Incorrect manufacturer declared U Values

Jimbo

I was just calculating a u value for a cavity wall and noticed that the u values declared by some manufactureres on their brochures seem significantly exagerated.

Take for example a standard rendered cavity wall (medium weight block) with 60 mm rigid phenolic insulation in cavity:

Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
External resistance - - 0.0400
External Render 0.017 0.57 0.0298
Block 0.103 0.57 0.1807
Cavity - - 0.1800
Insulation 0.06 0.021 2.8571
Block 0.103 0.57 0.1807
Internal Plaster 0.0125 0.43 0.0291
Internal resistance - - 0.1300
∑ Thermal Reistances = 3.6274

U-value of wall = 0.276
W/m2/K

(Not taking wall ties into account)
Compare that to say: http://www.insulation.kingspan.com/uk/pdf/k8.pdf (Page 4)

Their tables state that using dense blocks, which would increase the u value, 60mm insulation would give a U Value of 0.25.

Are people being misled, or are my calculation incorrect?

d'Oracle

They state the assumptions made for their calculation.

The assumed conductivity of the block and the width of the block (probably a lot less significant in this case) are different to those in your calculation.

Jimbo

There's only slight differences though.
They shouldn't effect it that mcuh

nmg-lky

They take into account alot more factors than what you have shown and work it out over a large area. You should not use there values, always go by your calculations. This is the advice i got from SEI anyway

d'Oracle

Jimbo wrote: »

There's only slight differences though.
They shouldn't effect it that mcuh

I must be missing something.

Your calcs has a lambda value of 0.57.
Theirs has an assumed Lambda of 1.13.

Thats huge.

Jimbo

I only gave an example calc.
A higher Lamda value for the blockwork would increase my u-value even more.

Pkiernan

Insulation 0.06 0.021 2.8571

Quick question, where are you getting the 0.021 value above from?
Is this a Kingspan figure, or a standard from the Part L manual?

It may be that Kingspan have a better value than 0.021 for some of their products.

Jimbo

I got that from the Kingspan docs.

It's generally about that for that type of insulation.

recedite

They have a new IAB cert out for Kooltherm products which gives a TC of 0.02 for the 60mm thick board.
But that still only brings the overall U-value down to 0.26.
I notice they are claiming 0.24 for standard density block with brick.

Jimbo

recedite wrote: »

They have a new IAB cert out for Kooltherm products which gives a TC of 0.02 for the 60mm thick board.
But that still only brings the overall U-value down to 0.26.
I notice they are claiming 0.24 for standard density block with brick.

Im getting 0.27:
Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
External resistance - - 0.0400
Brick 0.103 0.77 0.1338
Cavity - - 0.1800
Insulation 0.06 0.02 3.0000
Block 0.103 0.57 0.1807
Internal Plaster 0.0125 0.43 0.0291
Internal insulation 0 0.021 0.0000
Internal resistance - - 0.1300
∑ Thermal Reistances = 3.6935

U-value of wall = 0.271
W/m2/K


Something is wrong there. Big difference between 0.27 and 0.24

buzymc

I contacted them before about this, the silver foil on the insulation creates a low-e cavity and gives the cavity a resistance of 0.644
I think if you update your calculations this may be the difference.

Jimbo

buzymc wrote: »

I contacted them before about this, the silver foil on the insulation creates a low-e cavity and gives the cavity a resistance of 0.644
I think if you update your calculations this may be the difference.

Never heard that before, it changes things fairly substantially:
Fabric Thickness (m) ÷ Conductivity (λ) = Thermal Resistance
External resistance - - 0.0400
Brick 0.103 0.77 0.1338
Cavity - - 0.6640
Insulation 0.06 0.021 2.8571
Block 0.103 0.57 0.1807
Internal Plaster 0.0125 0.43 0.0291
Internal insulation 0 0.021 0.0000
Internal resistance - - 0.1300
∑ Thermal Reistances = 4.0347

U-value of wall = 0.248
W/m2/K

Not quite the 0.24, but considerably better.

If you're right, Kingspan should really make it clearer.

recedite

That was interesting. By the way, why use 103mm for the width of a block instead of 100? If anything materials are smaller than their nominal size.

Jimbo

recedite wrote: »

That was interesting. By the way, why use 103mm for the width of a block instead of 100? If anything materials are smaller than their nominal size.

It's a standard value I have in my spreadsheet.

4 inches is slightly more than 100mm, but your probably right - block manufacturers have proably rounded their dimensions to metric by now.
I was probably trying to squeeze what I could out of a wall construction when I set the spreadsheet up.