U-values of roof space
Around 25% of heat is lost through the roof, so insulating your loft is one of the best ways to improve the energy efficiency of your home. Luckily, it is a pretty cheap and easy DIY job!
- An uninsulated roof space has a U-value of approximately 2.5 W/m2k
- To achieve a U-value of less than 0.15 W/m2k you can use the following
- 175mm of expanded polyisocyanurate / PIR (Celotex or Kingspan)
- 195mm of Rockwool insulation
- 225mm of Sheepwool insulation
- 270mm of Fibreglass
- 270mm of Knauf space blanket (fibreglass wrapped in space blanket)
To achieve the 0.15 W/m2k U-value in your loft you can expect to pay the following prices per square metre.
- Celotex – £21 per square metre
- Sheepwool – £20 per square metre
- Fibreglass – £7 per square metre
- Knaufs Space blanket (fibreglass in metallic space blanket) – £15 per square metre
U-values of windows
New build windows must achieve a maximum U-value of 1.6 W/m2k under Part L of the Government Building Regulations (for new builds).
- A single glazed window will have a U-value of around 4.8 W/m2k
- Double glazed window with a 6mm gap between panes filled with air – 3.1 W/m2k
- Double glazed window with a 12mm gap between panes filled with air – 2.8 W/m2k
- Double glazed window with a 16mm+ gap between panes filled with air – 2.7 W/m2k
Now, a new double glazed window will typically have a U-value that adheres to building regulations – i.e. 1.6 W/m2k.
The issue with windows though is that for a long time these have been seen as the weak point in a property’s thermal envelope. No matter how low legislators set the required U-value for windows they are never going to get close to that of walls, since you can’t simply add insulation to them. This has resulted in house builders reducing the size of windows in properties – to the detriment of the homeowner, since it means less light entering the home.
If you’re looking for a cheaper way to increase and improve your windows U-values, Ecoease secondary glazing is an option that is simple to fit and costs only about 15% of double glazing. It has a calculated U value of 2.7 using a 20mm gap to the existing single glazing. Find out more Ecoease here.
So there you have it – a guide to U-values – we hope you find it useful when looking to insulate your home!
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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
This is very useful, thanks for sharing!
Thanks for your comment scott!
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
“get close to that of walls or windows” mistake?
Thanks bob – yep that was an error on our part – now updated!
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
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
What would be the u value equivalent to R 20 rating ?
0.05 – (1/R=U)
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
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
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?
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?
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
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
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.
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.
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 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?
this was so clear and easy to understand thank.
would have been nice to have floors included too.
Mark
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
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,
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 don’t see mentioned anywhere here the risks of condensation developing when insulation is fitted in upgrading work. This is an absolutely vital area to fully consider.
I heard that u-values need a full accreditation to be official, is this true?
It is true, Connor. If you need an official U-value, you can give us a call. We have the necessary qualifications to give official U-value calculations 🙂
Really useful article, thanks
Do you use the thickness of only the insulation board for example 60mm or do you use the thickness of the full system so 60+12 = 72mm rather than 60?
Very useful site. Many thanks.
My house was built in 1951 with cavity walls. The cavities were filled with some kind of insulation round about 1980. What is the U value liklely to be? Lots of unknowns I realise so just trying to get a rough idea.