs, not side stapled which compresses the insulation. The fiberglass insulation should be cut and fit around electrical wiring so that it is not compressed behind the wire. And it needs to fit fully into the cavity, touching both sides of the studs, as well as, the top and bottom. All too often insulation is installed by the lowest paid guy on the job site. It’s one of the most critical pieces to get right, but it’s nasty work and therefore done by the new guy. In basements, the Kraft paper side needs to be up against the warm floor above, not stapled to the floor joists below– I know this is easier to install, but it’s putting the vapor barrier on the wrong side. And in the North East we strap our ceilings, which makes fiberglass insulation the worst type of insulation to use in your attic. The ¾” strapping leaves a ¾” gap between the ceiling sheetrock and the insulation above allowing air to carry the heat away from the sheetrock without the protection of the insulation. That moving air also reduces the effectiveness of the insulation above. So make sure that your insulation is in full contact with your sheetrock ceiling. The proper way to solve this problem without adding a lot of extra expense is to pick up your fiberglass insulation, blow in 3 inches of cellulose, cut the vapor barrier on your existing fiberglass insulation and lay it back down on top of the cellulose. If you need more insulation to meet the code minimum, blow an additional couple of inches of cellulose over the top of your fiberglass insulation to make a fiberglass sandwich. The density of the cellulose minimizes the airflow through the insulation and makes the fiberglass more effective.Where would I use fiberglass? Well it makes a great sound barrier, so I would use it around the master bedroom and around bathrooms to reduce noise levels. I would also use fiberglass insulation in conjunction with rigid insulation in basements if you were going to finish a basement, because it does not hold water, and basements can be moist. It has its place, in a completely sealed envelope it can add a lot of r-value to a system, but it needs to be installed correctly and in the right location.