From the beginning, we've planned on this high performance tiny house on wheels (THOW) being airtight. That's a no-brainer, and everyone else wants airtight homes too, whether they know it yet or not. If you want to see how bright-eyed and bushy-tailed we were when we started planning the Tiny Lab, check out this webinar on the HVAC design:
After airtight construction, the next important step is always making sure the home is VENTILATED right, with fresh air in just the right amount. Too little fresh air, and you get staleness, odors, condensation, and a deep-seated disgusting feeling. Too much fresh air, and you're flushing conditioned air down the toilet. The calculation for determining how much fresh air any house needs is called ASHRAE 62.2-2013, and you can see a simplified breakdown of that here:
And finally, you need to decide How Exactly You're Going To Do This. That's always the sticking point, and designing and building this tiny house on our own has taught us that all day, every day. The devil's in the details. So began our ventilation adventure, and we immediately talked with our longtime friends at Panasonic, who recommended three pieces of equipment:
- WhisperGreen Exhaust Fan
- WhisperComfort 20/40cfm Spot ERV (Energy Recovery Ventilator)
- WhisperSupply 10/20cfm filtered supply fan
The ventilation strategy we were planning on initially was to exhaust air (20cfm minimum) from the shower (at the top right), which is only a few feet away from the kitchen (the L-shaped cabinet just below it), and supplying fresh air at the rear of the tiny house, in the underloft bedroom (at bottom).
We immediately faced a challenge with pressure equalization. Since this tiny house would be so airtight, the results of the blower door test would likely be less than 67cfm@50Pascals (2.0 ACH50). Without all the mumbo jumbo, that means that if I ran a regular exhaust fan in the bathroom or kitchen at anything close to 70cfm, I'd be sucking so hard on the house it would be as if a 20 mph wind was blowing on every single surface of the house (and that's a lot of surface- about 1000 sq. ft.)
Also, while the WhisperGreen bath exhaust fan can be 'manifolded' (which means connecting a duct system to the fan to distribute the air wherever you want it), the spot ERV cannot. A really cool MacGyver hack was recommended by Panasonic's technical team, which I think is awesome and could easily be done by anyone else per the instructions:
Although this is all very cool, it was deemed too iffy to work properly for the Tiny Lab because of all the customizations we're making with the on-wheels aspect, the tightness, the tiny size, etc.
To keep this story from being ten pages long, let me just say that we are now on our FOURTH and final ventilation plan. Lawrence Berkeley National Laboratory's Brett Singer and our favorite dehumidification guru Lew Harriman both strongly urged us to use a fully-ducted HEAT Recovery Ventilator instead of the ENERGY Recovery Ventilator (which exchanges both heat and humidity). We'll be fighting the humidity the whole way, since every shower, cup of tea, and exhale will add to the moisture in the tiny house.
Also, we all agreed it would be infinitely better to exhaust the stovetop gases directly to outdoors with an ENERGY STAR kitchen exhaust hood. We're keeping the pressure imbalances in check with a hole in the wall controlled with a mechanical damper that's activated by pressure imbalances.
Our final ventilation strategy is from our Product Partner BROAN, and we'll show you exactly how that miracle cure works when the system arrives next week! Thanks for tuning in, and for your support in these trying times. Whew.