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.
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.
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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