online U-Value calculation questions

[Thorfun]

Hi all,

I've been going through an online U-Value calculator to try and figure out my external wall make-up to get an idea of what I need to get a U-value. I have a few questions on the output:

 

my questions on the screenshot below are:

 

1. what is the U-value (uncorrected) and U-value (corrected) differences? i.e. what is corrected U-value and which one would the TF manufacturers use in their marketing materials. so would this be a 0.14W/m2K or a 0.15W/m2K external wall?

2. on the right there is an image of the make up of the wall with a cross denoting "DP" within the 40mm of PIR. can someone please explain what "DP" is and is it being in the PIR an issue? also, where should it ideally be?

3. I have specified 140mm of mineral wool/glass wool (e.g. Frametherm 35) on this calculation. my architect has said that you shouldn't fully fill the stud work and should leave an air gap between the PIR and the mineral wool. is that true? if so, why would you need that? as leaving an air gap will reduce the U-value.

 

thanks in advance.

 

 

[ADLIan]

Its often simpler and more accurate to get the manufacturers to run these figures for you - only takes a phone call or email. Looking at the calculation (I'm not familiar with the use of this software) I would comment;

1. Corrected/uncorrected - may be correcting the U-value for gaps around the insulation (as per the BS). Not required in this instance as you have 2 layers of insulation and the mineral wool is normally deemed to fully fill between the studs.

2. DP = dew point? Does not seem right in this position if it is.Especially with the inclusion of the AVCL

3. Architect is wrong. No need for an airspace. It will impact the U-value but making it higher! Leaving a 25mm airspace will give a low emissivity airspace with resistance 0.44 m2K/W. Filling this gap with 25mm of mineral wool gives a much higher resistance of 0.025/0.035=0.71 m2K/W

 

Other comments;

1. The airspace between the plasterboard and AVCL is not low emissivity. It is high emissivity with resistance 0.18 m2K/W

2. You do not appear to have accounted for the thermal bridging of the studs in the140mm mineral wool and the battens in the service void

[Thorfun]

2 hours ago, ADLIan said:

Its often simpler and more accurate to get the manufacturers to run these figures for you - only takes a phone call or email. Looking at the calculation (I'm not familiar with the use of this software) I would comment;

1. Corrected/uncorrected - may be correcting the U-value for gaps around the insulation (as per the BS). Not required in this instance as you have 2 layers of insulation and the mineral wool is normally deemed to fully fill between the studs.

2. DP = dew point? Does not seem right in this position if it is.Especially with the inclusion of the AVCL

3. Architect is wrong. No need for an airspace. It will impact the U-value but making it higher! Leaving a 25mm airspace will give a low emissivity airspace with resistance 0.44 m2K/W. Filling this gap with 25mm of mineral wool gives a much higher resistance of 0.025/0.035=0.71 m2K/W

 

Other comments;

1. The airspace between the plasterboard and AVCL is not low emissivity. It is high emissivity with resistance 0.18 m2K/W

2. You do not appear to have accounted for the thermal bridging of the studs in the140mm mineral wool and the battens in the service void

 

thanks for the response. I'm not in a position to buy stuff at the moment and so I haven't asked the manufacturers yet but I will do so once I get to that stage. I'm really at the planning stage to see how much I can reduce my reliance on PIR. 

1. corrected/uncorrected: ok, this makes sense. so I would use the 0.14W/m2K U-value in this instance?

2. I thought this was dew point as well. I'll ignore this for the time being then and wait until I get proper calculations from the manufacturers or architect.

3. this is what I thought and no airspace is required. I will stick with fully filling between the studs.

 

other comments:

1. is this true if using something this http://www.protectmembranes.com/protect-vc-foil-ultra-insulating-avcl/p/12? or do you just mention it because I have just got a basic polythene layer in the example? my intention is to use something like the Protect VC foil VCL/Airtight membrane but I couldn't see a way to accommodate that in the u-value calculator. 

2. I haven't a clue how to do that in this calculator!

[A_L]

1. The correction is a default value which may/may not be appropriate.

2. Probably indicates a dewpoint, however my software does not indicate any dew point within the wall as given, even with the polythene removed there is no dew point. It may indicate where the temperature is low enough to start to condense but this does not allow for the vapour pressure reducing effects of the materials in the wall.

3. No airspace required

 

The unventilated airspace thermal resistance would be correct if the plasterboard had a foil surface.

 

The calculator appears to have the ability to accept custom values

 

More important is no allowance for the timber fraction in the timber frame and less importantly in the service cavity. I estimate with a 15% timber fraction (default value) the build up has a U-value of 0.179, without allowing for this the U-value is 0.155

 

The external resistance R-value is already allowed for in the ventilated cavity R-value, although there probably is no way to prevent it being added by the calculator. The timber cladding R-value should not be included as our recent conversation (outside a ventilated cavity)

[Thorfun]

thanks for the additional information on points 1, 2 and 3.

52 minutes ago, A_L said:

The calculator appears to have the ability to accept custom values

 

the calculator does indeed allow custom values and I've changed the lamda value of the PIR and Mineral Wool Batts to reflect the chosen insulation (defaults are 0.023 and 0.038 respectively).

 

54 minutes ago, A_L said:

More important is no allowance for the timber fraction in the timber frame and less importantly in the service cavity. I estimate with a 15% timber fraction (default value) the build up has a U-value of 0.179, without allowing for this the U-value is 0.155

 

if I'm reading this correct, the timber frame itself has a massive impact on the overall U-value of the wall! I would assume that increasing the thickness of the PIR would reduce the impact the timber frame has as there is more insulation reducing cold bridging?

 

56 minutes ago, A_L said:

The external resistance R-value is already allowed for in the ventilated cavity R-value, although there probably is no way to prevent it being added by the calculator. The timber cladding R-value should not be included as our recent conversation (outside a ventilated cavity)

 

I read somewhere (apologies but I can't remember where!) that the timber cladding would add an amount of insulation even if it is outside of the ventilated cavity. is that not the case at all then?

[A_L]

56 minutes ago, Thorfun said:

if I'm reading this correct, the timber frame itself has a massive impact on the overall U-value of the wall! I would assume that increasing the thickness of the PIR would reduce the impact the timber frame has as there is more insulation reducing cold bridging?

 

Yes to both points. If you change the mineral wool batt layer to 85% does the calculator not offer room to enter 15% timber or put in such a custom layer?

 

57 minutes ago, Thorfun said:

I read somewhere (apologies but I can't remember where!) that the timber cladding would add an amount of insulation even if it is outside of the ventilated cavity. is that not the case at all then?

 

No, 4.8.6 on page 11 of the document from me that you downloaded recently

[Thorfun]

2 hours ago, A_L said:

 

Yes to both points. If you change the mineral wool batt layer to 85% does the calculator not offer room to enter 15% timber or put in such a custom layer?

Not tried this but will give it a go tomorrow. Thanks for the tip. 

 

2 hours ago, A_L said:

No, 4.8.6 on page 11 of the document from me that you downloaded recently

 

Ok, cheers. Will take a read again as I must’ve misunderstood it fully. 

[Thorfun]

Thanks @A_L I adjusted the calculator to 85% mineral wool and 15% timber stud and it worked. I also adjusted the internal services cavity to include 15% timber batons. I didn't even know you could do that so thank you very much. it makes my rough calculations just a little bit more accurate.

 

what's interesting now is that the corrected U-value is now failing the test and is above the Max U-value of 0.16W/m2K.

 

 

Also, what's interesting is this wall make up is the same as MBCs 0.14W/m2K wall apart from them using the VC Foil Ultra VCL/Airtight Membrane (see below)

 

can you, or anyone, explain the differences in the U-values received? although I guess it's that they use a different type of insulation as if I change the lambda value of the mineral wool and PIR to 0.032 and 0.020 respectively then I get an uncorrected U-value of 0.144W/m2K so that must be it.

 

so, I'm still a bit confused but I think I'm getting there!