U Value Calculation - help please

[GeorgeWBush]

Hi - Would be grateful for any feedback on the following...

I've been googling for what seems like for ever now and have come up with some u value calculations for the walls of our extension. I would like to forward this on to our BC officer. Do they look OK?

Do i need to worry about wall ties (if so how do i calculate?). Anything else I need to add?

--External pebbledash render 15mm (0.57w/mk), resistance = 0.015m/0.57 = .0263m2k/w
--Plastor Fibolite Blocks 100mm 7.3N/mm2 (0.28w/mk), resistance = 0.1m/.28 = .357m2k/w
--Dritherm (32) 100mm (0.032w/mk), resistance = 0.1m/0.032 = 3.125m2k/w
--Plastor Fibolite Blocks 100mm 7.3N/mm2 (0.28w/mk), resistance = 0.1m/.28 = .357m2k/w
--Plaster 13mm (0.57w/mk), resistance = 0.013m/0.57 = .0228m2k/w

--Total m2K/w = 0.0263 + 0.357 + 3.125 + 0.357 + 0.022 = 3.8881
--u value = 1/3.8881 = .257


Thanks.

 

[freddiemercurystwin]

Just phone your insulation supplier and tell them the construction and they'll tell you the U Value. That's easier than messing about. Expect someone who has some software will be along to do a check though. I haven't done a U Value calc for about 20 years!

 

[mikric]

As above apart from the 20 years
Why do you need so much detail ?

 

[GeorgeWBush]

Going on what the building control officer said to me. I told him we were using fibolite 7n - 100mm dritherm-7n fibolite. He was asking us to dry line to meet the required U value but I wanted to show we did'nt need to.

 

[RonnyRaygun]

Do your calcs take into account the mortar joints?

 

[GeorgeWBush]

nope... do they need to? What do i need to add for that?

 

[RonnyRaygun]

Ermm...dunno...never done it...

But...logically...mortar is going to have the same resistance per mm as render...so you need to work out the area of mortar joints per square metre...then subtract that from 1 square metre of wall to give your block area per square metre...add it all together and hey presto!!

Going to give you a very slightly worse answer than the one you've got I would think...

 

[freddiemercurystwin]

Just phone Knauf, they should be able to email you a nice little print out for BC.

 

[hotrod]

Do your calcs take into account the mortar joints?

You're a cheeky monkey Ronny

I've not done a U-value calc "long hand" since I left Uni - not quite 20 years. We used to have a Building Services Engineering module with a CIBSE bod and he used to make us do them over and over and over....zzzzzzzzzzzzzzzzz.

 

[GeorgeWBush]

Just phone Knauf, they should be able to email you a nice little print out for BC.

Brilliant! I'll give them a call.

 

[wavetrain]

I find doing U value calculations a very enjoyable part of building design.

The thin bed morta system is used to overcome the morta bridging problem. As seen on Tommy Walsh's 60 day eco house build last Monday on Freeview Quest channel @ 6pm

 

[JoeMar]

use the celotex one, it's really good and you can print it out for the BCO. it does floor, walls and roof.

http://www.celotex.co.uk/Other-Resources/U-value-Calculator

 

[freddiemercurystwin]

That would be great ......................

if he was using Celotex!

 

[ajrobb]

Anything else I need to add?

Outside surface R-value = 0.04
Inside surface = 0.13

As said above, check with Plasmor that K-figure for Fibolite is for chosen mortar thickness. Get them to give installed R-value rather than material K-value. Their brochure gives typical U-values, which would be great if one of them matched your wall. Given that you are using 2 skins of Fibolite, they should provide a U-value for BC.

Edit: just checked R-value for 100mm DriTherm Cavity Slab 32 Ultimate is quoted at 3.10 The difference might be due to typical installation.

 

[GeorgeWBush]

Just had the following sent through from Knauf:
View media item 31686
I'll forward this on to Building Control first. I'm hoping this will be enough for them...