Our second day of the tour started at what Paul and Jocelyn call the “Fisher Price House”. The Fisher Price House (FPH for short) is a double-wide mobile home that has been basically permanently installed on a granite slab that was cleared in the side of a mountain. It’s mostly made of plastic and chemicals, hence the moniker FPH. Paul also calls it “an air-tight baggie” because all of the doors, windows and joints are sealed so tight there is almost no air-flow in the house.
There was quite a bit of discussion while I was there about how homes should “breathe” and how after years of mandating minimum levels of insulation and tightening up on home regulations the government finally got some studies done that showed there really should be a minimum rate of air exchange to keep people in structures healthy and reversed their policies. All of that was mostly an aside, the air-flow discussions primarily centered around the effect of proper air-flow and draw to keep Rocket Mass Heaters working correctly.
There is a beautiful proof of concept Rocket Mass Heater in the FPH.
The barrel on this RMH is made of stainless steel and is really quite striking. The thing about this RMH that makes it special is it is the first “pebble style” RMH build. For those not familiar the “Mass” in Rocket Mass Heaters is used to store the heat produced by the system and release it slowly over time, that is part of what makes them so efficient. Typically the mass is made out of Cob, which is a mixture of sand, clay, straw and water. In a pebble style heater the mass is primarily large-ish rocks that are surrounded by pebble size rocks contained in a wooden frame. Paul’s also has a granite top to the mass “bench” so it also looks very nice and is useful at the same time. I’ve been told in the winter they put their clothes drying racks above the bench (Permaculture function stacking at it’s best)!
When Paul, Donkey and Ernie Wisner built the FPH RMH (how’s that for acronyms?) they originally tired venting it out the wall, just to see if it could be done. It turns out it can, but it did not draw well on cold days. They re-routed the chimney through the roof, as is typical with most wood stoves, and that did the trick. Paul figures that his pebble style bench and the stainless steel barrel make this stove about 30% less efficient in actual heat generation and retention than a cob style with a normal metal barrel but he still typically only runs it a few hours every couple of days in the middle of a Montana Winter.
We spent a bit of time in the house, firing up the Rocket Mass Heater, and discussing design tricks and I can personally attest that it didn’t take long for the room to be cozy and it stayed that way long after the fire was out.