Ken Levenson from 475 High Performance Building Supply explains what overheating is, why it happens and how to deal with it.
Interview with Ken Levenson
Ken Levenson ran his own architecture firm in Brooklyn, New York, for 20 years. As he focused more and more on low carbon solutions – and eventually Passivhaus – he became aware that it was very difficult to get hold of appropriate materials. That's when he decided to set up online store 475 High Performance Building Supply.
We Each Experience Overheating at Slightly Different Temperatures
In general terms, overheating is where the internal temperature of a building rises to such a point that it causes discomfort for the occupants. Therefore, with different people that comfort threshold is likely to be at different temperatures.
The Passive House Institute has a more specific and technical definition of overheating: no more than 10% of the hours of a day can be above 25º Centigrade (77º Fahrenheit).
Overheating of Buildings Has Happened Throughout History
Most people have experienced overheating in buildings at some point in the summer months.
What's unique to Passivhaus is identifying overheating as a problem and defining it, in order to address it successfully.
There Are Several Reasons a Building Might Overheat
On hot days if there are uncontrolled solar heat gains a space can quickly warm to uncomfortable temperatures.
Ken continues: “Another way is just from the thermal envelope from an insulation point of view. If you have a very hot temperature the heat transfer through the enclosure is going to occur coming into the structure just like it would be in the winter time going in the other direction.”
Internal Heat Gains Are a Huge Liability in the Summer Months
In a Passivhaus internal heat gains – the heat from our bodies, equipment, lights, cooking, etc. – are used to great benefit in the winter months.
However, in the summer they are a constant heat source that can significantly contribute to overheating if the space is at the borderline.
When a Passivhaus is being designed with the Passive House Planning Package (PHPP) internal gains are being taken into account and a balance is being struck so that the risk of overheating is being kept to a tolerable level.
Occupants Can Often Prevent Overheating
Some designs might use night time cooling as part of the strategy to avoid overheating. This requires the occupant to flush the air in the house by opening windows at night time when it will be cooler outside.
Closing external window shades is another strategy that actively involves the occupant.
Ken says: “If the occupants aren’t behaving per the design and the PHPP they'll be experiencing much greater incidences of overheating.”
Once Overheating has Occurred, Active Cooling May Be Necessary
If the temperature outside is hotter than it is indoors, there is no way to regain a comfortable temperature than by actively cooling (air conditioning) the space.
Prevention is therefore better than treating the problem. This can be done passively with architectural elements such as overhangs and awnings or with external blinds.
It's Easier to Control Solar Heat Gain on the Southern Exposure (in the Northern Hemisphere)
Ken says that as windows are added to the east and west facades, solar heat gains become more difficult to control than on the southern exposure.
This doesn't mean eliminating windows on these aspects of the building, just paying close attention to how they are shaded.
Use the BBQ for Cooking in the Summer!
Having good indirect daylighting allows the occupant to keep out direct solar gains in the height of summer.
Ken has previously advised clients to be much more cognisant of their incidental heat gains. This entails switching off lights and as many electronic gadgets as possible, and avoiding activities such as baking – instead opting to use the barbecue!
Passivhaus Buildings Have the Ability to Stay Cool in a Blackout
When so much of the USA is reliant on air conditioning, a Passivhaus has extra resilience.
Ken recalls that when there was a blackout in Virginia a couple of years ago most houses quickly became unoccupiable due to the heat. The Passivhaus in the neighbourhood, however, was not only habitable but comfortable for many days.
Passivhaus May be More About Cooling in the Future
When a building is air conditioned the heat is removed and put outside. In cities, therefore, as more people require active cooling the urban heat island effect only gets worse.
Ken says: “It’s going to be interesting to see how this evolves. Passivhaus really started from a cold climate heating genesis, beginning, but really what I think will drive the conversation, the science, and how we’re approaching it is going to be cooling dominated.”
A Heat Pump is Very Handy in a Climate Like New York
Many Passivhauses in New York are being heated by air source heat pumps. However, this simple technology is very useful in the extremes of summer for cooling too.
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