22 January 2015

Using R-value Calculator to Compare Timber and Steel

I have designed our tiny house to be very thermally efficient. To achieve this I have reduced the thermal bridging and used efficient insulation materials. I have posted previously regarding R-values and it has been brought to my attention that its not the easiest process.

Calculating the complete R-value for an entire house can become complicated rather quickly, especially when using parallel materials. To assist with the calculation I have used a free NZ R-value calculator website called Design Navigator. Initially this website can look complicated so below I have run through a simple example to show how easy it is to use.

Design Navigator website

The following information is an example regarding flooring. But a similar principle could be used for walls and roofs.

Timber Beam Floor R-value Example 

Select the following.

Floor: Suspended Floor
Selection for different floor types.

Flooring: Plywood 18mm
Selection for different floor materials.

Timber Frame & Cavity: 90mm joist @ 600mm
Selection in this case for different joist configurations. I have chosen a common 90mm by 45mm  timber at 600mm centres.

Insulation: 2
The is the R-value for the chosen insulation. Links to most if not all New Zealand insulation products are to the right of the page. Most manufactures websites will offer R-value information. I have chosen an arbitrary value of 2.    

Perimeter type: Exposed floor (pole house)
This calculator is designed for fixed houses. A 'pole house' is the closest equivalent for a mobile houses.

Resulting R-value is 1.99 which is very close to the in-putted value of 2. The timber framing reduces the R-value because the thermal conductivity is more for timber than insulation. As explained in this post a thermal bridge is occurring. So at this stage the R-value is under 2 because the timber is conducting better than the insulation. So how is the result of 1.99 close to 2 achieved? Well the internal surface has a R-value of 0.09. Air attaches itself in films to surfaces adding a insulating properties. Also the chosen flooring has an R-value of 0.13.

Steel Beam Floor R-value Example 

Now I will use the same calculator using steel beams instead of timber and compare the results. I have kept all the other information the same. I have used steel which is as close as possible in size to the timber used in the previous example. Its worth noting that varying the wall thickness will change the resulting R-value. 

The resulting R-value is 0.96, which is half as good as the timber example above. Why is this? If we look at the the thermal conductivity of the two materials you can see that steel conducts at a rate which is 385 times greater than  timber. (also see my previous post on this subject here)

Timber 0.14 W/(m K)
Steel 54 W/(m K)

Steel is a logical choice for the the trailer but to my thinking not for the sub floor beams if you want an efficient house. If you are using steel or other high conducting materials a thermal break is recommended. I am not saying don't use steel because it has some excellent properties for tiny house building.

These calculations may be off putting to start with but if you use this website it makes it much simpler to understand thermal loading. If any of your information is not an option in the drop down boxes use email or the message board and the website creator Albrecht is more than happy to help. 

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