Green building pitfalls of tiny homes

Amy Musser

What could be greener than a tiny home? They use so many fewer resources than a traditional home that they should almost get a free pass in the green building world, shouldn’t they? Almost, but not quite. With tiny homes becoming more popular, let’s explore some green building pitfalls that are unique to them.

When we talk about green building we usually include energy efficiency, efficient use of materials, indoor air quality, site sustainability, durability, and water conservation. Tiny homes have some of these in the bag: they use dramatically fewer materials, need less heating and cooling, and have a smaller impact on the site than traditional homes. This shouldn’t be a free pass to be wasteful or careless. Builders of tiny homes should still try to use recycled or low embodied energy materials and to create a building envelope with similar insulation levels to standard-sized homes. They should also take care to make whatever footprint they do have on their building site be a sustainable one. The strategies they would use for doing these things aren’t really different from those that builders of larger homes would use.

Indoor air quality is probably the most difficult and important area where builders of tiny homes need to be MORE careful than the builders of standard-sized homes. These homes have such a small volume that there is less air to dilute contaminants. They also tend to be more densely occupied, so there can be more contaminants generated per square foot. Because their building envelopes tend to have simple shapes, they tend to be comparatively airtight as well. All this means that both ventilation AND reducing the indoor contaminants that you put into a home are more important in tiny homes than in standard-sized homes. It can also make moisture control more tricky.

One of my biggest concerns with tiny homes is indoor combustion. Fireplaces, gas cooking, gas heating and water heating potentially concern me. I don’t think that an unvented or atmospherically vented gas appliance can be reliably safe to use in a tiny home. A range hood vented to outdoors is a must for all homes, but in a tiny home I would go the extra step of cooking with an electric range (ideally induction), electric oven or microwave. A direct-vent tankless gas water heater makes a lot of sense in some tiny homes because they take up so little space and they can be used for both water and space heating. But I also think there’s a good argument to be made for making a tiny home all-electric, especially if you don’t have a natural gas hookup available. After all, if your energy bills are tiny, why pay service charges to two companies?

Another big concern that I have is formaldehyde-free and low VOC materials. It’s always smart to use formaldehyde free wood products and insulation and low VOC paints and adhesives. But in a tiny home, it’s more important. Cabinets (especially the more affordable ones) and closet organizers (popular in tiny homes) are one of the building materials that still often contain added formaldehyde. I think it’s worth splurging to get cabinets and other wood products that are at least CARB-2 compliant. You’re going to be living in tight quarters with these cabinets. It will be worth the money.

It’s also super-important to think about moisture control. In the summer, the best way to control humidity in most climates is to have an option for air conditioning. The wall-mounted ductless minisplit is a great way to heat and cool these homes (unless you’re in a super-cold climate), and many of these come with a “dehumidify” setting. What may surprise many people is that tiny homes in cooler climates have an above-average risk of developing wintertime moisture problems. This can happen because moisture generated by occupants and their activities (like cooking and showering) is held in by the tight building envelope and there’s not as much indoor air to dilute its effects. The first sign of it is usually moisture condensation on the interior of the windows. I would consider all of the following strategies to address this: consider a good fiberglass window with a low U-value (you only have a few to buy), install a good bath fan (less than one sone, more than 70 cfm rated flow) directly over the shower enclosure, and make sure that have an enclosed shower (either with shower doors or just a bathroom that closes off from the house).  Finally, make sure that you actually use your bath and kitchen fans every time you shower or cook something.

Mechanical ventilation is important for all homes, but you certainly shouldn’t skip it here. The good news is that ASHRAE Standard 62.2 is based on both occupancy and square footage, so you’ll get a reasonable airflow if you use it to calculate ventilation. The one exception to this is that it calculates the number of occupants as “bedrooms plus one”. I would always install ventilation for at least 2 people in a tiny home, but if you know that you’ll have more (kids sharing rooms, sleeping lofts that may not fully qualify as bedrooms), you should do the calculation based on the actual number of people.

Finally, tiny homes can have durability issues that are unique to them. When space is at such a premium, it tends to drive decision making on things like the type and location of heating, cooling, domestic hot water systems, and other appliances. This means that there is a tendency to want to move equipment outside or into crawlspaces when possible, and that can put homes at higher risk for frozen pipes or inefficiencies that come with these systems being outside the thermal envelope. Ideally, all plumbing pipes should be kept inside the conditioned envelope of the home. If they need to go in a floor or exterior wall, it’s not enough to just insulate the pipes. A rule of thumb that I like to use is to have at least as much insulation on the exterior side of the pipe as you have on the inside. For example, if you put a pipe in the middle of the floor insulation, I would use pipe wrap insulation, AND I would also make sure that if you have 4 inches of insulation above it that you also have at least 4 inches of insulation below it. This “half and half” rule is usually pretty effective for climates where it gets as cold as zero degrees Fahrenheit. If it gets significantly colder than that, you really need to move your pipes inside the building envelope.

It’s also a good idea to make sure that any water heater tanks or clothes washers have drain pans under them, so that if there is a leak it can’t flood your whole house or rain down on something that it shouldn’t.

Tiny homes are new, so I’m sure there are even more pitfalls that I haven’t thought of. I’d love to see you discuss yours in the comments.

Copyright 2016.  Amy Musser