Desk Study 4

Let’s say you’re building your dream property and want to maximize the size of your new rear doors, but your problem is you only have a certain number of watts of heat loss allowable in your SAP calculations.  For the sake of this example, we’ll say you’re limited to 12W/K.

To give you good door performance, triple glazed bi-fold door units can have a u-value as low as 1.0W/m2K.  With a standard 2.1m door height this gives a linear transmittance of 2.1W/mK.  If you have a beam and plate lintel as explored in Desk Study 1 it will increase by 1W/mK (almost 50%!), totalling 3.1W/mK.

Dividing the 12W/K by 3.1W/mK would mean your maximum door width would only be 3.87m.

Now let’s see how wide the door could be with one of our thermally broken lintels.

With a lintel psi value of 0.07W/mK, the total door plus lintel transmittance would be 2.17W/mK.  Dividing 12W by this value would be the doors could be 5.53m wide!  That’s a 43% increase in door size.