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! 🙂

The bathroom fan turns on and makes noise—it must be working!

Your bathroom has an exhaust fan.


While not required in every instance in some jurisdictions, in modern construction, in Washington State, they are required in all bathrooms and  I think they are essential.

For this exercise let’s assume you have one. How do you know it works? “Well, I can hear it running,” you might say.

OK—let me be a little more precise. How do you know it is “functional.” By functional I mean how do you know it is pulling air from the room. (For now we won’t discuss where it is going after it leaves the room–that will make the post too long.)

Home inspectors will not generally pull out expensive testing equipment to determine if the fan is functional. A simple test, that is highly unscientific, that does give us “some” indication of function, is whether the fan will suck a tissue paper up against the grille.

Another test is to put same tissue paper on the floor right at the bottom of the door to the room and then turn on the fan. If the fan sucks the tissue across the floor that is a pretty good indication the fan is “functional” to some degree.

So let’s say the fan does not hold a tissue paper and it does not move away from the door during operation. Both of these conditions can be caused by either a blocked exhaust on the fan or too tight a seal on the bottom of the door. We do not want to recommend replacing the fan if it is a simple matter that air cannot get into the room. One can open the door a little and then see if the fan will hold a tissue.

Another common problem that this simple test can reveal is what is pictured in the following picture.


The fan is indeed pulling air–but is blowing it right back into the room on the other side of the grille. Ideally when running this test, the entire grille should be covered with tissue, but with experience the inspector can figure it out with partial coverage.

As you can see, this simple tissue paper test can tell us a lot about the true function of the fan—not simply that it turns on and makes noise.

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 home a sieve?

We can never totally stop air movement in and out of homes. Not only are our attempts to seal homes never perfect, materials change over time and sometimes create new gaps where there were none to begin with.air-infiltration2  Some homes are more forgiving than others because of the choice of materials used.

air-infiltration1In this first picture we can see the very common black staining that happens around the edge of the carpet where air is finding its way in and out of the living space at the wall bottom plate.  The carpeting is merely acting as a filter.

In this next picture the black staining on the soffit around this beam of a home, with a flat roof, is where air is leaking in and out of the building.

air-infiltration3I have long held that since it is going to be difficult to completely stop air movement, why not do our best to at least torture the heck out of it as it moves?  In other words make it difficult for air to flow.  We can do this by choosing really good quality insulation like cellulose fiber insulation–it gets into every little nook and cranny making a very good barrier to air movement.  We can caulk the connection of bottom plates and sub-floor.  We can use weather-stripping on doors and windows and access hatches. We can glue drywall to studs and top & bottom plates. We can seal around pipes and ductwork that penetrate the building thermal envelope.

If we can stop or significantly slow down air movement something else happens in the process.  One of the major ways moisture moves through the building envelope is in moving air.  If we can stop air movement we can eliminate or greatly reduce moisture movement as well.

Fiberglass insulation is notoriously bad at stopping air movement–and thus moisture movement.  In fact, in the early days (early 1970’s) of making homes more energy efficient, one of the main reasons we went to such great lengths to install plastic vapor barriers behind the drywall was because fiberglass insulation could not do the job adequately.  Everyone has experienced the cold drafts that can occur around electrical outlets of homes insulated with fiberglass insulation.  This does not happen with cellulose fiber insulation.

In my opinion stopping air movement is perhaps the single most important quality of insulation. Its “R-value” is a moot point if it can’t stop air movement.  How much money are we really saving by insulating our homes if we cause structural damage by moisture related to air movement?

Your home inspector will often be on the lookout for signs of air infiltration/exfiltration during the inspection.  An important thing to keep in mind is that air moves both in and out of homes depending on atmospheric conditions.  At times your house can be pressurized and at other times depressurized–close to “neutral” is ideal but not always easy to manage.

For example, if you turn on all of your exhaust fans, where will air come into the home that is being displaced by the exhaust fans? 

Well, if it can’t “easily” find a way in, a negative pressure in the home will be created and the fans will actually not move any air–won’t do their job.

As an example of this effect, my sweetie and I had checked into a little motel a few years ago and experienced this first hand.  The motel was brand new and very well built.  The motel had a little kitchenette and when my sweetie decided to make toast in the morning, she did her typical method of making toast–which is to have the smoke alarm tell here when it is done.  Well, NOT REALLY wanting the entire building’s alarms to go off, I thought it more prudent to turn on the kitchen exhaust fan and get rid of the smoke before the alarms went off.  I turned on the fan and the smoke just sat there like a dense motionless fog.  It occurred to me that perhaps if I opened a window it would help and immediately the fan became functional and pulled all the smoke out of the unit and the alarms never went off.

In most homes, air does find a way in-somewhere, but fans will be much more effective if they can obtain air easily.  Air will find its way into the home around poorly sealed doors and windows, around the bottom of walls, around electrical outlets, around crawl space access covers, down the chimney or even back in through un-dampered exhaust fans.

Another important factor to remember is that we want air to come into the home–we just want control over how, where, when and how much.

Energy efficiency is compromised when we lose control.

Take this picture of a heating system supply-air duct in a crawl space.  This is a great example of how inadequate fiberglass insulation is as an air barrier, as well as a good demonstration of air movement at a location where we don’t want to see air movement.


All duct connections should be air-tight and on my planet ductwork would not be allowed to be installed outside the building’s thermal envelope.  If the connections are not air-tight, when the heating system is operating, warm air will be pushed into the crawl space–right through the insulation.  When the system is not running, these leaking ducts will become a place for air to enter the home when the home is under negative pressure (as when exhaust fans are operating or due to the home’s stack effect).  As you can see from the black band around the ductwork, the yellow fiberglass insulation is acting like a “filter” as air moves through it.

These are just a few of the “visual” indications of poor control of air movement in the home.

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 :-D