What is an U-value and how to read it?

Thermal transmittance or U-values of tiny houses is the rate of transfer of heat through its structure. This is the rate, with which heat transfer through the walls, floor and roof (single material or a composite), divided by the difference in temperature across that structure.

For example a wall with U value of 0.215 W/(m²K) is better insulated than a wall with U value- 0.3 W/(m²K).

As the European Union is aiming for better performance, if not passive buildings, in the recent years the standards for U-values has significantly grown (U-values are required to be lower). This is usually enforced by governmental building regulations and norms specific for each country.

Public buildings will soon be obliged to produce as much energy as they consume in order to leave less of a carbon footprint. While private housing regs tend to be a bit more loose in terms of thermal performance. And this is especially the case for buildings meant for holiday stays or non-permanent living such as cabins, retreats, villas etc.

U-values and different components of a building.

The overall performance of a building depends on the U-values of all its main structures that divide the inside from the outside – so these in general are: walls, roofs, floors(slabs) and very importantly glazing.

As even the best glazing tends to perform way worse than an insulated wall both its quality and quantity matters. This is the reason why passive houses have less windows and pay attention to their orientation.

Typical U-values for conventional buildings.

Passive houses have U-values of main structures ranging from 0.10 to 0.15 W/(m²K). In Central Europe these values can vary depending on the climate.

A typical, built in the recent years European building has U-values that range between 0.2 and 0.3 W/(m²K).

A house with no insulation at all could have a U-value of 2 W/(m²K).

Where does tiny houses stay in the scale of what is considered reasonable U-values?

If a tiny house is built with 10cm walls/which is the average standard/, meaning that this is the thickness of the structure and following the overall layering as shown in the scheme:

The U-value of such wall would be about 0.3 W/(m²K), if a good performing rockwool is applied.

If a tiny house is built with similar structure of the wall but with 6cm structural layer, then the U-value goes up to 0.44 W/(m²K),

And finally, if some measures are taken in order to improve the thermal performance of the wall and an insulated installation cavity is applied on the inner part of the wall, then the overall U-value could go down to 0.215 W/(m²K).

If we have to compare overall thermal performance of tiny houses to the one of typical housing then it will look like that:

The comparison chart shows that tiny houses built with 10cm structures of the walls perform as good as a conventional recently built house.

U-values of tiny houses and weight.

Although the U-value comparison looks optimistic, there is one more consideration to be made when choosing the best structure for your tiny and it is inevitably the weight.

As tiny houses that are built on legislated trailers, such as Vlemmix for example, are typically limited to 3,5tons overall weight incl. the trailer itself, this means that a very delicate balance between size, structure thickness and weight should be found.

So if you are after better thermal performance, your house should be on the smaller side in order to fit into the weight restrictions.

Better thermal performance = Smaller tiny to fit the 3,5 tons restriction.

On the other hand, if you want to get a house as big as possible and till fit the 3,5 tons, then U-values need to be sacrificed.

Big tiny house to fit the 3,5 tons restriction = Worse thermal performance.

But what happens if you are greedy and want the best from both worlds – have a well performing tiny house that is spacious as well?

Then we recommend you to think about building it on a non-legislated trailer, so that the trailer will be constructed to support the weight of a big and well-insulated tiny house. In this case there are less limitations in terms of size and you could still benefit from the 0.215 W/(m²K) U-value, but as the house is no longer suitable for moving on the roads, you will have to consider either docking it to a specific location /which most of the tiny house owners do anyway/ or moving it seldom and with the help of a truck for oversized loads.

Big tiny house + better thermal performance = Non-legislated trailer.