Metal framed Tiny House construction article

I was reading JLC online (Journal of Light Construction, mostly for home builders) and hit an article the Tiny House folks here might find interesting.

A Troubled Tiny House-JLC online "Building without considering basic physics is a recipe for a building-science disaster"

It talks of a tiny house that was built out of wood on a metal frame without accounting for condensation from the cold metal hitting the wood beams. Basically the place was rotting from the floor upward.

Interesting reading if you are considering building a wood house on a metal frame (like a trailer base.)

Pearl Sutton wrote:It talks of a tiny house that was built out of wood on a metal frame without accounting for condensation from the cold metal hitting the wood beams. Basically the place was rotting from the floor upward.

That can be an issue with a wooden shed built on a concrete base also. The "industry" approach here is to use a strip of foam between the concrete and the sill plate that's designed for that job. In our climate, they often use rocks for the bottom foot of cob houses, and similarly, there's a metal flashing between that knee-wall and the cob to stop the moisture from the earth from migrating up into the cob. (we're wet for months on end in the winter with minimal sun to dry things.)

Water is the elixir of life - and death! It can move to the top of 200 ft Cedar trees by magic, and condensation isn't just on your glass of ice-water on a warm day!

I read this and am scared by it, as I have already purchased a trailer to build on, and house plans, that seem to mimic this structure!

Trying to research what I can do at this point, to mitigate such condensation. Can you elaborate more on this: "The "industry" approach here is to use a strip of foam between the concrete and the sill plate that's designed for that job."

I do plan to move the house several times, at least in the first few years, so secure attachment to the trailer is as important as insulation/thermal bridging is to me.

Thank you!

Molly Wisconsin wrote:I read this and am scared by it, as I have already purchased a trailer to build on, and house plans, that seem to mimic this structure!

Scared is good - that way you will find people who *know* how build right for your ecosystem, and read about industry standards, and come up with a plan that won't be a disaster in the medium, let alone long term.

This is key in the article:

The main culprits leading to the floor damage were cold, moist air infiltrating the framed floor through seams in the sheet-metal bed that had not been air-sealed and, more critically, warm, moist air from the interior condensing on the inside surface of the trailer’s sheet-metal bed and saturating the vapor-open Rockwool batt insulation on top of the metal between sleepers.

Every insulation I've read about has pros and cons. Rockwool is held up as safer than some for the planet, but as noted here, it can absorb and hold water. Water is often better at getting *in* to places, than getting back out, because it often goes in as a vapour and then condenses into a liquid. It takes a lot of energy to reverse that process.

Most of the motor homes I've met, use rigid foam panels for insulation, often with the expandable foam to fill in the gaps. You make sure you get the version that doesn't absorb water. The downside, is that carpenter ants think it makes a fine home, so keep an eye out for them.

Other key words in the article are "vapour barrier" and "air-sealed".  Humans give off a lot of water vapour just breathing, and then add more to a small space through cooking and hanging wet coats to dry etc. So the smaller the space, the more important it is to have responsible air exchange for the climate. Warm air holds more moisture, so any place where warm air can sit and cool, will drop that moisture off and that will show up as mould and mildew. (I'm betting the word "dew" in mildew wasn't an accident of linguistics!)  

Let me introduce you to my bed in the morning! Never an issue in Ontario in a house with a furnace and everyone complaining about how dry the air is in the winter. Now I'm a mile or so from an ocean in a very wet climate and the old-fashioned, pull the blankets to the foot of the bed to "air the bed", needs to be done (Anne of Green Gables maybe???) I go one step further now. I have a dehumidifier in the bedroom which I point at the bed in the morning and let it run for an hour. The only shower in the house is in a room off the bedroom, and we now make a point of pointing that dehumidifier at the bathroom for an hour after anyone showers also. The water from the dehumidifier goes into a bucket to be poured into the washing machine when we need to wash clothes - stacking functions!  

So yes - be scared just long enough to get yourself educated. Read books on house construction. Talk to people and watch videos about how to do things and how and why things go wrong. Ecosystem is *everything*. It took five years to convince Hubby who spent his life running a humidifier in the winter in Ont., to *really* believe that we absolutely have to do the opposite in our new climate. A house designed for this climate, might have fewer issues, but humans don't do that in North America anymore... but let's not go there!

Molly Wisconsin wrote: "The "industry" approach here is to use a strip of foam between the concrete and the sill plate that's designed for that job."

Not sure what you want elaborated on, so I'm guessing here.
When a standard construction house is built, the concrete foundation is laid, with bolts that are set into it to attach the house to. Then a strip of a very dense cell foam is laid on it, tight holes are cut for the bolts. The sill plate (which is a wood board) has holes drilled in it, and they go over the bolts, and nuts are put on. The house is built up off that sill plate, and that wood is not exposed to the problems caused by heat and cold and wet from the concrete damaging the wood.  It's a thermal break, as well as a vapor barrier, sized to fit below the sill plate.

Looks like this:


And on conventional construction it's installed here (a scribble I drew for you)


So on a metal frame that holds a wood floor, you'd put the foam on top of the metal frame, so it's not touching the wood floor, just like the concrete doesn't touch the wood.

In my words it would be "use a thermal break/vapor barrier between the metal and wood." The article came off of a conventional construction site, and it was meant for people familiar with that pink foam that is used between the concrete and the wood.

Does that make it make sense? I can explain more if I know more what part is not understood.  

Pearl Sutton wrote:In my words it would be "use a thermal break/vapor barrier between the metal and wood." The article came off of a conventional construction site, and it was meant for people familiar with that pink foam that is used between the concrete and the wood.

Maybe - where I live in Canada, the foam is white not pink. I would go by the description and purpose rather than relying just on colour.

Lots of post and beam cabin foundations have insulated floors.
They don't use metal .

This is something that would probably not happen in a building that went through inspection.
There are standards that have to be met, and a vapor/moisture barrier is usually one of them.
Building regulations rarely require the best engineering or design, but they do tend to prevent the known hazards.

Wood flooring over a metal "foundation" is kind of odd for a building, but less odd for for some vehicles, namely boats.
The metal of the hull is probably a lot cooler than the conditioned interior.
Boat builders and more importantly rehabbers, will have some tricks for dealing  with this.

Thank you all for the advice, especially Pearl that picture about the foam strip! I have taken a Tiny House building workshop, a woodworking class at our tech school, and read every book our library has on tiny houses and this has never come up. Onward and upward this gives me more to investigate.

Molly Wisconsin wrote:Thank you all for the advice, especially Pearl that picture about the foam strip! I have taken a Tiny House building workshop, a woodworking class at our tech school, and read every book our library has on tiny houses and this has never come up. Onward and upward this gives me more to investigate.

It was because it was a detail being missed that the article was written. Contractors are being called to fix things that wouldn't have needed fixing if the thermal break was installed when it was made.