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Piers

Help with u-value calculations pls

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I'm struggling to get to grips with the various u-value calculators online. They seem to accept differing degrees of inputs. I'm really not sure where I'm at.

 

The timber wall construction we're considering is (In to out)

 

Plaster 5mm

Plasterboard 12.5mm

VCL

150mm rockwool or similar (in 150mm stud)

OSB 11mm

Breather membrane

30mm Kingspan or similar (this is flexible)

Batten & Counter batten => 50mm cavity

softwood cladding 20mm

 

I'm getting u-values from 0.16 to 0.196 using the various calculators (see below)

 

The most worrying thing, is that some calculators claim to show where the dew point is. In one of the calculators (http://www.changeplan.co.uk/u_value_calculator.php), if I enter 150mm of rockwool the calculator tells me the dew point is within that layer of insulation. However if I change the input to 149mm of rockwool (yes, 1mm less) then it's not a problem!

 

So can anyone please help me by telling me what my actual u-value is and whether I genuinely have condensation issue.

 

Calculators:

http://www.thermalcalconline.com/u-value-calculator/u-value-opaque/u-value-opaqueExcel.html

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

http://www.changeplan.co.uk/u_value_calculator.php

 

 

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Try this, it's simple, but OK for just working out a basic build up U value.  It doesn't make allowance for thermal bridging, and assumes that thermal bridges will be designed out (as they should be, really).  For interstitial condensation risk you need to use something like WUFI, but with an internal VCL the risk should be low.

 

 

Simple U value calculator.xls

 

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That's a strange build up, having a relatively vapour impermeable layer (the Kingspan) outside the very permeable rockwool. I can't explain the different U-values but it doesn't surprise me that you're getting odd results for the dew point values as it'll all be very sensitive to assumptions made on the gas tightness of the VCL and the Kingspan (i.e., any allowance for possible leakage points).

 

How about using EPS for the outer layer? It's a lot more vapour open (though not as much as rockwool) so ought to make things quite a bit safer. You'd need a bit more and might have to think about fire safety, though.

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@Piers

My calculator shows a U value of 0.2 for the wall build-up that you described. I've assumed that the ratio of timber stud to rockwool is 15% - 85%.

 

I agree with @Ed Davies that it is usually recommended to have the least vapour permeable insulation on the inside (your Kingspan) and the most vapour permeable ones toward the outside (your Rockwool)

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Will depend on grade of rockwool and polyurethane assumed. Remember to include 15% bridged proportion. Should give approx U of 0.20 W/m2K. Condensation should not be a problem if care taken with the VCL & PUR not sealed though not much safety margin.

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Thanks.

 

@Ed Davies I hadn't considered the permeability of the different types of insulation. We'd thought of rockwool as being a cheaper option than putting polyurethane between the studs. Maybe we should do two layers of kingspan, thicker one between studs and thinner over sheathing to deal with thermal bridging?

 

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Posted (edited)

Yep, rockwool is cheaper per R-value both financially and environmentally. Have you considered putting the PIR/PUR on the inside? I.e.:

 

Plaster 5mm

Plasterboard 12.5mm

30mm Kingspan or similar (this is flexible)

VCL

150mm rockwool or similar (in 150mm stud)

OSB 11mm

Breather membraneBatten & Counter batten => 50mm cavity

softwood cladding 20mm

 

That's essentially what my house design is, though with thicker layers.

 

VCL could probably go either side of the PIR/PUR (or even be its foil face if appropriately taped). Advantages of putting the VCL against the studs: holds the rockwool in place until the inner layers are added and the PIR/PUR protects it from some damage from screws in the wall, etc.

 

No service cavity? With the PUR on the inside you'd at least have something to bury cables in (after checking compatibility, I think PVC cable sheathing is OK in PUR, though not in EPS).

Edited by Ed Davies

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7 minutes ago, Ed Davies said:

Have you considered putting the PIR/PUR on the inside?

In all honesty, no I hadn't. I'd only thought of dealing with the thermal bridging by keeping the studs warm from the outside.

 

Is there much difference between your suggestion above and using insulated plasterboard?

 

Any thoughts on the alternative below?

 

Plaster 5mm

Plasterboard 12.5mm

VCL

120mm PUR/PIR or similar (in 150mm stud)

OSB 11mm

Breather membrane

EPS 50mm

Batten & Counter batten => 50mm cavity

softwood cladding 20mm

 

 

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PUR pushed in against the OSB with the VCL wrapped in to form a 30mm service void?

 

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28 minutes ago, Ed Davies said:

PUR pushed in against the OSB with the VCL wrapped in to form a 30mm service void?

 

Yes

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I think your way is probably superior. Trying to wrap in the VCL is going to be fiddly and I imagine we'd end up with an imperfect seal.

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10 hours ago, Ed Davies said:

 

Plaster 5mm

Plasterboard 12.5mm

30mm Kingspan or similar (this is flexible)

VCL

150mm rockwool or similar (in 150mm stud)

OSB 11mm

Breather membraneBatten & Counter batten => 50mm cavity

softwood cladding 20mm

 

 

As Ed suggests, but fix the 30mm kingspan (or similar) with 25mm batons to give you a service cavity. 

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