I built my two storey extension for 20% of builder's quotes

This site shows how - step by step from design to completion

Calculating Insulation U Values


These days, energy efficiency is the keyword, so when specifying any part of the structure, you also have to calculate and specify its U value (how thermally efficient it is), listing the u value of materials used in concstruction. I have met builders that find this a nightmare, so I'll talk a little of my experience with U values on the next page.

Building Regulations L1B Conservation of Fuel and Power in Existing Dwellings, lists target U values for renovated structures (maybe dry lining an existing wall) and different U values for new structural elements of your extension. The U value table below is the basic guide, but I recommend reading the Building Regulations as there are caveats that may come into play depending on circumstances.

Thermal Elements
Pitched roof - insulation in ceiling
Pitched roof - insulation in rafters
Flat roof or roof with integral insulation
 At first, I found this a nightmare. If I thought - how to calculate U value of cavity wall - I decided to build my walls with brick cladding, mineral wool cavity bat and concrete block inner with plaster finish - what's my U value? Well working it all out can be worse than re-sitting your 'O' level maths exam because first you need to know the R value of each material, add them together and find the reciprocal. To make it worse, wall ties and mortar may have to be taken into account too depending on your choice of materials. It's also worth noting that when trying to calculate a floor U value, the Perimeter/Area ratio needs to be taken into account. Trying to work it all out is horribly reminiscent of one of those head mangling questions like - How long does it take two men to fill a bath with the plug out when two trains are heading toward each other at 35mph with a 15mph headwind?!!

That said, I found that insulation manufacturers websites helped out a little here. Naturally they are in the business of meeting these targets with their products, so if you go to an insulation manufacturers website, they may well have a U values calculator available as well as a U value calculation example. You specify brick, (choice of their insulation) block, dot and dab plasterboard for example, and their calculator generates a U value. A great guide but most do have a caveat that says it is just that - a guide - and not suitable for Building Regulations application. I think this is because they do not take into account wall ties and mortar fractions. I e-mailed several manufacturers to see if I could get more detail. I found knauf very helpful, e-mailing me back the calculations for chosen structures.

I also found software called build desk U. This is professional software aimed at those in the business, and is not free. However, as they offer a free 30 day trial, I downloaded it and gave it a go. Along with their online training videos, I managed to get on with it pretty well, creating my own calcs for wall, floor and roof (I was pleased to see that my calcs pretty much matched those done by manufacturers, so was confident I was doing something right). I was actually amazed how much difference the choice of wall tie makes to the overall U value. Build Desk U has the facility to pick specific manufacturers products, which was really useful.


And so eventually I was armed with some new detailed CAD drawings dotted with notes and U value figures. I collated this plan with the structural calcs supplied by the engineer, site and block plans, a page of U value calcs, truss layout drawings supplied as part of a free quotation from a truss design company and then e-mailed it all off to the Building Control department of my Local Council.

Cost of Building Control is based on area of your extension. As I was having some internal work also, this did not count as part of the extension cost and so would be an extra £136. That said, I'm pleased I first phoned the Building Control department to check this because they then agreed that as my extension at 27M area fell quite a bit below the maximum of 40M for my price bracket, the cost of the small amount of internal work could be absorbed in the main cost of £495. However, I then got a call back from Building Control, confused by my 27M calculation. I had just calculated ground floor area; they pointed out that for a two storey extension the area is double - TOTAL floor area. I guess that's pretty obvious really, doh!

I  really was not sure if I had gone over the top or not with the detail in my plans, but I waited to see and report back. Normally they take 5 weeks to check over the plans. I was entitled, however, to start work before then at my own risk - (i.e.) if they don't like what I've done, it's my job to put it right. My first task was to have the internal wall removed - a job I did not wish to undertake, so though I was going to do as much prep as possible to keep cost down, I employed a builder to do that for me. I guessed I would start making a mess when he had a free slot to do the work.