U value calculator for cavity wall

[Dannyboyski]

Before I go and spank 70 notes on an energy assessor to calculate this for us.

Does anyone know of a free U value calculator for a cavity wall?

Building control have asked us to calculate this as part of our renovation.

I'm guessing LBC will probably only accept something official.

Dimensions: Single skin brick wall Externally 115mm thick, Empty cavity 60mm wide, double skin brick wall 220mm thick on the inside.
(Different bricks used on inner and outer walls)
Victorian wire cut bricks on the outer skin and pressed bricks on inner skin. All bonded with Lime mortar.

Any advice gratefully received.

Dan

 

[tony1851]

The U-value of that will be around 1.2 W/m²C, but why is building control asking for that?
Legally they cannot insist that an energy assessor do the figures - their job is to check any figures you send.

 

[Dannyboyski]

Thanks for your reply Tony,

They didn’t exactly insist.

Basically we’re restoring an old Victorian cavity house that was covered in paint. And had many damp issues when we got it.

we’ve tediously removed all the paint. Ground out all the cement, unclogged the cavity.

anyway long story short, I believe that in order to keep it damp free we need to let the walls breathe..

House needs a new roof so we’re converting the large attic while we’re there. Obviously this then needs insulating to current standards. (I’m keen not to insulate the external walls In the loft as I’ve read the heat helps to keep the wall dry and healthy in old buildings) we are not insulating any of the exterior walls elsewhere in the house.
LBC officer has been really helpful and says if it meets 0.7 w/m2k Then we can apply an area weighted u value. But it must hit 0.7

we are self builders doing a building notice and as yet I haven’t submitted any u values.

cheers

dan

 

[tony1851]

You are correct in saying that an old house needs to breath, but remember that in the old days when many rooms in your house would be heated with cheap coal, the draft caused by the fires would be sufficient to keep the house dry and free of condensation.

But that was then - this is now and without having some form of insulation applied to the walls, you will really struggle to reach 0.7. Even by doing area-weighted calcs (ie putting far more insulation in one area to allow for a reduced U-value in another) it will be difficult to get what you need. The problem is that if there is too much disparity between the U-values of - say - roof and walls, it encourages condensation and mould growth on the element with the lower U-value.

If the inspector will not budge, you need to consider some form of internal wall lining such as Kooltherm. If the cavity is to be kept clear, you shouldn't have any problems. What you should be wary of is applying insulation in the cavity itself - not usually a good idea as it can cause decay with some types of brick used on the outer skin; wirecuts can be problematic.

 

[Dannyboyski]

Kudos Tony, that is very helpful. The LBC did say the same regarding moisture. we have a wire cut outer skin. pressed inner.

having dug all the muck out of the cavity (came up to first floor) I do not intend to fill it with anything.

Thanks

Dan

 

[Dannyboyski]

The U-value of that will be around 1.2 W/m²C.

Tony, is this accurate enough to use as a benchmark figure, to give to BC? Do you mind me asking how/ where you got it?

Thanks

Dan

 

[DevilDamo]

You could use this...

https://www.vesma.com/tutorial/uvalue01/uvalue01.htm

Using your build up, it provided a figure of 1.17 so basically what @tony1851 said.

 

[Dannyboyski]

Cheers Damo, that is a very useful link.

Dan

 

[mrrusty]

We have fairly recently completely restored an edwardian semi - back to bare brick, new roof etc. Ours is also cavity wall. The cavity is well ventilated. We discussed internal insulation with BC and decided this was a good idea. our BC wasn't particularly interested in actual u values in the restored areas (but the extension had to be to spec) - just worked on the basis that whatever we could practically do was better than it was.

We used 72.5mm insulated PB wherever we could and 25mm where we couldn't (stairwell). It has been well worth it - house is toasty and the warmer internal surfaces assist in avoiding condensation. Energy consumption is really low.

We discussed interstitial condensation risk, and the conclusion we came to was if it did occur in the inner skin, the ventilated cavity would keep things dry.

I wouldn't internally insulate a solid wall without a ventilated cavity, but it does seem to be a good solution otherwise. IMHO it's all very well keeping original materials in restorations, but you also then need original ventilation - open chimneys, draughty windows. If you create a more sealed box to modern standards, you also need the insulation to keep the internal temperature up to modern standards and avoid condensation on cold surfaces.

 

[Dereekoo]

THis may help
http://www.cba-blocks.org.uk/u-value-calculator/

 

[Nakajo]

Several insulation manufacturers have online calculators, but they'll do the calculations for free if you phone them.