Accounting for Air Infiltration

Q: I am having a very difficult time finding HVAC contractors who know how to calculate the load requirements for insulating concrete form (ICF) structures. The HVAC contractors all come back with estimates based on a general load calculation, which does not take into account the tightness of the ICF construction method. Do you know of a software program for HVAC contractors that includes air-infiltration calculations?

Ron Hollingshead
MainStreet Properties

Software guru Bion Howard responds:

A: Your question raises issues that go beyond simply picking the right software package. Currently I am not aware of any program that has an ICF-specific module or a dedicated approach to handling ICF construction. Estimating air-tightness in buildings that are yet to be constructed is always somewhat of a gamble. Here is some guidance that may be helpful.

Your question suggests that the HVAC contractors you deal with need more training in building science issues to become better-informed users of whatever software they use for HVAC system sizing. Unfortunately, few vendors of building energy software packages go the extra mile to train users on what inputs make sense and on how to deal with newer construction approaches. One exception is Energy-10 (see "Software Secrets," HE May/June '00, p. 5). Its vendor, the Sustainable Building Industries Council (SBIC), offers a national workshop series called "Designing Low-Energy Buildings". More should follow the lead of SBIC by presenting workshops integrating primary building science education with advanced energy-design software user training.

There are now many full-shell ICF homes out there that have probably had blower door testing. The supplier of your ICF building system may share field data on finished homes' leakage tests. Armed with historical airtightness data from examples of the same ICF construction system you use, you could provide guidance to the mechanical contractors. I would also encourage your commitment to blower-door testing of your own production. Doing so will build up an airtightness knowledge base on your own products. Also, observing blower door testing and sealing operations is a great way to find and repair pesky systematic air leakage sites in any construction system. (Please forward your results to Lawrence Berkeley National Laboratory's Nance Matson at nematson@lbl.gov, so they can be added to a national database.)

Your contractors very likely use a computer program like Manual J, which has air leakage assumption settings that can be edited. Failing the availability of historical blower door data, ask that your HVAC contractor use the "tight" setting or the equivalent leakage category in their software. They may have an input tab or window for "air-change per hour" (ACH) values. Without specific data, they should select a value in the range of 0.25-0.35 ACH (the equivalent average "natural" leakage) for new housing. On the low end, 0.25 ACH is a typical backed-out value from blower door tests that we have seen in the literature concerning new homes, while 0.35 ACH is the recommended average ACH level for residential buildings (ASHRAE Standard 62). Their program's input side may also include editable assumptions about mechanical ventilation rates that combine with air leakage rates to create an overall average air change rate.

Alternatively, you could use a volumetric rate (CFM per unit area, or length), either by estimating the lengths of leakage cracks in the proposed design (which is tedious) or by estimating openings normalized to the building surface areas. The Energy Efficient Building Association (EEBA) Builder Guide criteria appendix recommends the latter approach. Unfortunately, there are vagaries in these types of estimate, and residential load calculations are very sensitive to assumptions as to how leaky the building is. In an efficiently insulated home, air leaks account for a proportionately larger part of the design loads.

When building with the ICF system, remember also that it creates an integral air-barrier, provided that the joints are properly sealed. This will lead to low air-leakage rates that are insufficient for natural ventilation. You don't want your buildings to be insufficiently ventilated by failing to provide sufficient fresh air. The building then becomes "accidentally ventilated"--which will not provide good indoor air quality despite the most accurate HVAC calculations. I recommend that you provide controllable mechanical ventilation in tight ICF homes. With tight construction, providing good ventilation for indoor environmental quality may be as important to your customers as getting the HVAC contractor to fine-tune the equipment-sizing calculations.

Bion Howard is principal of Building Environmental Science & Technology (BEST), an environmental building consulting firm in the Washington, D.C. area.

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