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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

Number 2My 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.

Airstream Exterior

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. 

Airstream Interior

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.

Ice Smiley

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. 

Compared to the thick metal framing of a 1964 Airstream and the ribs of trailer beds used under most portable, tiny houses, the metal framing typically used in today’s mainstream construction is very thin. This stuff’s very unlikely to be the cause of anything nearly as bad as the thermal bridging problem I had in my home, especially if insulation is added to the outside surface of each member.

Check back on Wednesday for Mistake #3: “Well, I Guess That Brilliant Idea I had Wasn’t so Brilliant, After All”. 

If you missed Mistake #1, You can read it here: I Made My Bed Wrong, and Then I Had to Sleep in It

Posted by Jay Shafer — February 25, 2014

Comments

Shelly Cassidy:

How well would radiant heated floors work for a tiny home on wheels? Would this source be sufficient enough to be the main heating source?

February 25 2014 at 09:02 PM

Marsha Cowan:

So true! The tiny house in which I live now is well and properly insulated, and so even on the coldest nights (9 degrees F), I warm up the house before bedtime, then shut off my propane heater, crawl into my down comforter, and sleep like a baby til morning. Yeah, its chilly when I get up to cut on the heat again, but never below 40 something, and it does not take long for my very tiny house (6×6 not including the bay window) to heat up. Yes, insulating properly is very important and makes the difference between living happily ever after in your tiny house, or being miserable with every climate change. Thanks, Jay!

February 26 2014 at 05:02 AM

Jay Shafer:

Hi Shelly.
Heated floors are a great idea, and they work great in a house on wheels. The ideal sub-floor heating system would have some thermal mass to it to more evenly distribute the warmth (i.e. a heavy flooring material like concrete, stone, clay, etc.), and massive materials don’t generally work very well in portable structures for which weight is a concern. That said, I’ve seen sub-floor systems installed under the wood floors of these little homes that work very well. Because heat rises, the floor of a home is most often the coldest place in it. Heating the area beneath your feet first, then letting that heat work its way up to warm the rest of the house makes perfect sense.

February 26 2014 at 10:02 AM

BOB HENRY:

When I designed my caboose style 8 × 24 tiny house I placed 2×6 sleepers flat on the frame at 2 foot on center. The trailer was an rv in a prevvious life and had an aluminum splash panel already in place so the sleepers were protected. I did seal them completely in fence post protectant. It is a heavy tar like substance. All the voids were filled with 2 layers of 3/4 blue closed cell foamboard. It was stuffed tight and taped. Over this went the 3/4 tongue and grove flooring that had also been completely encased in the same fence post protectant. Giving me an R9 – R10 value in the floor. I chose to give up a little better than 6 square feet of floor space so I could have 2×6 walls. These were also 24" on center and the cavities were also completely filled with the blue foamboard . As 1" of blue foam is rated at R 5
I have walls with greater than R 27. The ceiling is 8 inches deep and is also filled with the blue foam for an R value approaching R40.
We have had record lows this year with wind chills of -30 however as I enter the unheated caboose it remains jacket comfortable. I have noticed a few wind leaks that will need some attention around the windows and doors. AC wiring is complete and upon completion of the DC circuits it will recieve a vapor barrier and yes I am drywalling the interior. Some say it will be a big mistake adding not only extra weight but adding a potential for cracking. I am approaching the drywalling a bit unorthodoxed by bedding all seams in 100% silicone caulk instead of the hard drying wall board compound. The elasticity in the caulk wil offer some give with movement. All in all these construction methods should offer great thermal protection and the drywall will offer far greater fire supression than a thin sheet of plywood paneling or even wooden car siding.

Planning begins long before the first board is cut.

February 27 2014 at 07:02 AM

Ronni:

Hubby and I are thinking about buying a friends old GMC tour bus as a bridge home until we buy an acre and put our tiny house/micro farm on it… and this might have solved the cold bus problem. I would hate to totally gut it as it’s got bunks and everything already in it but it’s SO cold and almost all metal… I’m thinking if we at least do the flooring and put 2×4 walls with paneling sheeting and insulation in between the outer walls that should do it? Do you think we should put a vapor barrier too? (it’s currently pretty sealed, but it’s built like your airstream almost…).

February 28 2014 at 11:02 PM

Casey Friday:

I’ve been stuffing three 3/4" sheets of rigid insulation in all my tiny house’s wall cavities and floor/ceiling. It made an instant difference in how the house maintains thermal temperature vs. just having open studs.

Although freezes aren’t often occurring in south Texas, I’ll be glad to not have to scrape the windows, for sure! I’ll also be happy to be keeping the heat out.

March 06 2014 at 08:03 AM

Leecille Keasling:

How would spray foam work? I was thinking this would work because it is made to keep moisture out and also more pliable, stays in place with the movement of the house. We will be at the stud stage so the thought of spray foam would be a good idea, Has anyone else used it?

March 06 2014 at 10:03 PM

coffeewitholiver:

Interesting article series! I appreciate your sharing your experiences very much. It was difficult to decide how much insulation I should plan for in my own Tiny home, as there really isn’t much information available on heating needs of these spaces. I went with 2×6 walls and floor, and will use 2×8s for the roofing, and am using wool insulation….perhaps overkill? But perhaps not. I’ll be living in a cold climate with long winter temps of near/below zero at times. It seemed to me in designing my home that it would be better to overdo the insulation than under….

Funny about your original tiny bed too, and how you lurk about different spaces with a tape measure, I love it! I have been known to wander around with a tape in my purse, “just in case”. But I’m a bit of an oddball.
coffeewitholiver@wordpress.com

March 07 2014 at 05:03 AM

Cory :

I’d like to hear the answer to Leecille’s question about spray foam insulation as well. I likes the idea of it because of ease of application and it would be lightweight. I’d like to know how it might work to insulate the floors too? Our home would be on wheels if we can do it.

March 12 2014 at 12:03 AM

Kris:

Where are all 10 of your mistakes? I want to read them all!!!

May 13 2014 at 04:05 PM

Judy Murray:

You make some great points about insulation that I don’t think the common consumer knows. Just adding more insulation is not always going to make it better! Having an honest expert come in, tell you what you have and what you should do to make your home more efficient is the way to go!

October 30 2014 at 07:10 AM

Angela:

Has anyone here successfully insulated an Airstream during refurbishing for full-time living? What us the best way to do that? Thanks!

November 19 2015 at 08:11 PM

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