Microhouse indoor air quality is of great importance given the tight spaces and tight structures many are building. There has been some piecemeal discussion about microhouse indoor air quality, but after many months of reflection on this topic (and as a BPI Energy Analyst and former DOE guy), I offer some basic guidance.
In short, indoor air quality might be thought of as a product of a) product off gassing, b) combustion appliance functioning, c) moisture levels, and d) natural and mechanical ventilation. Let’s start with the least frequently discussed.
1. Chemical off gassing/breakdown
Any product in the home that off gasses as it ages will contribute negatively to indoor air quality. While not chemically sensitive, I’ve developed a few personal rules for clean air and clean dust.
- No unnaturally scented products (cleaners, air fresheners, soaps, candles). Use only natural products for cleaning, and stay away from anything artificially scented.
- Use only no-VOC paint and wood treatment. No-VOC paint can now be commonly found at major paint stores. Much more difficult to locate are all-natural oils for wood finishing such as tung oil, Rubio Monocoat, or my favorite, AFM Naturals oil wax finish.
- Use formaldehyde-free plywood: it’s now very easy to find formaldehyde free plywood (such as Purebond) for any interior construction, thus avoiding any formaldehyde off gassing.
- No off the shelf furniture or mattresses with chemical foams. Furniture cushions and mattresses are almost always made of polyurethane foam, which is highly flammable, and are mandated to use a variety of flame retardants, which end up in household dust, and include penta-BDE’s (for furniture before 2005) and chlorinated tris, TCEP, TDCIPP and other chemicals listed as carcinogens. There are volumes of information available on this (NPR, Scientific American, the My Toxic Couch video, etc) but the simple solution (while we work for new standards) is to not have manufactured furniture and mattresses with treated polyurethane foam in your tiny home (or any home). Even if you can get untreated polyurethane foam, it’s still made up of a slew of chemicals identified as carcinogenic, and when it’s being sat and slept on daily, the foam oxidizes (the Oecotextiles blog is particularly informative here). To sum up:
- Mattresses: Some bed manufacturers will make you a foam bed without fire retardants with a doctors prescription, but then the fire hazard remains. One natural (but pricey) alternative is 100% natural Talalay or Dunlop latex foam, which is a renewable resource (rubber trees), mold resistant, not highly flammable, with no off gassing, and lasts for 40+ years. Manufactured mattresses made with 100% latex must have a fire retardent layer added (typically wool, which is naturally flame retardent). Make sure there are not other chemical foams added- it’s very common to see a 10” mattress with 2” latex and 8” of polyurethane foam. The cheapest deal I found was a 6” full size soft latex mattress from Miracle Sleep– it’s one of the more comfortable beds I’ve slept on. There is at least one other economical natural sleep option: futon mattresses made with cotton, but note that the cotton is usually treated with boric acid to meet fire codes.
- Sofas: Any upholsterer will sell you non-treated polyurethane foam for cushions (this is typically their default foam unless they are working for commercial clients). Making your own sofa + cushions (even with polyurethane) still beats a purchased couch in terms fire retardent chemicals. But note that the fire hazard of untreated polyurethane still remain, so read up and weigh the cost/benefits. Another option is 100% latex foam- all foam in couches prior to the 1960’s was natural latex until cheaper polyurethane came along (and now they are difficult to find latex sofa cushions- foamsource.com has them). But note that without a natural or chemical resistant barrier natural latex can also be a fire hazard. Other economical and safe options for seating: wood bench, leather, wool, futons.
- Use certified cabinet fiberboard (MDF) and particleboard: these can potentially be loaded with chemicals that can offgas, so it deserves careful research. It appears IKEA cabinets are now CA Carb2 compliant (one of the most stringent standards, as seemingly verified here).
- Insulation: Fiberglass, cellulose, rockwool insulation appear to be relatively harmless (though they have relatively low R-values per inch). Most debate centers around use of sprayed-in urethane foams (see this discussion on Green Building Advisor). Officially these foams are ‘stable when cured’, but there are doubts. In the case of SIPS panels using foam insulation, the foam is mixed and sprayed and cured offsite (when most of the off gassing occurs). The foam is sandwiched between relatively impermeable, formaldehyde-free OSB, and is structurally rigid and does not break down readily when fully cured and left intact. While there is still some uncertainty about foam offgassing, there is no question that ensuring adequate ventilation levels is good practice, which could mitigate any off-gassing once the foam is cured (see below).
2. Combustion appliances
Non-combustion appliances such as electric space heaters, water heaters, cooktops should not have any indoor air quality impacts. However, for off-grid living (or any cook that can’t bear the thought of an electric cooktop), certain LP or natural gas appliances may be desired. Burning gas for heat or cooking releases moisture when LP combusts, and also requires oxygen from within the home if not provided with an outdoor supply line. Here are some recommendations:
- Gas Cooktops: use only indoor rated cooktops (no camp stoves, etc), but note that these require makeup air.
- Gas Heaters: use the Dickinson closed loop marine heater commonly found in many tiny homes. The vent pipe on these is double walled, which allows fresh air from outside to enter, and exhaust to exit.
- Hot Water Heaters: use only indoor rated heaters (some of the smaller/cheaper units are made for camping use only). Any indoor units will require makeup air from the living space.
- Small fireboxes: These are generally not a good idea, as most all draw combustion air from the living space (not through a separate outside air intake). So look for ones with a separate external air intake.
Note that all combustion appliances should really be tested for incomplete combustion (carbon monoxide) and proper venting (drafting) where applicable. Pressure diagnostics are also recommended when there are combustion appliances. Typically this involves a ‘worse case depressurization test’ — for example, if the bath fan and gas-fired hot water heater are both running, the pull of the bath fan may backdraft combustion gases from the water heater into the house, etc. A BPI or RESNET trained energy auditor will be able to perform these tests and make appropriate recommendations.
In addition to combustion appliances, bathroom use is a significant contributor to moisture levels. All baths should have a simple bath fan that operates during and after showering. Buildup of moisture can quickly lead to significant mold and mildew problems, negatively impacting air quality.
Every building needs a natural rate of air exchanges per hour (ACH), currently .35 (essentially, 35% of the cubic air is brought in/exhausted each hour, though this rate varies depending on occupancy). This allows for adequate moisture control, oxygen levels, and dissipation of any remaining household pollutants.
Ideally each building should have a simple blower door test (by a BPI or RESNET auditor) to determine air exchange. Typically, larger older buildings are naturally leaky, and can achieve adequate ventilation rates without trying. Others, among them tiny homes, are potentially much tighter and require careful attention to air exchange. Most simply, ventilation may be achieved by keeping a window permanently cracked open, but this has drawbacks in terms of safety, noise, weather, and energy efficiency. Perhaps the best solution is to install an energy recovery ventilator (ERV), which essentially is a bath fan with a heat exchanger, allowing cool/warm inside air to stay cool/warm, all while bringing in fresh outside air at a pre-set rate. One of the smallest and highest rated ERVs on the market, the Panasonic FV-04VE1, is recommended. It is adjustable so you can do a simple calculation of house air space and ensure proper ventilation requirements are being met. We recommend an ERV as standard equipment on every micro house– in addition to a CO, smoke and propane alarm (where applicable).