The Range Hood Exhaust–as Air Intake

Modern tight houses can easily become depressurized when exhaust fans are turned on. What this means is there is no place for the air to come into the home to replace the air that is trying to leave. If there are gaps around door weather-stripping, or gaps around window sashes or similar locations, the air will come into the home at these locations.  Sometimes even chimneys might be the path for this air.

If we operate and exhaust fan in a bathroom the house becomes depressurized, or an area of “lower pressure.” Areas of higher pressure will tend to make balance with areas of lower pressure, so the air outside the building literally “pushes” its way into the area of lower pressure.

Most houses are not tight enough for the air to not find its way in somewhere, and general infiltration was once allowed to be the source of this air replacement.

This small condo unit was too tight for general infiltration to be the source of make-up air, as was evidenced by its finding a path through the range hood exhaust.

Most range hoods have a back-draft damper in them, but there should also be one in the cap at the exterior of the building as well. You can see in this picture there is no damper—but there is a screen.

Exhaust fan vent termination with no back-draft damper

With two bathroom exhaust fans and the laundry exhaust fan running, the purple/violet colors of the thermal image of the chase and microwave/hood shows cold air cooling the chase and the area around the microwave.


The screen at the exterior cap location did hold a tissue paper to show that indeed air was pushing its way through the microwave/hood.

So, let’s say we “fix” the cap at the exterior with a proper back-draft damper. Where will replacement air come from? General infiltration may still be adequate, it is just easier coming from where it is now. If it is not adequate, the functionality of the exhaust fans will be reduced. In other words, they will make noise but not exhaust enough air from the room. It is like turning a 100-cfm fan into a 50-cfm fan.

For exhaust fans to do their job, replacement air is necessary and is required by modern codes when houses get to a certain point of air-tightness. This one may be at that point, even though it is an older home in that respect.

Some “positive” means of allowing exterior air to enter the home may be indicated if exhaust fans do not function properly after the exterior cap is repaired and its back-draft damper installed.

By Charles Buell, Real Estate Inspections in Seattle

If you enjoyed this post, and would like to get notices of new posts to my blog, please subscribe via email in the little box to the right. I promise NO spamming of your email! 🙂

Two wrongs still don’t make it right.

Two wrongs still don’t make it right.

As you can see in the picture, the cut truss (wrong #1) is pointing right at the whole house fan (wrong #2).

While whole house fans may have been a good idea, once upon a time in some climates, they have virtually no place in modern energy efficient construction. In northern climates they are of particular concern as they are typically NOT adequately insulated and sealed against heat loss in the winter. When they are not insulated and sealed, the natural stack-effect of the home will pull warm air into the attic more or less around the clock.

The idea of these units is to flush the warm daytime air from the home at night, and then pull in the cool night air to cool the home off. Not a bad idea really. While this principle works in older poorly insulated homes, in modern well insulated homes they should not be necessary. Merely opening a few windows on opposite sides’ f the home should achieve the same result.

If the home is overheating during the day, one should look to the causes of that overheating and fixing the overheating as opposed to installing a system that should not be necessary. If one’s home is overheating and one considers it “well insulated,” I would argue that one should perhaps re-think one’s definition of “well insulated” or that perhaps there are other factors contributing to the overheating.  A good question to ask might be, “what are the air sealing abilities of the insulation?”  Not all insulation is created equal.

As a side note, I can pretty easily argue that even newly constructed homes in areas of the country with high cooling needs are NOT adequately insulated to appreciably reduce energy costs. Code requirements for energy conservation are “minimum” standards, and make no distinction between the air sealing characteristics of the various kinds of insulation.

If installed properly, and if used properly, and if maintained properly in the off season, these fans can help reduce air conditioning costs, improve comfort and improve air quality. Please note that this statement includes a lot of “ifs” and their installation can more often result in increased heating costs in the off season.

Another issue that arises from these fans is that if they are not sized properly (and they rarely are) they are capable of drawing more air into the attic than the attic space can get rid of. This can result in pressurizing the attic and minimizing the effectiveness of the fan—oversized or not. Regardless, even if additional venting is installed to compensate, there will then be compromised and possibly inadequate venting of the roof structure for that part of the year when the fan is merely wasting energy. It might be possible to balance these differences, but the reality is that often the different requirements for the different functions are simply not taken into account—or, worse yet, not even possible to take into account. More often than not, when I see them installed in the Northwest, they seem to be installed on the insistence of someone that has moved here from a climate where they worked or were possibly even necessary.

For the installation above, someone is now going to have to incur the cost of removing the fan and repairing the damaged truss. These costs will now need to be added to the increased energy costs created by the installation in the first place

By Charles Buell, Real Estate Inspections in Seattle

If you enjoyed this post, and would like to get notices of new posts to my blog, please subscribe via email in the little box to the right. I promise NO spamming of your email! 🙂

Is your insulation doing what you think it is doing?

I do not want to tackle the huge ugly topic of whether fiberglass insulation should be used at all–as it frequently is in attics.

For today, I just want to talk about one obvious issue with fiberglass insulation that prevents it from performing as expected.

The short version of what is wrong with fiberglass insulation is that it is not an air barrier, therefor, if it is not encapsulated and air sealed on all six sides its performance suffers. In an attic, at most, only five sides is likely to be sealed leaving the entire top not sealed.

This post is about the sides, which can and must be air sealed. The vented sides are typically not adequately air sealed, especially along the eaves. Insulation baffles, designed to keep insulation out of the lower roof venting and to allow for air flow into the attic, rarely gets adequately sealed.

Soffit vent that allows air flow into the attic

Because the baffle/top plate connection is not air sealed, and because fiberglass insulation represents very little resistance to air flow, air pushes its way into the attic right through the insulation. As it does this, it either cools the ceiling in the area, or warms the ceiling in the area depending on the season and/or side of the house.

In the winter and/or the north side of the home, the air will tend to make the ceiling cooler in the area of the vent. In the summer, especially on the sun side, the air will tend to warm the ceiling in the area of the vent. This will increase both heating and cooling loads of the home.

This next picture shows what that area looks like at the interior ceiling with Infrared camera on the South side of the home. Warm air is moving through the insulation and warming the ceiling.

In the same house on the North side we can see how the ceiling area near the vent, as indicated by infrared camera, is “cooler.” In the actual picture we can see the fungal growth present because this vent happens to be in the area of the bathroom. The moisture in the bathroom condenses on the cooler surface creating a perfect environment for mold growth.

The only real repair for this condition is to pull back the insulation and properly air seal the gaps where the insulation baffle and the house framing meet.

Spray foams are good for sealing the areas where the baffle makes contact with the framing as indicated in the circle in the picture above. Of course, in a perfect world, we would not use fiberglass insulation at all, and instead use types of insulation that are much better at stopping the flow of air. Cellulose fiber insulation can do a much better job at this.


Charles Buell, Real Estate Inspections in Seattle