U-value calculation for solid concrete walls

Diarmuid

I created a spreadsheet to calculate the U-value for a solid concrete wall (ie 1940s council terrace house). I'm looking for someone to validate the numbers.

I'm trying to get an idea of how effective dry lining will be and what depth of insulation make sense.

My calculator is suggesting that the U-value of an uninsulated 215mm solid concrete wall is 2.2 w/m2k

Then adding 25mm PIR brings it to 0.64 and 60mm, to 0.3. Those numbers suggest to me that anything more than 60mm is well into the diminishing returns area and even 50mm PIR is probably good enough.

Does this sound right? The guys installing the dry lining is pushing 100mm because bigger is better, which is true but it just seems that I'm well into diminishing returns long before 100m

Thoughts?

https://docs.google.com/spreadsheets/d/19DtEp6DTG4EyhviWpgUucVi_IrZgl59fG8Joy7vSPTg/edit?usp=sharing

sydthebeat

where did you get the resistance / thermal conductivity of the concrete wall?

also your external render resistance seems way off.

you need to get your thermal conductivities from Table A1 of TGD B

ive had a quick look on Uvaluate and get 3.15 as the u value of a 215 cast concrete wall.

Diarmuid

Thanks for the comment

I got the cement from various reports I read, but more reading between the lines. Anyways thanks for the suggestions, if anything it makes the case for not going too deep with the PIR

My baseline is now 3.13 and PIR50 will get to 0.38, PIR74 to 0.27. To me that's diminishing returns, considering all those walls have windows with Uvalues of 0.8 or more?

sydthebeat

https://www.boards.ie/discussion/comment/121071574#Comment_121071574

have you considered if you need to comply with 'major renovations' section of TGD L ?

if so the u value you need to meet will be prescribed for you during a preliminary Part L assessment.

personally, in 2023, i wouldn't even consider any wall u value worse than 0.2

A wall of 0.27 is 2002 standard. Diminishing returns really only kicks in when you are below 0.15 (which would be around teh standard wall construction these days)

and im not sure what you mean by correlating the wall u value to the window u value.

Windows and doors are necessary holes in your walls, but you dont negate on your wall insulation because you have holes in your walls. if anything you makee the walls MORE efficient to allow for the poorer performing window and door elements.

Diarmuid

I don't see my work as falling under major renovations "means the renovation of a building where more than 25% of the surface of the building envelope undergoes renovation". I'm rewiring the house and replacing the rads. As the rads were off the wall, I am considering placing insulated plasterboards. So right now the default is nothing on the wall, so anything is an improvement.

The consideration is not just uvalue. It's also room area, which in a small terrace house, is already at a premium.

I have ~50m2 of outside walls, so adding up the uvalues for a 20 degree differential between outside and inside, my calculations suggest I go from 3237W (right now) to 1037W(walls 0.38) to 857W (walls 0.2)

Could it be better, sure but it's all tradeoffs.

sydthebeat

if your doing the front and back walls of a terraced house, you might easily find your covering more than 25% of the surface area

anyway, something is better than nothing, but "diminishing returns" occur immediately when your starting point is nothing. The optimal point, in my opinion would be 0.2 because, as ive said, 0.27 would be 2002 standard, and 0.38 would have you in the region of the 1991 building regs.

Diarmuid

That's a fair point

Tim Robbins

https://www.boards.ie/discussion/2058312303/u-value-calculation-for-solid-concrete-walls

Are you doing anything to increase air tightness e.g. sand cement?

Might be worth considering how much space you are losing inside? My house was bought 2012 and had 90mm internal insulation - and was also fine.

I think the point about diminishing returns is good but also - some house go for mega low values because they might be passively heated or have a heat pump. One thing I cannot understand is how this can make any sense if the rooms also have open 4 inch window vents that are not electronically controlled?