Calculating U value

housebuyer7

Hello,

I am removing blown render for the rear of my house and re-rendering. It will be over the required threshold triggering the need for adding thermal insulation. 

I don’t want to add the thermal insulation so would be grateful for any advice for how I can get around this. 

Building control advised me we don’t have to apply if our U value meets their target of 0.30W/m2K. But despite looking online and trying to use calculators, I do not understand how to get the calculation. 

Internally, previous owners have added a plasterboard stud wall to every external wall and filled the gap with polystyrene. Obviously not the best but May count towards something. We are also replacing all our windows which will increase our EPC.

Please can anyone advise? 

Another option would be to calculate the 15 year payback - again I don’t understand how to calculate this.

@FreeBear

FreeBear

I use this calculator - https://www.changeplan.co.uk/u_value_calculator.php - Close enough..

tacpot12

If you don't understand how to use the U Value Calculator, if you post the some details here, I have used that calculator before and could input the data for you:

You (or I) would need to know: 

• how thick the polystyrene is,
  
• how deep the studs are (their thickness from the front of the wall to the back)
• how wide the studs are (their thickness side to side)
• the number of studs there are along the whole wall that is being re-rendered
• the length fo the wall that is being re-rendered 
• how thick the plasterboard is
• how thick your brick wall is, and whether there is any cavity.
• if there is a cavity, how wide the cavity is, and how wide each brick leaf is

You will need to specify the Element Type as "External Wall" and the Build Type as "Refurbishment" in the Change Plan U Value Calculator, and then add the 

The calulator will then show you what the "Max U-Value"  is, and it should say 0.30 W/m2K if you have set the Element Type and Build Type correctly.

With this information, you can use the calculator to calculate the U Value of the wall.

To calculate the payback period, you need to:

- get three quotes to apply wall insulation. The firms that quote will need to do the same data in my list above  (you can save them time if you provide this rather than making each one take the same measurements). They will then work out how much insulation needs to be added to get down to 0.30 W/m2K  (assuming the wall doesn't already meet this standard), and will provide a price for the work. Take the average of the three quotes (This is the cost of doing the work)
- engage an energy assessor or heating engineer to work out how much heat is lost through the wall today and how much would be lost through the wall once it is insulated, and then work out the cost buying the energy to get the heat over 15 years.  (This is how much you will save by doing the work).

If the cost of doing the work is more than the savings you will make over the 15 years, then you provide the calculations showing this to Building Control rather than a design for how to insulate the wall. 

I don't know if you have to include some assumption on the amount that energy prices will rise in future, but doing so would make it MORE likely that you would have to insulate the wall, so you may not want to do this. However, if you think that energy costs will increase during your time of ownership of the house, you might want to factor this into a seperate calculation that you don't share with Building Control. 

0.30 W/m2k is a very unambitious target, and I think it likely that you could met it without making any changes; you just need to gather the data to show that this is the case. You might need to drill a few test holes in the stud walls to get all the data, but these can easily be filled and covered over.

Doozergirl

You don't want to add insulation? 

That's incredibly short sighted.  What's the logic there? 

FreeBear

Doozergirl said:

You don't want to add insulation? 

That's incredibly short sighted.  What's the logic there? 

Adding insulation on the outside of the wall often entails extending the window sills, moving downpipes, and can look piggin' ugly around bay windows. if the detailing around the roof line is done poorly (a problem with gable walls), it can store up problems in the future.

But most of these problems are easily overcome without too much difficulty (except round bay windows). 50mm of Celotex/Kingspan type boards should be enough with a cavity walls to hit the 0.30 W/m²K target. A solid wall will need 65-75mm to get there, but breathable materials would be better (and would need to be much thicker).

When doing the u-value calculations on the current setup, check to see if there is an air gap behind the polystyrene sheets. This will affect calculation slightly - If you don't want the insulation on the outside, and the calculated u-value falls short, another option would be to replace the polystyrene with Celotex and/or overboard with insulated plasterboard.