Confused about U-Values

[alittlerespect]

Humm

Mountainwalker you are hitting a nerve with such outlandish criticism but I will hold my breathe for a little while. Nothing constructive can come of such brash comments! Think twice - answer once, and respect others contributions!

Have a good evening!!

 

[mointainwalker]

@ alittlerespect

Leaving aside the critical comment which is presumably what you don't like, please say what is :

a) Inaccurate
b) Overstated

in my appraisal.

 

[alittlerespect]

Hi

In your statement you are inferring that I am demented and I am bloody well infuriated that you should make such inference when you do not even know me!

To say I am ****ed off with you would be an understatement! Why am I so ****ed off? I will tell you - I have spent the best part of my professional career being criticised by people who are less knowledgable than myself and I have since learnt that the only reason for them pursuiing such tactics is to preserve their own position of seeming superiority and being seen to be in control of a situation by their peers (who are sometimes just as bad) the whole situation being driven by nothing more than their own insecurity.

There is a whole world of difference between criticism and constructive criticism. There being nothing constructive in your comment.

Regards

 

[ajrobb]

Mineral Wool

0.145 12 0.042

0.145/0.042 = 0.345

Timber studs ( 145*50 )

0.050 700 0.180
0.050/0.180 = 0.277

Looking at your original question again, you seem to have chosen 50mm as the thickness of the studs. However, it is normal to have the wider dimension (145mm) as the thickness (this also fits in with 145mm insulation thickness).

Also, you must calculate what proportion of the area is covered by the studs and the rest covered by insulation between the studs. With studs at 600mm centres and noggins, you can typically use 10% stud and 90% insulation. With studs at 400mm centres and no noggins, you get 12.5% stud ( 1/8 ) and 87.5% ( 7/8 ) insulation.

Lets assume 10% stud area. Calculate the average U value for the stud+insulation layer alone:

(0.042/0.145)*90% + (0.180/0.145)*10%

= 0.260 + 0.124 = 0.384

To combine this with other layers, calculate the R value (1/0.384 = 2.6) for the stud+insulation layer and add it to the R values for the others layers.

Surfaces = 0.18
Cladding = 0.015
Plywood = 0.142
Plasterboard = 0.093
Stud+Insulation = 2.6

Total (R value for wall) = 3.03

The U value for the wall is 1/3.03 = 0.33

Sadly, this misses the target of 0.3 by some 10%.

 

[ajrobb]

The U value for the wall is 1/3.03 = 0.33

Sadly, this misses the target of 0.3 by some 10%.

I'd consider changing from plasterboard with a separate vapour barrier, to something like Kingspan K18 32.5mm board, which incorporates the vapour barrier. This has an R value of 0.9 (about 0.8 more than plasterboard alone), which would make the overall R value for the wall 3.84 or a U value of 0.26