Don’t Try This On Home: Insufficient Insulation for full-time living
At first, buying an ice scraper for my home’s interior, instead of properly insulating its walls for the Winter, seemed to be in perfect keeping with its quasi-survivalist motif
My first tiny house with its way-too-tiny bed and lethal heater was, actually, a 1964 Airstream travel trailer. I gutted, refurbished and, then, inhabited it for two years on the edge of a hayfield six miles north of Iowa City. It was an impressive remodel, if only in the relative sense that almost anything I could do to this derelict RV I bought for $2,400 would be an improvement.
Far more challenging would have been the, heretofore unknown, need to thoroughly re-insulate. To my own credit, I had taped a layer of metalic-bubble-wrap to the vinyl walls before covering them with a pine plank finish material. But it turns out that a quarter-inch of air and aluminum foil coupled with another quarter-inch of wood isn’t nearly enough to combat outdoor winter temperatures that often dip well below zero.
It was about a week before Thanksgiving of 1997 when my home’s interior first started to look like the belly of a whale with ice formations on the vaulted walls and ceiling delineating a rib every 24”. The windows still allowed plenty of light to come in, but the view of the hayfield and treeline outside they’d provided in warmer months soon disappeared behind the ice on their inside surface. Metal and glass are great conductors, and my home’s single-pane windows and aluminum framing were no exception. The problem was, at least, localized to the windows and places where my trailer’s metal framing carried outdoor temperatures directly thru the exterior walls to the bubble wrap and wall paneling inside. I would come to learn that this was the result of something called “thermal bridging”.
When a surface is sufficiently cold, moisture from the air starts condensing on it. If the temperature of that surface is below freezing, then the water’s going to freeze. Anyone who’s spent any time in a car without a defroster on a cold, damp day already knows how quickly water can build up on a windshield or any other cold object. Anyone who’s done the same thing in a sub-freezing climate with no defroster and a broken heater knows how important it can be to have a sporting passenger with an ice scraper and decent upper-body strength.
In my Airstream, I came to understand that dense materials, like the metal framing in my walls, make great thermal bridges, while light ones tend to be better used as thermal barriers. This is why the insulation for exterior walls is typically made out of things like fluffy fiberglass batts, foam board and wool. It’s also the reason objects designed for readily conducting temperatures, like an iron skillet, for example, are made out of things like… well… iron.
To be more specific, insulation, itself, doesn’t even really insulate. It’s the amount of dead air trapped in its fibers that does all the work. Folks who think they’re adding extra R-value to their home by compressing a piece of 5 ½”-thick fiberglass insulation into a 3 ½”-thick wall, aren’t doing themselves any favors. They’re just displacing valuable air space with a denser material.
Cold Floors all around
My memories of scraping ice off the walls and windows of my little Airstream have me concerned about the increasing use of metal floor joists in small houses on wheels. A lot of folks are starting to lay their flooring directly on the steel ribs of the flatbed trailer below. Putting a ¼” piece of Sill Seal (i.e. styrofoam) and ¾” of wood flooring between the metal joists and the floor isn’t doing much to improve the problem, either. Once some of the air gets squished out of the styrofoam by the material on top of it, this composition will only provide ¾ R of insulation, which is just a little less than of the bubble wrap I’d used on the inside of my trailer.
To guard against condensation, ice and any resulting water damage to your structure in anything but the most temperate climate, you really need at least 3-Rs between your interior and what’s going on outside. Even today’s residential windows tend to have enough dead air space between their double panes to provide that much protection, and your windows really should be the weakest link in the envelope, as there’s relatively little you can do about thermal bridging there.
Whenever and wherever possible, create a thermal barrier of no less than 3-Rs between your homes interior and exterior. This is easily accomplished by using wood framing members spaced at 24” O.C. with no less than 2”-worth of high-quality insulation stuffed (not too tightly) into the cavities. 2x4 studs have an R-value of about 4.6. * Along these same lines, opening up no more than 30% of your exterior wall’s for windows isn’t a bad idea either.
Toes already cold?
If you already have a house with thick metal floor joists, you can insulate your existing floor to maintain a temperature above which condensation occurs. This can be done by covering the floor with sheets of ¾” thick, closed-cell polyisocyanurate foam board. Be sure to lay the stuff from wall to wall and under any cabinetry, and be sure that it all fits pretty tightly together. Then seal all the seams and edges of the foam with foam board tape and/or caulk. It’s important that the joints be airtight, or the relatively warm, moist air from inside the house will work it’s way thru gaps to the cold, original flooring beneath and condense right in the middle of your new flooring system, which would create a mess worse than the one you started with. Fasten ¼” flooring or PureBond plywood over the foam with 1 ¼” screws to distribute any weight that’ll be on top of the insulation and help to make the whole thing pretty. Your new insulation will provide about 4 Rs of protection after it’s completely done degrading and getting a little squished.
Don’t try insulating the floor from underneath your trailer. Chances are good that you wouldn’t be able to cover all of the trailer’s metal parts, and, then, your new insulation would just be trapping the condensation caused by the resulting thermal bridging inside your new floor’s composition.