It's so hot the cats are streaking. A massive heat wave is covering the US and many people are wearing as little as possible inside their houses in order to beat the heat. For some reason the question of cooling a passive house comes up frequently almost as if people expect them to be designed as little ovens that are warm, but would be miserable when it is hot outside. So… does it get hot enough that you will be running around au natural in a Passive House because of the weather?
I just read in interesting discussion on whether Passive House would work in New Orleans and the concerns were that super insulation might be overkill and actually in some situations might result in poorer performance. While a very interesting article, it still comes back to my blog posting of last week – what is the recipe for soup? There is no one Passive House set of building plans that will fit for every location or even just every climate region that we have here in the United States. That is where the STRENGTH of Passive House comes into play. Each project – every single one, is modeled for performance on its lot with its surroundings and its occupants usage and its climate. So the point in the GBA article that extra insulation worked in New Orleans for cooling but failed in Colorado missed the entire concept of Passive House. You would not design the same house to be built the same way with the same materials in both regions. (If you would like to read the GBA article there is a link below.)
The article did make one extremely important point and that is what I want to expand on. “The extra insulation may not be particularly cost-effective but it will never increase your energy use for cooling as long as outside temperatures are higher than the interior. The AC pumps the heat outside: the insulation stops it getting back in. If it's cool enough outside that you're not needing to use the AC then standard natural ventilation options apply.”  The insulation stops the heat from getting back in once the AC pumps it out. In a climate with very hot summers there may be a need to add mechanical cooling and perhaps humidity control as well - that is allowed in Passive House design just like windows that open are also allowed. What ends up being different is just how much cooling is needed and of what type, but that is determined by the amount of insulation and air tightness factored into the location and style of the building. That is where the planning to keep the heat in or keep it out comes in to play.
Let’s review how heat works. In a very simple definition the 2nd law of Thermodynamics explains how heat will seek to equalize temperatures with cold. (Actually, my technically irritating friends will remind me that there is no such thing as cold, just the absence of heat. I must remember to remind them they are not cold in December when their teeth are chattering.) Comfort within a building then comes down to being able to control the transfer of heat. Insulation slows down that transfer and air tightness keeps airflow from whisking heat in or out when you don't want it to.
In the winter you want a thick coat on your house to keep the heat in, just like you would put a coat on to go outside. We understand that. The word “coat” doesn’t work so well for summer, but what if instead you thought of your house as a cooler. The better the insulation AND airtight seal, the more the heat stays out, the better your drinks stay cold in the cooler, and the longer the ice lasts. Apply that concept to your house. You keep the hot air outside through insulation AND airtight construction just like the really good cooler designers planned for their coolers and you will stay much more comfortable.
So the point then isn’t a bunch of insulation or not a bunch of insulation, it is the correct amount of insulation and air tight construction planning for that specific building type and location. Once the numbers are crunched it may become evident that mechanical cooling will be required, but the size of that unit will not be anywhere near the size of what homeowners in the South are used to, just like the folks in Minnesota won’t need the giant furnace or wood stoves that they are used to.
So, will you end up streaking in the summer because of the heat in a Passive House, probably not… but use your imagination – maybe you can come up with an even better reason!