Why you should Build a Tiny House Subfloor

In NZ it is common place to direct fix the flooring to the steel chassis of a tiny house trailer, in most cases some sort of thermal break is added between the steel and the timber. This is not always common practice in the building industry, so let’s explore why it might not be the best idea in tiny house building either. 

 
Example of tiny house subfloor attached to a steel trailer

Example of tiny house subfloor attached to a steel trailer

 


Initially it seems like a good idea to direct fix flooring to the trailer for these reasons:

 

·         Reduced weight

·         Reduced height – need more space in your loft anyone??

·         Reduced cost – less materials and labour

 

So why do some builders insist on building a subfloor on the trailer? Well firstly what is a subfloor? A subfloor can have different makeups in terms of materials and construction but essentially it’s a thermal/structural sandwich between the trailer and the finished flooring inside. Similar to a permanent house which commonly is made up of say timber joists, insulation and flooring.

 

Why do permanent houses with steel framing not just add flooring directly to steel joists and save a heap of coin? Simple, the building code requires a minimum insulation value measured by r value, hence the subfloor approach for permanent houses. But mobile tiny houses don’t need to be built to code I hear you saying. As it stands right now that may be true but we need to explain why R values are important.

 

Firstly, let’s explain what R value is. To do this we need to understand thermal conductivity which is how easily heat flows through something. Some tiny house related examples:

·         steel 54

·         glass 1

·         plywood 0.13

·         polyurethane 0.02

The lower the number the harder heats finds it to travel though the object. For example steel conducts 54 times better then glass does or 415 times compare to plywood, is that a wow moment?

Steel of a tiny house trailer

Steel of a tiny house trailer

 

Understanding thermal conductivity we can now look at R values. R value is a measure of how well it resists the flow of heat depending on its thickness. Shopping for insulation you will see R values stated on the products, for example you may see an insulated batt product, 90mm thick with a rating of R2.2. This is just a number and needs something to relate to. The building code specifies minimum R values for different parts of the country for floor, walls and roofing. If we look at a wall for example it is made up of more than just insulation and may include cladding, lining, wraps etc which all have r values too. So we need to look at the total R value of the wall not just the insulation.

 

Building code minimum R values

Floor R1.3

Walls R1.9/2.0

Roof R2.9/3.3

NZ minimum insulation, R value zones

NZ minimum insulation, R value zones

 

Back to tiny house floors, there has been a lot of science and research go into the minimum R Values and in our mind it’s worth using as a point of reference for a tiny house. It’s also worth noting in residential buildings these numbers are not targets to meet but indicators that your house should be well above these numbers for a comfortable and efficient home, personally our own tiny house floor is R4.2. 

 

So let’s take some real world examples of floor assemblies and compare them. To do this we are going to use Design Navigator a free NZ website designed to compare different floor, wall and roof assemblies in terms of R value. We will make a few assumptions to simplify the comparisons. Plywood floor 18mm and joists at 50mm x 50mm @ 600mm centres.

Example subfloor inputs from design Navigator

Example subfloor inputs from design Navigator

 

Typical Floor

•       Plywood 18mm floor

•       Polystyrene EPS 50mm S grade - between steel joists

•       Steel joists 50x50x3 @ 600

 

Example of direct fixing plywood to tiny house trailer

Example of direct fixing plywood to tiny house trailer


  • No thermal break = R 0.8

  • DPC 2.5mm =  R 1.0m

So with no thermal break between the steel and floor which is a big no no for condensation issues in itself is just R0.78 still a long way from minimum building code R1.3. Adding a DPC or damp proof course of 2.5mm as an example rises to around R1 which is an expected improvement but will this aid to living in a warm comfortable home? Lest run another example to compare.

 

Subfloor


•       Plywood top 18mm, bottom 7mm

•       Timber joists 50mm @ 600mm

 

Example construction of a tiny house subfloor on a steel trailer

Example construction of a tiny house subfloor on a steel trailer

•       Polystyrene XPS 50mm = R 1.5m

•       Polyurethane 50mm = R1.9m

 

This time around we have added a thin layer of ply above the steel trailer joists. Sandwiched with timber joist and EPS insulation. Now we achieve a higher than minimum r value of R1.54. If the polystyrene (EPS) insulation is changed to the less conductive polyurethane the R value increases to a more impressive R1.94.

 

Like any design decision there are compromises but we hope this helps point the spot light more on balancing thermal conductivity in your tiny house design. The thing we can’t see often gets forgotten about and can make one the biggest differences in terms of how your houses performs aiding your level of comfort and reducing energy bills. Armed with this info you can apply it to your floor, wall and roof assemblies also. Let us know if you have any questions in the comments below or send as an email.

Mike PickeringResearch