November/December 2006 Editorial

Posted by Home Energy on November 24, 2006
The Cost Of Air Tightening May Go Up

by Max Sherman

I grew up in an old, leaky house. I’ll bet a lot of Home Energy readers did too. When the wind blew, you could almost feel it inside. No one worried about getting enough ventilation then, though. After all, the Latin root of “ventilation,” ventilare, means to expose to the wind.

Houses were said to breathe. Despite the back-to-nature feel of such descriptions, we have known for a long time that such breathing, less poetically known as infiltration, is not particularly sweet. Infiltration causes uncomfortable drafts; infiltration is greatest when it is most expensive to heat and cool; infiltration is least when we would like it most. Nevertheless the vast majority of current housing stock gets its outdoor air from infiltration and spends 30%–50% of its space-conditioning bill on it.

All houses need sufficient outdoor air to dilute indoor contaminants. In new construction, the mantra Build Tight; Ventilate Right has led to new homes that are tighter than existing homes by factors of 3 to 10 times. Such houses get little of the outdoor air they need from infiltration and need substantial mechanical ventilation. Standards such as ASHRAE Standard 62 provide guidance on what that should be.

New, tight construction clearly needs mechanical ventilation to meet minimum requirements, but does old, leaky construction? That leaky old house many of us grew up in may have cost a lot to heat and cool, but we got plenty of air to dilute indoor contaminants. Is that OK? From an energy standpoint, perhaps not. From the standpoint of indoor air quality (IAQ), why not?

For a long time, energy auditors and retrofitters have used ASHRAE standards for guidance in answering the question, How tight is too tight? For example, ASHRAE 62-89 said that homes should have 0.35 ACH, and that this could be made up by infiltration or natural or mechanical ventilation—but did not say how. Conscientious retrofitters used various methods to figure out how much infiltration they should count, but really such a requirement was meaningless; it enabled anyone who wished to “meet” that standard. The current version of the applicable ASHRAE Standard, 62.2-2004, uses a slightly different formula than 62–89 but says that about half of 0.35 ACH must come from mechanical ventilation; the rest is assumed to come from infiltration.

With the current ASHRAE standard, you have the option of taking more credit for infiltration in that old, leaky house, but you have to measure airtightness and use a protocol that seriously discounts the resultant infiltration to account for its variability. This procedure, based on basic building science, is flexible, and it does allow the decision makers to optimize retrofit resources with their program objectives. It ensures that infiltration credit will only be given when there really is ventilation.

Following a ventilation standard like ASHRAE 62.2 provides not only guidance but some amount of protection for the retrofitters who follow it, because they are following a standard of care accepted by the profession. The current standard fills a valuable need in the residential retrofit industry. However, changes are being proposed for Standard 62.2 that will substantially increase the cost of following that guidance and providing that protection for retrofitters working on old, leaky buildings.

The current 62.2 committee, with a focus more on new construction, is proposing to completely eliminate the infiltration credit from the standard. This means that if one wants to meet 62.2, a full-size, continuously operating mechanical ventilation system must be installed in an older home in exactly the same way as in a new, tight home.

This proposal is apparently intended to force retrofitters to tighten buildings to the maximum in order to conserve energy. But the real incentives will cause the opposite to happen. If you can in fact regularly get that leaky old house very tight (for example, 3 ACH50), then this change is for you. But if you can’t, then you have only poor choices left.

One choice is to do what you can to tighten and then install the redundant mechanical ventilation. That uses up some resources you could have applied to other efficiency measures, but more importantly, the occupants are not getting the energy efficiency they deserve because they are running and paying for extra mechanical ventilation and the associated space conditioning.

Another choice is to walk away from using ASHRAE Standard 62.2, by applying some other standard of less repute or none at all. Here you take a bit of increased liability and the occupants are less assured of getting good IAQ. One way to avoid this, of course, is to do no tightening whatsoever, thereby passing up cost-effective energy efficiency options.

I recommend a third option. Let ASHRAE know how you feel about this proposed change. When this new proposal comes out for public review, which should be happening as you read this, tell the committee what you think of it by writing a public review comment through the ASHRAE Web site, using the Standards for Public Review shortcut.

Infiltration is the form of ventilation that we love to hate, but ignoring it as a ventilation source does not make it go away. Certainly it will not go away in our heating and cooling bills. It should not go away in a minimum ventilation standard. If we could make existing construction as tight as the new homes the 62.2 committee cares about, it would not matter; but for the world of existing construction, it certainly does.

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