Would you keep all the windows open with the heat on in your car?

Bet you didn’t know you are heating and cooling a wind tunnel!

Certainly the biggest contributors to moisture in the attic are breakdowns in the building envelope. (Of course I am assuming that water is not getting in from outside the envelope–like a leaking roof) These compromises also make the building less efficient and will empty your wallet more as you attempt to heat and cool your home/tunnel.

air bypasses To understand this problem, it is helpful to think of one’s house like a Tupperware container. Clearly if we leave the lid a little bit ajar (like an attic access that is not weather-stripped), or start drilling holes in the bottom, sides and top, the container will no longer do its job–or at least less efficiently—relative to the number of holes, the size of the holes and where they are located.

Of course, if the holes are so the snake (or whatever else the kid catches) does not die, that is another matter.  It is still important to keep the lid on though.

There are other factors–but let’s keep it simple for today.

We can build the most super-insulated house in the world but if we don’t control air movement in and out of the building our efforts at insulating can be wasted.

One of the most common, obvious, everyday sort of “by-passes” that I see in homes is dampers in fireplaces that are left open. These chimneys will pull conditioned air from the home 24/7–with an occasional pause for atmospheric inversions that can happen. The screen on the fireplace filling up with lint is the first clue that this is happening. Even closed dampers in most cases will not stop this movement entirely.

Eliminating open flame solid fuel appliances altogether is a good idea in the context of building more energy efficient homes.

I have heard people argue that this natural draft is a good way to exchange the air in the home–without a mechanical fan. The truth is that this works–but at much greater cost than running a simple exhaust fan periodically. It is the 24/7 aspect of the chimney that makes it a problem, as it vents conditioned air that we are also paying for. On a windy day the venting might be much more than on a still day. We need “control” over this air exchange if we are truly going to control energy use as well as maintain a healthy indoor environment.

I want to stress that this is a LOT more complicated than I am willing to address here. For example rates of ventilation will not always guarantee good indoor air quality. While outside air is generally of better quality than indoor air, that is not always the case and in some areas of the country, outside air is becoming worse every day. At some point the political aspects of outdoor air quality will be forced to reconcile with the private aspects of indoor air quality. Some will argue that if you want “quality” drinking water you had better be prepared to solid-block-carbon-filter the water where it comes into your home. The same thing is perhaps coming for the air we are bringing into our homes.

Welcome to the 21st Century.

Back to air by-pass issues.

As previously mentioned, the attic access hatch is a common by-pass but the list is almost endless. Here is a partial list of some common breaches: plumbing pipes running through walls and ceilings, can-lights, HVAC equipment/ductwork in attics, crawl space hatches at the interior of the home, wiring holes in top and bottom plates of walls, chimneys, b-vents, improper framing techniques, skylights, pull down stairs, drop ceilings, exhaust fans etc.

“Stack effect” is something else that affects our homes.

Stack effect is relative to temperature/pressure differences. It is relative to the fact that warm air is buoyant. It is further driven by the lowering of pressures inside the home which then allows for air to be pushed into the lower levels of the home (from outdoors and/or crawl spaces). The taller the home, the more pressure differential as the buoyant air moves to the exterior (attic) of the home, bringing with it the moisture in the air. The colder the outdoor environment and the taller the building the more that hot air will be trying to get into the roof structure or outdoors to get to that cold. Perhaps the perfect storm is to have a leaky floor system over a vented crawl space in conjunction with serious breaches in the attic floor.

In this scenario you can think of your poor heating system as attempting to heat a wind tunnel. You will have to be willing to throw a whole bunch of energy at this wind tunnel in order to feel comfortable in your home.

In a very well sealed home there will be less stratification of temperatures and less “driving” of the stack effect–even when doors at the lower level are opened. Opening and closing windows on upper and lower levels in conjunction with each other is a way to manually control stack effect to change the air in the home. This is not rocket science, but can be as expensive as rockets.

At an inspection a while back I had one of the most egregious examples of a home with a functional wind tunnel. The defect was created when part of the forced air heating system was removed. If you could zoom in on this picture, you would be able to see the furnishings in the room below. There were three of these vents into the attic. The missing insulation around the vent is not even consequential in relation to this breach.

Closet vent open to attic

Closet vent open to attic

Sealing these air by-passes, even in older inefficient homes, can drastically reduce heating and cooling costs. Remember , heat tries to get to cold and high pressure moves to areas of lower pressure. So if the attic is really hot in the summer and we are cooling the home we have made the job of the AC unit all the more difficult. Better sealed homes accounts for why the size of heating and cooling systems have halved since the 40’s–remember—back when oil was free?

Sealing and eliminating all kinds of air by-passes is perhaps the most important thing we need to do in making our houses more energy efficient. Insulation alone will not do it and in fact in many cases will only filter the air as the air moves through it. This is especially true of fiberglass insulation–even 18 inches of it. All air by-passes must be found and sealed (or otherwise eliminated) prior to insulating. Choosing types of insulation that are in themselves good air barriers is also recommended.

By Charles Buell, Real Estate Inspections in Seattle

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Raining and 35°—-why shouldn’t I cover my crawl space vents in the winter?

It is OK to leave them open—that is how it works in the Northwest.

I know that it may seem counter-intuitive, but you can actually lower the humidity in your crawl space by bringing in that wet air in the winter.  I will attempt to keep it as simple as possible by speaking in “generalities” as opposed to specific temperatures and humidity levels.

humidity1Lets take air that is very cold and saturated (as in raining), and bring that air into the crawl space that is warmer than outdoors.  When that cold wet air is heated to the temperature of the crawl space, not only can it easily hold the moisture in that air (and not rain in your crawl space) but it will mix with the warm humid air of the crawl space resulting in a net lower humidity of all the air in the crawl space.  As that air then moves out of the crawl space the humidity of the crawl space will thus be lowered.

Temperatures in the winter in the NW are generally favorable for this to happen and reverse slightly in the summer.  In this sense, general humidity and moisture levels can be expected to rise in the summer and then drop back down in the winter.  Of course without proper venting the moisture levels can build to far above acceptable levels, resulting in wood decay/rot and infestations of Anobiid Beetles etc.  And that, in a nut shell, is why it is a bad idea to cover those vents in the winter.

This same principle can happen in the indoor environment if we bring too much of that wet cold air into our very warm indoor environment and result in humidity levels inside the home becoming lower than desired.  This can be especially pronounced in two or three story homes where stack effect, induced by pressure/temperature differentials as well as by winds blowing by the house, come into play.  A simple way to think about this is that the house is having the air sucked out of it at the top and sucked in at the bottom (your house is acting a bit like a chimney).

This can be especially problematic with houses that have window inlets as part of whole house air ventilation systems. 


These air intakes at the windows are one of the ways to meet modern energy code requirements to bring fresh air into the home.  The problem with them is that in multi-story houses, they do not just bring fresh air into the home whenever an exhaust fan is running, they also continually vent outward at the top and inward at the bottom (due to stack effect) when fans are not running (see drawing that shows window vents at “1” and “2”).  This creates WAY more air changes in the home than necessary, wasting considerable energy, and it can lower the humidity in the home by the same principles that reduce humidity in the crawl space.

humidity3There are other factors that can contribute to loss of control of house humidity due to stack effect.  Openings anywhere in the building envelope can result in air moving in and out of the building where we don’t want it to.  This is why it is so important to keep the “Dotted Line” (see drawing) as perfectly sealed as possible so that we are not pulling crawl space air into the home.  In many homes a very large percentage of air that is being drawn into the home can come from the crawl space.  In the drawing we can see that penetrations like plumbing pipes (marked “V”) are sources of such leaks—but “V” should also be seen as symbolizing b-vents, chimneys, wiring holes etc. 

Inadequately sealed attic hatches and crawl space hatches can represent very large breaches in the building envelope. 

Obviously exhaust fans can represent breaches—but they are designed to be breaches.  Unfortunately, even when the fans are not running they can still have inadequate dampers and leak around the fan housings themselves.  Of course running them 24/7 would result in pulling all that cold/wet air into the home around the clock.

Can-lights or any electrical junction boxes in the building envelope—especially at the attic level—can represent significant breaches of the building envelope as well. 

While right now we are primarily discussing the effect of bringing cold/wet air indoors, we can obviously create problems for the home if this heated/moist air can find its way, or is vented directly into the cold attic space.

A real-life story of how this works happened on a recent inspection.  My client complained that he was not able to keep the humidity in the home high enough in the winter to protect some very expensive guitars.  I suggested that he correct the missing weather-stripping on the attic access hatch and close the upper window air intakes.  Humidity levels quickly returned to normal levels and the guitars are now happy.


Charles Buell, Real Estate Inspections in Seattle


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