“Build Tight – Ventilate Right”

This was the mantra chanted at all Energy Star / EEBA conferences for years to quell the concerns of naysayers who recalled the horrors of the “sick building syndrome” from the early 70’s attempts at sealing up office buildings. I joined the chorus of chanters, but in truth only really delved into the build tight part. But how does “ventilate right” actually translate into design?

This question of technical detailing become painfully pertinent as the results of the second blower door came in on my super insulated house. The first round was done prior to sheetrock, and was followed by several days of painstaking work in caulking, painting and taping to tighten up all the leaks identified by the thermal imaging camera. This baseline was ACH .39, pretty good for a remodel. This second round came at the project completion, and was anticipated to knock our socks off. Which is did, but with disappointment - ACH .35. Still very admirable, and just on the line of requiring ventilation, but certainly not a reasonable return on the investment of time and labor spent.

What happened? The house is riddled with holes in the form of exhaust vents which connect directly to the outside. All 7 of these bathroom fans are Energy Star fans, but this only refers to their energy efficiency in exhausting air. There is minimal protection in preventing unwanted infiltration when the fan is not active.

The solution? Heat Recovery Ventilators (HRV). All the bathroom fans can be fed into the HRV, which both prevents the problem of the unregulated holes to the outside, and also couples the amounts of air exhausted out of the house with return air brought into the house. This requires an HRV with a humidity control, such as the UltimateAir by Sterling Technologies, which has a 95% heat recovery, higher than US norm, and sufficient to meet PassivHaus requirements. The supply can to into the return side of the HVAC system.

Note that only a few HRVs have some level of humidity control, which can maintain existing levels, but are not able to reduce humidity.  Also, they can recover heat, but not generate it.  An HVAC system will still be required based on the climate, the insulation, and the size of the house.

Laundry and kitchen exhausts are the exception, as they must be vented separately and directly to the outside, per the IRC. Options for laundry are to install a condensing dryer, an outdoor drying line, or an indoor drying room with an exhaust outlet within the balanced ventilation HRV system. Kitchen exhaust fan covers can keep air from leaking in when the exhaust fan is not in use. The covers typically attach via magnets for ease of replacement.

The second portion of the air leakage was through some well-hidden gaps in the outer envelope. Once air penetrates the envelope, it moves freely about the rest of the interstitial space. And that is precisely why our construction methodology needs to move to panelized construction, or solid concrete. Wood frame construction just has too many opportunities for air leakage, too many workers who need to perform flawlessly, and too susceptible to change and shifts from the wood drying.

Build tight and ventilate right. My next project? ICFs / SIPS roof and HRVs. Overall faster, with more certain outcome, and less expensive than chasing air.