What are the Causes of Mold in Tiny Houses?

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1. Built by Non-Experts

Highly skilled builders are rare. They tend to focus on upscale houses because that extra work and time they spend to build something right does translate to more expensive homes.

Most tiny house companies I have seen were started by contractors who don’t have much experience, they certainly don’t have the expertise in building science (which is mold prevention), and high craftsmanship.

Most small companies I have looked at started with someone who had not even managed a whole house build before! That was the case with my builder.

Even if a company is well established, it’s rare that an architect or other building science expert is recruited as a designer and consultant on a tiny house build. It is necessary to have that building science expertise to building anything that will hold up to mold, no matter the size.

My resource page contains links to architects, builders, and building science experts who I think are good.

I certainly wished I had chosen an experienced builder who really cared about details, and hired an architect to design and manage the project.

2. Building Codes not Followed 

Building codes are not perfect, they are only the bare minimum requirements and even though they are just the absolute bare minimum of what needs to be done on a house, they are not usually followed.

To build a truly mold-resistant (regular) building you have to hit many of those codes right on and in many other areas go above and beyond those codes if you want the house to hold up to moisture and mold.

Tiny houses don’t even meet those codes on some of the most basic building standards. If you are having trouble finding a regular house that does not have mold, well, I don’t have good news, tiny houses are built even more poorly.

For some areas of the build, RVIA certification could help ensure things are done better. But some RVIA codes are not mold preventative (like the requirement a vapor barrier), that could make things even worse!

Look at RVIA codes and see if that will work with your moisture management system. You may have to forgo that certification. There may be important guidelines to follow there for general quality (plumbing, electrical, fire safety), but not necessarily for mold prevention.

You can build a mold preventative build (like this one) and also meet the International Residential Code (IRC) (which would be a good idea).

 3. The Trailer is Tricky to Insulate

All Tiny Homes on Wheels are built on a metal trailer – that’s a tricky interface for condensation in most climates. Building the house by using that trailer as a floor cavity is a bad idea.

The house should be built up on top of the trailer, and even then there are many very detailed decisions and details to consider and execute. See this post on a Mold Preventative Tiny House for details on the trailer base.

4. Metals Frames 

Many THOWs are also made with metal framing. That is really tricky too! Now you have a whole house with thermal bridging and the possibility of condensation in the wall. Even if you go with all foam for insulation, this has to be well thought out and designed well.

I have seen trailers with a metal frame and then an organic insulation! No!

The wheel wells, another metal area, are also particularly difficult to insulate and are usually done wrong. The wheel wells in my tiny house went moldy within the first 3 years. They need to be thought out and detailed right.

Hire a building science expert to design your building envelope.

5. Which Climate is it Built for?! 

Tiny homes are often moved throughout many different climates in the US and Canada. Which climate was it designed and built for?

Even if some thought was given to design the building envelope and HVAC for that climate, will it now be moved to many different climate zones? How will the house perform in those?

If the house is made for multiple climates that has to be factored into the design of the entire envelope and moisture management system as well as the HVAC right from the start.

WUFI is one program that you can use to model the moisture in the house and see how it will perform in various climates.

6. Movement is not Your Friend!

Taking a house and then jostling in on the highway does not lead to something very durable. All those little details that need to add up to have things perfectly sealed and flashed do not benefit from a lot of movement.

You can easily lose the integrity of your sealing or flashing here, you may even have trouble with your framing or siding. Problems with your framing could lead to problems with your doors and windows and more…

7. A One Year Warranty??

Many tiny houses are very expensive and only come with a one year warranty! What!

What other 60K item would you buy that only has a one year warranty? Builders can get away with a lot because major mold problems will take longer to show up than one year.

My house had a few problems in the first year, but there were other major problems that took more than a year to show up. Mold above the shower in the ceiling, mold on the framing, an insulation system in the floor that was not done right – I fixed these myself on my own dime.

Ben Garrett who built my house only paid for things that went wrong in the first year. He then changed the name of his business (and so I could not sue him). That’s another reason these small companies are very risky. I lost 100k from his mistakes.

Though I did learn a lot about building science when I took it apart! And this did lead me to become certified as Building Biologist to help others build low offgassing, mold-safe houses.

8. Siding is Prone to Mold

Siding is frequently done wrong on tiny houses. Mold preventative siding almost always has a rainscreen with very few exceptions.

Regular wood-framed houses almost always require a vented rainscreen siding system to prevent water damage and mold. This is rarely done on a tiny house.

Most houses I have seen have siding right up against the sheathing. Sometimes it’s permeable siding, meaning solar vapor drive can drive moisture in, but even if it’s not, it prevents drying from the inside out. And you are losing your layer of protection from water entering behind the siding.

A rainscreen helps water that will get behind your siding drain out.

I highly recommend Cheryl Ciecko’s course on building a mold preventative house which has opened up now for the January session.

To hear about future course dates you can sign up for my email list:

9. Roofing Prone to Mold 

Regular sized houses have a fairly complex roof system that often has venting. An unvented roof would have to be done really carefully.

In a small space, your roof cavity can start to mold fast. Mine did over the shower as the builder had no interior vapor barrier and the breathable roof sheathing membrane was enough to trap moisture there.

Many tiny houses also have roofs that are flat or don’t have the right material for the slope.

Here is an example of a highly detailed mold preventative roof.

10. Details not Done Right 

The devil and mold prevention are in the details. Window flashing, house wrap installation, house wrap taping, metal head flashing, the detail below the rainscreen, flashing on any permeations going through the wall, details around the door- all of these are super crucial.

It’s especially important to get right due to the lack of overhangs over windows and doors on most tiny houses. This means everything has to be done perfectly. The overhang on a regular house is insurance against too much rain hitting these vulnerable areas.