Building Standards

my guide to building standards / regulations

Ok , so building standards/regs is a very big, complex issue. I'll add info here where I can/when I have time. As I'm going through the build I'm getting a better and better understanding of how the whole thing works. 


U-values are a strange one. Please believe me when I say that the internet is a minefield of disinformation on this, which is a very odd and difficult situation. I'll try and summarise what I've learned so far, for both the Scottish and english/Welsh systems.

DISCLAIMERS: : Last updated 13/2/23. BE WARNED that misinterpretation/misuse of the regulation tables is commonplace online. I have nothing to gain here, just trying to explain things as well as I can, based on my understanding. However, I am not trained in anything building/architecture related. Also please note that, in my experience to date, the below detail is most relevant in Scotland, england has a much more lax system when it comes to proving thermal performance. In Scotland you must document and prove everything to the council before you lift a spade. In england, you must prove it to your chosen private inspector at some point along the way. Private being the key word here; greater flexibility and somewhat less scrutiny is inevitable. The Scottish V-values changed on the 1st of February 2023. This is typical; both systems have been stepwise reducing their u-value limits over time to reduce national energy consumption. Scotland is, in general, one step ahead/harsher in regulations compared with e/W. In Scotland the u-values that apply are those which were active when the building warrant application was submitted. For e/W it's those active when starting construction.

I will now warn you that what I've tried to summarise below is complicated, I can't lie. But I've tried my best to summarise it in a digestible format. Hope it helps. I will mention where relevant the differences between Scotland and england/Wales(e/W below). But for this discussion they're pretty similar, in methodology at least.

Firstly, what is a U-value? Basically, it's the degree to which a part of a building allows heat transfer. A U-value of zero would mean zero heat transfer through a building element, and the further away from zero, the worse performing that element is. 

New buildings, in both the english/Welsh and Scottish systems, have to have a thermal performance as good as what is called the 'notional dwelling'. This is basically a theoretical house of the same size and shape which adheres to every one of the U-value targets for each element of the building (walls/roof/floor/openings). The theoretical/notional house has a rate of heat loss for each element of the building calculated like this:

Rate of heat loss (W/K) = Exposed surface area (m2) x U-value (W/m2K)

W = Watts = Energy with Time (Joules per second). Basically loss of energy rate.

K = Kelvin - this is effectively the same as centigrade/celcius in this application, the two units are the same but offset on different scales. So an increase of 1°C is the same as an increase of 1 Kelvin. And only temperature differentials are important here, not absolutes.

Ok, we're in deep already, bear with me if you're not familiar with this kinda thing. The 'rate of heat loss' is important only as a number you must match or beat, by getting a lower number, when you calculate this for your building. Exposed surface area is simply the area of the element, so roof/floor/wall/door/window in metres squared. And U-value for your specific build we'll worry about later in this discussion.

Let's try an example, my build, and calculate the notional dwelling example for it. So, my building has the following areas for each element:

Floor = 23 m2

External Walls = 39.2 m2

Roof = 25.6 m2

ALLOWABLE Openings (25% of floor area) = 5.8 m2

In the last line we've come across a complication. For the purposes of this calculation, you are required to assume a maximum area of openings (again, this means external doors and windows) of 25% of floor space. This may be the case occasionally, but realistically most extensions/new builds have more glazing than this. You're allowed significantly more glazing/doors in reality, but for the purposes of the notional dwelling you must assume 25% maximum.

So, we have the areas, now we need the U-values to use. These are detailed in the building standards/regs, and are very dependent on what you're building and where. Below are the u-values relevant to new buildings only (NOT extensions, these are different):

These values were taken from table 6.1 in the Scottish building standards and table 4.1 in the e/W regs. IMPORTANT - A lot of people online reference other tables in section 6 (S) or part L section 4 (e/W) but this is not correct. There are tables detailing maximum u-values, but these are 'capping' values only, they are not to be used in overall notional dwelling calculations. If a single element exceeds the numbers above it's allowed to, upto the stated maximum/cap, but this must be counteracted with better efficiencies achieved in other elements. The building must always achieve the notional dwelling heat loss figure calculated using the numbers above. Sorry, I told you this was complicated...

Ok, so we have everything we need now to work out the 'notional dwelling' example for the build:

Floor heat loss = surface area x u-value target = 23 x 0.15 = 3.45 W/K

Walls heat loss = 39.2 x 0.17 = 6.66 W/K

Roof heat loss = 25.6 x 0.11 = 2.82 W/K

Openings heat loss = 5.8 x 1.40 = 8.12 W/K

To calculate the total heat loss for the notional building, we just add these numbers together:

Total heat loss = 3.45 + 6.66 + 2.82 + 8.12 = 21.05 W/K

So this is now our target, and the actual calculation for our actual final construction make-up must match or better this.

So how do we calculate it for our actual building? Well, it gets even more complicated :)

Let's start with something simple. I know how many doors and windows I'll have, and how big they are. For my build they totalled 8.6 square metres. If we wanted to go for the U-values stated in the standards/regs (1.4 for me), this would result in a heat loss of 12.04 W/K. Which would mean we've 'already used' up 57% of our allowable total heat loss!

This is where a balance/battle begins, between cost/efficiency (they increase together) of the 'openings' and amount of insulation required in the rest of the build. This is because we have more than 25% (of floor space) openings. In my case it's around 37%. And we need to achieve better U-values through the rest of the build to account for this, to make sure we still achieve that important total heat loss figure. In reality, if my openings were all a U-value of 1.4, I would likely need to use a very excessive amount of insulation in the rest of the build to conteract the effect of this on total heat loss, resulting in thickening walls, reduced floor area and increased cost of build. So there's a balance between spending money on efficient doors/windows and spending money elsewhere on insulation.

In Scotland this also complicates your pre-build process, since all your U-value calculations must be checked and approved by the council before you start building. Your architect will specify U-values of doors/windows which they believe to be achievable, whilst minimising the amount of insulation required in the rest of the build. For me, the table below shows my architect's final specification for the build:

Ok, so a few things to note here already:

1. Wall type 1 is the back and sides of the Micro house, these need fire proofing inside and out because they stray within 1m of the boundary. Wall type 2 is the front, which doesn't need any fire proofing.

2. The roof u-value is worse than the notional dwelling target. This is because I had my building approved with a slightly different roof notional dwelling U-value target of 0.13. I think this is because my architect had my building classified as an extension to my main house's insulation envelope, which raised this target u-value only. I'm not certain though, will update when I confirm. Anyway, this will become less relevant below, since I ended up quite significantly improving on the legal minimum.

3. The rest of the main U-values are quite a bit better than the target U-values. This is again because I have more than the specified 25% of floor space worth of 'openings'. The negative impact of the respectively-high U-values of the openings must be counteracted by improving the performance of other elements. 

4. My architect specified quite high U-values for door 01 (french doors) and door 02 (single glazed door). This was on my request, since I thought I may be able to buy cheaper doors. However, this wasn't the case, and I ended up with lower U-values, and much more expensive doors, than specified/wanted. I'll talk about this in another section when I discuss doors/windows WRT building standards. 

So that was what was specified pre-build, and what was approved by the council. But, as is probably common, I ended up with some changes as the build progressed, resulting in the following actual final spec:

More notes, we're getting there, don't worry:

1. Door 03 is set to 0.11 since I can't afford this door ATM, so I'm blanking it off for now. It will be added at some point, and this will have a negative impact on the total heat loss here (will increase total heat loss to 18.2 if I buy from the same supplier, glazed). For now it will be insulated/treated the same as 'wall type 1'.

2. The total heat loss is significantly better than required/theorised. This is basically because I ended up with expensive, high-quality doors in order to achieve the required safety standards. This was not purposeful, but I guess it's a good thing. I've actually ended up achieving the notional dwelling performance for the latest regulations (1/2/23 update would result in notional heat loss of 17.9 W/K). So a positive error I guess?

3. I added an unplanned-for roof window once I realised the micro house was a little dark at times. This was a cheap Screwfix one, so doesn't have the best of U-values. It's only small though, so the impact is limited. 

So we've seen how to calculate the notional dwelling heat loss, and how the manipulation of your actual U-values must achieve this same heat loss total. However, we're missing one key part of this - how do we know the U-value of the actual construction elements we're planning? And how can we adjust/manipulate them to achieve what we're aiming for?

This will be done by your architect most likely, this is a core part of their job, but it's still useful to know how to do it, so you can potentially make adjustments on-the-job. U-values for building elements can be calculated manually, but generally architects and others will use software for this, to simplify the process. There's many software makers you can use, but I use one of the membrane manufacturers software - Proctors. I won't go through the whole process, but it's fairly simple anyway. I'll show an example:

Here I've inputted all of the layers of 'wall type 1' to try and emulate what my architect would've done. You can see I've ended up with a slightly different final U-value of 0.12. There's a lot of numbers here and it looks quite complicated, but in reality the process is quite simple. Basically all that needs to be inputted is the material and thickness for each layer, plus details of any thermal bridging or air gaps evident.

Thermal bridging is a part of the layer which has a higher thermal conductivity than the rest of the layer. For example, the 'polyisocyanurate 140 mm' above is in fact the main timber structure layer. Most of this layer is PIR insulation, with the timber structure acting as a thermal bridge through this layer. In this software this is entered with the bridge symbol shown in green and blue.

All the material properties of common building materials are pre-loaded into the software, so the hard work is done for you. The best thing about using this type of software is the ability to play around and adjust the make-up of the building element to try and get to the u-value you're aiming for.