A Post-Passivhaus Paradigm for Energy-Efficient Design
In hot, humid climates, the most important HVAC design variable is the latent load
Last night, I enjoyed an intense conversation with my friend Bill Updike. Bill, who has been closely following the developing partnership between PHIUS and Building Science Corporation, is the green building specialist at the Washington, D.C. Department of the Environment.
We were talking about cost-effective energy-efficient design, and Bill tossed off a comment that the key to any design — at least in our mixed-humid climate here in Maryland — should be the latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. of the building. When he said that, my mind lit up like a pinball machine showing three cherries.
Here’s why: The latent load of a building — in effect, the amount of humidity inside your home that has to be removed for you to feel comfortable — is largely independent of how much insulation you use in the floor, walls, and roof, as well as the qualities and disposition of the other components of the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. You can thicken your insulation as much as you want, but you will not substantially reduce the latent load.
So then, why not select the most efficient heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. that will handle a given latent load, and then scale the envelope components to the point where that same heat pump can just handle the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. during the winter months?
ARTICLES BY ALAN ABRAMS
The fatal flaw in the Passivhaus approach
This approach would be in contrast to Passivhaus methodology, where envelope and mechanical system design begins with the established value of 4.75 kBtu1,000 Btus per square foot per year for specific heat demand.
Generally speaking, using the Passivhaus system, the insulation values are beefed up until the predicted annual heat load is no greater than that value. In theory, that is the point at which the ventilation system — in our climate, a system based on an ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. — is capable of distributing not only sufficient fresh air, but also the heated air required for thermal comfort. At that point, as the theory goes, a conventional heating system can be eliminated — in turn saving enough construction cost to pay for the extremely high levels of insulation and other envelope components that were necessary to hit the magic 4.75.
The fatal flaw of this system, however, is that even when the envelope is designed to achieve Passivhaus certification, the ventilation system may not be adequate to manage the latent load. So a conventional mechanical system remains necessary — and the higher levels of insulation required for Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. provide what can then be considered unjustifiably diminished returns.
Elevating latent load to a new position of importance
So it seems reasonable to approach energy design from a individually determined performance standard — that is, latent load — instead of an arbitrary value, like the mystical 4.75 kBtu.
This provides a rational basis that hopefully avoids the rapidly diminishing return for extreme levels of insulation.
Reducing the small-house penalty
Of extreme importance is that it may tend to offset the penalty that Passivhaus imposes on smaller houses, where the higher ratio of wall area to floor area exacerbates the diminishing return phenomenon. Conversely, none of this negates the value of PHPP or WUFI-Passive as design tools.
It was Deep Throat who said, “Follow the money.” When it comes to designing energy-efficient homes, it’s still good advice: to follow the money into the thickness of your walls, roof, and particularly your subslab insulation. But I think the real story is to “Follow the water vapor.” That is the key, in my book.
So, thank you, Bill, for this epiphany. I’m eager to try this approach on the next project. It may be that the results will not be too far off from a straight-up Passivhaus analysis. I’ll keep you posted…
Alan Abrams is a Certified Passive House Consultant, a Certified Passive House Builder, a
Certified Green Professional (NAHBNational Association of Home Builders, which awards a Model Green Home Certification.), and a Certified Professional Building Designer (American Institute of Building Designers). He is also the owner of Abrams Design Build in Takoma Park, Maryland.
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