Getting to grips with U-values!
U-values might seem a pretty dull subject; however it is absolutely key that you understand them so that you can insulate your home with the most appropriate material. You can learn more about what a U-value is by clicking here.
The U-value signifies the heat lost through a given thickness of a particular material. You don’t really need to understand the mechanics of how it is calculated; instead it is useful to be able to compare different substances by their U-values.
The best insulating materials have a U-value of close to zero – the lower the better. Building regulations currently stipulate that for a new building, the elements must have maximum U-values as follows:
- Wall – 0.3 W/m2k
- Roof – 0.15 W/m2k
- Windows – 1.6 W/m2k
So in each section below we are going to examine each of the different elements and their typical U-values, we will then show you how to achieve the best possible U-values as stipulated in the building regulations (Part L).
U-values of SOLID WALLS
Solid walls were the norm in most properties built prior to the 1930s – unfortunately at this time energy efficiency wasn’t really on the radar, since fuel was very cheap. However, nowadays trying to keep this sort of home warm is very costly, so insulating them can considerably lower the heating bill.
- An uninsulated solid brick wall with a thickness of 225mm value will have a U-value of 2.70 W/m2k
- To insulate a solid wall you can either insulate internally or externally.
- 100mm of EPS insulation located internally or externally should bring the U-value of the wall down to 0.29 W/m2k in line with building regulations (in this example we have use EWI-PRO 100mm graphite EPS with thermal conductivity λ=0.032W/mk)
- To achieve this with rockwool insulation you will need to add about 110mm to either the inside or outside wall (in this example we have use rockwool 100mm with thermal conductivity λ=0.036W/mk) – obviously if this is done externally you will need to attach it firmly to the wall and it will need to be rendered. If you want to do this internally it is going to steal a large area of floor space.
- ThermaLine Plus is internal wall insulation (often referred to as dry-lining) attached to plasterboard and if you opt for the 40mm thick ThermaLine Plus you should achieve a U-value of about 0.65 W/m2k. This isn’t bad, but if you need to adhere to building regulations (i.e. it is a new extension for example) you are going to have to opt for something else.
U-values of CAVITY WALLS
Cavity walls became the norm in the 1930s. However until 1995, we assume that they were built but left unfilled (with insulation).
Unfilled cavity walls
- Unfilled cavity wall (built prior to 1900) will have a U-value of 2.0 W/m2k
- Unfilled cavity wall (built 1900 – 1975) will have a U-value of 1.6 W/m2k
- Unfilled cavity wall (built 1976 – 1982) will have a U-value of 1.0 W/m2k
- Unfilled cavity wall (built 1983 – 1995) will have a U-value of 0.6 W/m2k
Filled cavity walls – assumed post-1996
All cavity walls built after 1996 are assumed to have filled cavities as part of more regimented building regulations.
- Filled cavity wall (built 1996 – 2002) will have a U-value of 0.45 W/m2k
- Filled cavity wall (built 2003 – 2006) will have a U-value of 0.35 W/m2k
- Filled cavity wall (built 2006 – 2010) will have a U-value of 0.3 W/m2k
- Filled cavity wall (built 2010 +) will have a U-value of 0.2 W/m2k
Mechanisms for improving your cavity wall U-rating
- For a new build cavity wall to achieve a U-value of 0.2 W/m2k you would need to install 150mm of wool but this obviously means extremely thick walls – but this is the cheapest option available to builders.
- Using Celotex to fill the cavities is more expensive, but you will only need to use 100mm celotex to achieve a U-value of 0.2 W/m2k . The only thing to bear in mind here is that you still need to keep a residual cavity on the outside – but this means the cavity can be marginally thinner.
- The maximum U-value you can achieve by retrofitting cavity wall insulation in any property older that 1975 is 0.5 W/m2k since you are limited by the thickness of the cavity. In this instance to achieve a U-value of less than 0.2 W/m2k, you need to apply either internal or external wall insulation too, although you will only need to apply 50mm of celotex either internally or externally to achieve this.
- If you retrofit insulation in cavity walls built between 1970 and 1995, you should achieve a U-rating of 0.5 W/m2k.
- Any cavity wall after that date should already have cavity wall insulation as part of building regulations – the U-values of these types of walls can be seen above in the Filled Cavity Walls section.
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I have a building, a church, which I want to convert to a pair of semi detached houses, the walls are brick inside and outside with a 100mm cavity, what insulation would I
I need to meet building regulations.
Hi, I am building a house and am planning to use 100mm stone external wall, 100mm concrete block inner wall, and between them a 150mm cavity into which I plan to install 100mm Celotex boards. 2 questions please:
1- Will this give me the desired 0.2 W/m2 K?
2- Because of the un-uniform courses of stonework on the outside that I am planning to use, I’m keen to find a wall tie that will screw into the inner concrete block, hold the 100mm insulation board in place, and tie in the outside wall. I am looking at Bluebird Cavity Wall ties to do this. Can you recommend them, or indeed another product to achieve this?
Many thanks,
Your definition of U value seems to be confused with R value. U value is the heat loss per degree temperature difference for a given completed building element per square m (eg a brick cavity wall with 80mm of kinspan. Hence the thickness is not relevant. R value is thermal resistance per m of thickness for a given material. Eg a wall of given construction has a U value the brick the wall is made of has an R value.
Hi can you advise me the best way to insulate a internal wall the existing is 9″ solid and meets a extended cavity wall .Is a foiled foam insulation better than using batterns and multi layered foil insulation. I have no damp problems. Another possibility that has been suggested to me is place membrane against the wall then 25mm celotex fix by 25mm treated batterns over the top and inserting 25 mm celotex I between these battens then the plasterboard . Please would you advise
this was so clear and easy to understand thank.
would have been nice to have floors included too.
Mark
I am being offered free CWI usIng thermabead basic, my house is 1995!build but with no cavity infill even partially. I am apprehensive as i residenin a zone 3 location according to a windswept zonal chart and preTty much exposed on westerly southerly sides
I am told that thermbead does not alloW passage of penetrant rain but i am highly sceptical whats the view On Here?
I am thinking of treating My walls with breathable brick water repellant , would this alleviate the tHreat?
Is that 1983 date correct? The article you published later (February 6 2015) said the regulations changed in 1985, not 1983. Which one is correct? And do you know which month in the year the change came into effect? (I ask because I’m in a block that has a plaque saying June 1983 and I want to check which regulations were applicable when it was built.)
I am in the process of having my conservatory roof replaced using a WARM-ROOF conversion. Some vendors are promoting a 5% VAT rate but many are not. My preferred local fitters do not offer this but had they done so it would save me about £750. How do I go about recovering the difference, is it from the local authority or should I tackle the vendor/contractor. (Having only just discovered this fact and have already paid a large deposit.
Hi there, you need to go back to the contractor and find out why they are not offering the reduced VAT rate.
Can you help please. I have a court case soon whereby the builder on my extension, did not put enough insulation in roof or floor, despite it being done under building regs! Are you able to tell me in the difference in u value losses into a financial figure i.e. If door was shut from extension into remainder of house and heated to 21degress c, over a period of six months, what would the increased cost of heating be in that room.
The figures are 0.37 u-value in my pitched roof instead of 0.17; 0.32 u-value in my floor instead of 0.19.
Nobody seems to be able to work it out. Thank you ASAP please. Shirley Gale
Basically you have to work out what it would be for say an hour for each equation. Take one away from the other and multiply roughly by the number of hours you would be heating the room during the six months.
Sorry read your email wrong was just scanning but still an building inspector should know this.
It sounds like you need to get a building inspector into write a report for you. It is the same type of person who does a report for you when you are buying a house its just more detailed and room specific. But any estate agent, google or yell should help you find someone in your area. Can’t understand why your solicitor hasn’t organised this. You then need to check the paperwork and planning outlines from the builder who did the work. Your plans should state what thicknesses the builder planned to do your roofs and wall etc and the inspector should confirm whether or not it was done to this standard. Your solicitor should have sorted this out for you.
Hi There hoping someone can help me out?
Is 100mm thickness of an insulation material, thermal conductivity 0.04W/(m.K) equivalent to 50mm thickness of an insulation material with thermal conductivity 0.02W/(m.K)?
Thanks
Matt
Congratulations – delighted to see it. I spent nearly all my career in the construction industry – where I conceived a considerable respect for the Building Regulations and the people behind them. Then I retired to France, which enhanced my respect – oh, how they are needed here……
In my experience the builders and house-builders themselves failed to appreciate who their best friends were – there must have been times when the BR people were all too aware of this. In the meanwhile, though, you’re doing a splendid and desperately, increasingly, needed job (even though I don’t offer favourable odds on external insulation…..). The content of your website’s excellent, if I may say so; equally if I may say so, it would perhaps be good to balance the influence of the double-glazing salespeople with some objective figures on window insulation (which is to say its near-impossibility…..). Erm.. you do talk, under Mechanisms for improving your cavity wall u-rating, of achieving a maximum U-value – woops?
I think your technical advice is first-class
All best wishes –
Peter Firth
Thank you for a good article regarding U values. Currently building an kitchen extension, I am confident that 100mm Kingspan or Celotex insulation in cavity walls will provide a superior performance to glass/rock/earthwool of equal thickness. Now wool insulation has excellent acoustic properties, I do not find information regarding acoustic properties for Kingspan and or Celotex cavity wall insulation, which makes me wonder if a combination of wool and Kingspan insulation would provide the best thermal and acoustic properties? Say wool in the cavity and kingspan on the inside or outside wall?
Can you advise what effect that air circulation within the cavity wall will have on the U value, in both insulated (filled) and uninsulated (unfilled cavity) and especially when adding external insulation to a cavity wall structure?
I am currently converting a garage into a kitchen extension, wet room and office and the external wall is 100mm. What is you advice with regards to insulation to bring this wall in line with building regs??
I am planning to dry line the wall using plasterboard with metal studs.
Thanks
I live in a two level house built in 1910. It is of a random rubble construction and has a gap of between 3 and 8 inches between the inner face and the lath and plaster or gyproc dry lining (where that has been installed prior to our ownership). We also mainly have sash and case windows that I intend to retrofit with insulating units and draught beading. The house is listed which prevents changes that alter appearance.
I want a better wall performance without massive disruption to the family. Taking down plaster walls is absolutely the last resort and external cladding is a no no. I was thinking about PU foam or blowing in polystyrene beads. I’m worried about damp accumulation associated with PU foam so was favouring the beads. I’d value your thoughts and advice. Regards, Stewart
What would be the u value equivalent to R 20 rating ?
0.05 – (1/R=U)
Very useful site for the u values of construction of recent years. Made calculations for compliance with current part L of the Building Regs much easier thanks Graham
Hi James
Please could you confirm what the construction of the un-insulated cavity build is for the wall achieving 1.0 W/m2k and wall achieving f 0.6 W/m2k. I’m trying to work out what the u-value is for an existing wall with 100brick outer skin, 50mm clear cavity and a 100mm internal block work with 15mm plaster skim is?
Thanks
Tim
“get close to that of walls or windows” mistake?
Thanks bob – yep that was an error on our part – now updated!
Hey, thanks very helpful. Just found a case study of u-value measurements.
I am interested in the case stydy of U-value measurements. Please can you e mail me the site.
I am grtting frustered as i am not get anywhere.
Thanks
vimal
Hi Vimal,
What exactly are you trying to achieve? Maybe we can help?
Kind regards
James
This is very useful, thanks for sharing!
Thanks for your comment scott!
This is a very useful article – thanks! Who can I link it to R-values? Any ideas?
how can you link it to R-values?
i think, one is the reciprocal of the other. so to get R values from a U value you divide 1 by your U value. as in 1/U value
Thats spot on niall