We’ve been monitoring the temperature and humidity in the Arlington Passivhaus for about a week now. As part of a long stretch of heat wave a storm hit (they haven’t come up with a name yet, but let’s just call it the Big One for now), along with the PH, more than 1.5 million homes in VA/MD/DC area lost power, some areas are still recovering from a multi-day outage without air-conditioning. This actually provided us with an unique opportunity to monitor and report what happens in this situation.
Let’s start with monitoring.
Here are a few constants:
MiniSplit setting: cool 67 F
ERV: Low (approximately 75 CFM)
Transfer Fans: ON
A note on humidity: we noticed the indoor humidity has been consistently high (60s), since we’ve been monitoring. We attribute this largely to the moisture still in the building materials, especially that giant basement slab and seasonal factors. However, we also noticed the relative humidity level has about a 7% swings if several windows or doors are open and it is humid outside. This let us to consult the Bethesda Passive House team (David and Brandon JR.) since they probably experienced something similar. The advice they gave us was to turn the ERV down while the house is “drying out”. Because while the ERV dehuminifies the air that comes into the home, it does not take all the moisture out, so in effect, operating an ERV makes the humidity problem worse. See http://www.greenbuildingadvisor.com/book/export/html/15970 for more explanation on this. By turning down the ERV, we’ve been able to consistently drop the relative humidity levels a few percentage points a day from high 60% to mid-50%. It still likes to go back up on very humid days though…
On a Normal Summer Day
We don’t have a large enough data set yet but we can report our observations so far.
We’ve observed the following on a typical summer day (June 27, 2012 Outdoor Temp: 62-89F, Humidity: 23-40%)
At 12 pm when the outdoor temp was 88 F, the indoor temps were: 68F/70F/70F (Basement/1st Floor/2nd Floor)
At 3pm when the outdoor temp was 94 F, the indoor temps were: 68F/70F/72F (Basement/1st Floor/ 2nd Floor)
Two Degree Temperature Differences Between Floors
This experience was consistent with other days with similar levels of humidity and temperature. The temperature differences are typically about 2 degrees. Heat rises up so, the 2nd floor usually is the warmest.
Effects of Solar Screens
We have motorized solar screens on the 1st floor south-facing windows. This picture was taken around noon time and it is illustrative of the need to shade first floor south-facing glazings, the sun hits directly on it, while the 3ft roof overhang protects the 2nd floor windows from the sun. By the way, shading needs to be on the outside of the windows, or they just become radiant heaters!
One day, we wanted to see the effects of not shading, the result is that initially, the 1st floor becomes warmer than 2nd floor, then a few hours later, both 1st floor and 2nd floor are 2 degrees warmer than they would normally be. This makes sense because the lack of shading makes the 1st floor 2 degrees warmer, then as heat rises, the 2nd floor eventually gets 2 degrees warmer too. The basement is most consistent, the lack of shading on the 1st floor has no noticeable effect on the basement temperature. However, not shading creates a temperature difference of 3-4 degrees between basement and 1st floor.
On an Extra Hot and Humid Day (Outdoor Temp 71-103 F, Humidity: 50-66%)
At 12 pm when the outdoor temp was 95 F, the indoor temps were: 68F/70F/72F (Basement/1st Floor/2nd Floor)
At 3pm when the outdoor temp was 103 F, the indoor temps were: 70F/75F/77F (Basement/1st Floor/ 2nd Floor)
As you see, at 95 F, the indoor temperatures are still able to keep a nice two degree temperature difference between floors. However, when it hit above 100F, temperature differences widen between basement and first floor to 5 degrees. However, the difference between first and second maintains 2 degrees. Also keep in mind, conditions are never perfect. On this day, we had workers opening and leaving doors open all day long bringing in large amounts of hot and humid air.
After the Storm
The storm devastated Northern Virginia, Mayland and D.C. Here’s a picture of what we are seeing around town.
After the storm, the heat wave continued to rage on. We were curious how Arlington Passivhaus performs in a heat wave with no active cooling. Here’s what we observed on after 40 hours of power loss on another scorcher day:
While the outdoor temperature was 92 degrees, the basement was a comfortable 73 degrees. First floor was a warm but not unpleasant 81 degrees. Second floor was 79 degrees.
This tells me two things:
Insulation Really Works
First, all the insulation in the walls and roof is effectively isolating the indoor environment from the outdoor elements, slowing down the effects of extreme outdoor temperature changes, i.e. after 40 hours of power loss, the first floor only warmed up by 6 degrees (75F -81F) and second floor by 2 degrees (77F-79F).
Shading is Crucial
I think the 6 degree increase in temperature on the first floor can also be attributed partially to the lack of shading on the French door on the middle of the wall (see first picture above), which explains why the second floor only increased by 2 degrees.
We still have no power…
We anxiously await the power to return. This would be an opportunity to monitor how quickly a system like this recovers to desired indoor temperature and relative humidity level.
Hopefully, the next time we post, there will be power and interesting observations to report.