Tiny homes have become increasingly popular across the U.S. in recent years. As more people make the move to tiny living, contractors and owner-builders alike are faced with the challenges of building tiny spaces that include all of the comforts of conventional homes. Those challenges vary by project, but undoubtedly include issues surrounding the installation of utilities, specifically plumbing and HVAC systems.
There are two types of tiny houses: those built on ground-bound foundations, typically called a Tiny House on Foundation (THOF) and moveable tiny houses, typically referred to as a Tiny House on Wheels (THOW). Both are classified as 400 square feet or less in total habitable square footage. THOFs do not raise the same issues as those built to remain mobile. After all, a THOF is nothing more than a very small house. 

Before getting into the specific challenges of building a moveable tiny house, it’s important to consider the challenges that size alone has on housing when it comes to HVAC systems. The biggest issue is that most mechanical systems are not designed to service tiny spaces. Because average home sizes in the U.S. have grown over the last four decades to the world’s largest square footage, our systems have grown in size and capacity as well. Finding an AC unit or heating system small enough to service a 200-square-foot home can be difficult, and providing too many BTUs to a space can be just as damaging as not providing enough. 

There are smaller heating and cooling systems designed for mobile homes, RVs and apartments; however, those are often lower quality and, in the case of heating systems, non-vented. Many in the tiny house community are looking for small, but elegant and high-quality homes. The idea of using a sub-par heating or cooling system is not an option. Although the HVAC market is starting to expand to include tiny homes, the choices afforded to tiny house dwellers are still thin relative to the conventional housing market. 

THOWs have their own set of challenges, specific to the fact that the homes are moveable. Although still a grey area in much of the country, THOWs tend to be classified as recreational vehicles, and many of the HVAC and plumbing systems and standards used in those homes have come directly from the RV industry. The main problem with this approach is that many individuals living in THOWs want to use them as their primary residence, and most zoning jurisdictions won’t allow people to live permanently in RVs. As such, tiny house dwellers have to find ways to either change the zoning in their area or build their home to meet residential standards. 

As a means to that end, I wrote a tiny house specific appendix last September for the 2018 International Residential Code (IRC). Although it does not specifically address the issue of moveable tiny houses, it covers all of the other aspects that were once a challenge for building IRC-compliant tiny homes. In December of 2016, the International Code Council (ICC), the body that oversees most residential code development in the U.S., approved the appendix. Once adopted by state and local jurisdictions, the appendix will allow building officials to approve tiny houses as permanent housing. 

Even once the issues surrounding the legal construction and habitation of tiny houses have been reconciled, there are still challenges specific to the plumbing and HVAC system installations that need to be addressed. Moveable tiny houses are subject to height restrictions enforced by the department of transportation because they travel on public roads. In general, a THOW cannot be taller than 13 feet, 6 inches measured from grade to the highest point on the structure. The key here is that it is measured to the “highest point on the structure” not to the roof ridge. Because every inch of space matters in a tiny house; lowering an entire ceiling by even 6 inches simply to accommodate a roof vent stack is not an option. 

Design is key in tiny houses. As I mentioned, every inch matters. In fact, it matters much more than in conventional housing. What’s more, the location of a sink, toilet, shower/tub or other plumbing fixtures can have serious implications on the overall design of the house because of structural width and height restrictions. It is absolutely vital for designers and builders to work closely with each other to make sure that all of the utilities are placed in the optimum locations.

Another challenge to running plumbing lines is the use of open joist lofts and shallow roofing systems, often using 4x4s for rafters or loft joists. These small frames do not allow for large penetrations through the members nor do they provide adequate protection from freezing due to their close proximity to the external envelope. Further, running lines across the house can be difficult to do without leaving them exposed due to the open frame systems of loft floors. 

Once again, exceptional attention to detail is needed during the design phase to ensure that there will be adequate paths for the utility lines to traverse the house as necessary to supply all fixtures. Keep in mind that all utilities, not just plumbing lines, need to traverse the house. Finding space for electrical wires, low voltage lines, water service lines, plumbing vent lines, waste lines and gas lines can be a major challenge in such a small space. 

You may be thinking: “no problem, we’ll just run the lines in the floor.” Unfortunately, many tiny houses are built with the floor sheathing installed directly on the trailer frame as a way of maximizing interior ceiling heights. Remember that the overall structure typically cannot exceed 13 feet, 6 inches, and that interior ceiling heights are governed by building codes. Placing sheathing directly on the trailer bed saves headspace but causes significant problems for the installation of utility lines, especially plumbing waste lines. In cases where a framed floor system is used, it is rare to see anything taller than a 2x4 floor frame. So even in the best-case scenario, you won’t have much room to work with.

Consider installing waste lines underneath the THOW while maintaining acceptable slope on your main line. You may have a bathroom at one end of the house, a kitchen on the other and laundry facilities somewhere in the middle. Assuming a house that’s roughly 30 feet long, the amount of slope you would need would mean that the main line would end up running straight into the trailer axles. Obviously that won’t work, so now you’ll have to provide at least two separate waste line mains for the house, each draining to the side of the home and each provided with a rated shut-off valve for use while in transit. That’s a lot of effort for only 200+ square feet.

You have to be careful not to leave things so low that they get damaged while in transit. You’ll also need to pay special attention to any penetrations through the floor so that they are not damaged by rubbing, friction or impact during travel. Finally, any lines that are installed below the trailer flashing will be exposed to road conditions such as water, ice, snow, oil, gasoline, road salt and other chemicals or materials potentially present on the road surface. It’s important that your utility lines be capable of handling this kind of exposure. 
Both the scale of the tiny home itself and the potential for its mobility impact HVAC systems. Because tiny houses are so incredibly small, the impacts of interior air quality and moisture control are more immediate. It is not uncommon for tiny houses to have significant condensation issues, especially in cold climates, if not properly vented. In conventionally sized homes, there is ample space for moisture from respiration, cooking and bathing to dissipate. That is not the case in a tiny home, and the results of all of those actions quickly build up inside the home.

It is vital to provide tiny houses with adequate ventilation through the use of moisture sensor-controlled bathroom ventilation, range hoods and whole house recovery ventilation systems. It may be necessary to provide a passive make-up air vent as well in order to provide the make-up air necessary to allow the main ventilation systems to function properly. 

The most common heating and cooling system used in a tiny house is a mini-split. They are small, efficient and scaled properly for the homes. The major challenge with mini-splits when it comes to THOWs is the placement of the compressor unit. The entire system needs to move down the road with the house, so the compressor cannot be placed separately from the home. The most common placement is either on the tongue of the trailer or attached to the rear of the home, mounted on an exterior wall. 

Another challenge with heating and cooling systems comes in the form of the home’s power source. Most THOWs are either solar-powered or connect to land power via a main power cable; picture a 30-amp to 50-amp extension cord. If the solar system or land power is not large enough to handle the load of the mini-split, then the home will be without heating and cooling. This is easy to solve when a home remains in one place, but as the home travels across the country, inconsistent power sources can be a problem.

Whether you are concerned with the legality of tiny homes, pondering the installation of utility systems or are just interested in the idea of building tiny, you will need to pay more attention to design details than you might on a conventionally sized home. The impact of every inch and every detail is felt much more readily in tiny spaces. A single mistake, being only an inch off, may result in a system being excluded from the house. There simply isn’t room for those types of mistakes, whereas in a conventional house, they could potentially be absorbed into the “free space” of the home. Designing, building and living tiny takes a stronger attention to detail and a commitment to a different lifestyle, from the very first lines drawn to move in. 

Andrew Morrison has been a professional builder for 20 years and has been teaching people how to build their own homes in his hands-on workshops since 2004. In that time he has personally taught more than 2,500 participants how to hand craft their own homes and has seen again and again that anyone with passion and perseverance can build theirs too. Morrison and his wife, Gabriella, are the creators of “hOMe,” the 207-square foot (+110 SF in lofts) modern tiny house on wheels. They live and work in hOMe full time, off grid and debt free. With the extra time and money that they have, they travel and enjoy time together as a family. For more information, visit tinyhousebuild.com.