Calculated Wall Type U-Value ?

[MKEllis]

We usually run our own calculations for U Values using the BRE Calculator (UK Building Research Establishment).

I just carried out an experiment to check the value that appears in the Revit properties of a wall type (Heat Transfer Coefficient - U) for a given wall build up.

The result in Revit does not seem to correlate with the value that our calculator generates.

I have checked through the conductivity values for the materials (under thermal properties) - and using the exact same values the Revit generated figure is far more optimistic.

For a typical cavity wall construction:

Brick (102mm) @ 0.54 W/mK

Cavity/air gap (40mm)

Insulation (60mm) @ 0.020 W/mK

Block CME (100mm) @ 0.51 W/mK

Plaster (12..5mm) @ 0.51 W/mK

The value that appears in the Wall Properties Dialogue box (greyed out - so I assume it is a calculated value) is 0.1935 W/m2K

Our BRE calculation generates a figure of 0.268 W/m2K

The insulation manufacturer claims around 0.25 W/m2K for the given wall build up.

Any idea how we could have such a discrepency ? All the conductivity values are correct for the materials used in the wall build up.

I am beginning to wonder whethe the value in the Wall Type isn't a U-Value, although the units are consistent with it (W/m2K)

Thanks

Martin

[MKEllis]

I am wondering whether this is either a units issue, or simply confusion (on my behalf) on exactly what this 'Heat Transfer Coefficient' figure is.

'Heat Transfer Coefficient U' - is described in units as W/m2K, which is the same as the UK U-Value figure.

Does Revit somehow use the 'emissivity' ratio (a material thermal property) in its calculation  for each layer / material ? If so, this is not considered part of the calculation used to set limiting values in the UK covered by the Building Regulations (but I could see how it could reduce the calculated figure)

I must admit to being confused, we have compared a number of build-ups now with the Revit calculation vs. an external (approved) calculator - and Revit is consistently lower / more optimistic.

Anyone else encounter something similar - or have any ideas if there are units settings somewhere that need to be changed ?

Many thanks

Martin

[MKEllis]

OK - did a bit more investigation.

The discrepency in calculated values appears to be due to the way a cavity is treated / setup in the wall layers.

In the example above I had set up the cavity as a "Thermal/Air layer" or 40mm thickness, with the 'air' material identified.

In Revit the thermal properties for the 'air' material are setup as having a conductivity of 0.0250 W/(m·K) - which means that Revit appears to treat a 40mm air cavity the same as 40mm of insulating material - in other words a resistance (R) value of 1.6 m2K/W.

In the UK the standard cavity resistance is 0.18 m2K/W, so the calculation here is treating the cavity as having far more resistance to heat than it should have.

Taking the cavity out, and running the calculation with full-fill insulation is consistent with an external U-Value calculator / manufacturer information.

So - the question is - how should cavities be treated in the Revit wall layers to generate compliant U-Values ?

I could fudge it by setting up a cavity material that has a specific conductivity figure to generate the correct 0.18 m2K/W for the desired cavity thickness (ie for a 40mm cavity the conductivity would be 0.222) - but if someone then increases the cavity size it will screw up the calculation again.

Has anyone else come across this ? - I haven't found any other mention of it - so I am wondering if I have just overlooked something very simple here.

Any pointers would be really appreciated, thanks

Martin

[MKEllis]

Took a step back there.

I think it is probably better for the time being to set up a specific Shared Type Project Parameter "Thermal Transmittance" (CoBIE 2012UK label) and use values that we have calculated in an approved tool (that can factor in bridging, wall ties, specific types of cavity etc.).

We could use that consistently across various building elements, walls, roofs etc. and use the figure in schedules etc.

It is a shame, as it means resorting to a 3rd party tool to generate the U-Value, and I'm not sure whether that also throws a spanner in any Energy Analysis done through the Green Building Studio (does that automatically inherit the Revit calculated figure rather than a custom parameter ?).

Sorry for my ramblings, maybe it will help should someone else stumble across the same issues.

Martin

[smart_guy7989]

Howdy : I'm a Building engineer i'm faced the same problem , but i understand that the Ri and Ro values equal 0.17 and Autodesk refer that to Wikipedia and that clear in this link http://whatrevitwants.blogspot.co.il/2012/08/calculating-u-value-of-walls-in-revit.html its written in germany you should translate it . As i found there is a problems when i compare the calculations with what i learned in the Uni. such as: 1- the Rse value depend at wind speed thus it should be connect with the wind speed of the location and that's not exist in the software . 2- if you try to do the calculation's at a slab's , floors , and roofs it use the same values of the wall , and that's wrong as i studied .

[MKEllis]

Hi,

Thanks for the link, it shows what Revit is doing in the formula and confirms that it factors in Rse (0.04 m2K/W) and Rso (0.13 m2K/W), but not the cavity Resistance.

BRE guidance in the UK recommends the use of the following cavity/airspace Resistance:

Unventilated airspace: 0.18 m2K/W

Unventilated airspace (wall) with low emisivity faced insulation: 0.44 m2K/W

For traditional UK cavity wall construction, it means that the 'out-the-box' calculation does not appear to be correct (omit the cavity and it's ok).

It also doesn't appear (at least from what I have looked at so far) to be able to factor in bridging materials (ties or composite layers that comprise multiple materials such as timber rafter and insulation in a known ratio).

Hopefully these are things that can be addressed in future versions.

Martin

[smart_guy7989]

I wonder if we can use another website to exchange experience , i'm from the Middle East and you look's from the UK .

[bimandict]

Hi,

In no engineering terms then. . .

What do I change the Thermal Conductivity value to for the wall buildup exaple above? Cavity 40mm.

Image: Air Barier Thermal Settings, attached.

I found this:

http://archbps1.campus.tue.nl/bpswiki/images/2/22/H5.pdf

See the "U value for Air barier" image attached please.

From this image, can I set my Thermal conductivity to 0.1833 (average for the 50mm cavity) to get an ok estimation?

Thanks

[bimandict]

I get 0.2757 with the average value (0.1833). I would appreciate any comment a great deal however.

Side note. My value I set the thermal conductivity to zero and got 0.13xx, I set up my Wall Schedule to report the U value etc. Then I change the Thermal conductivity value to 0.1833 and my schedule didn't update (the properties did). I had to remove the U value field from the schedule and then add it again to get the updated U value.
Glitch?

[MKEllis]

Hi, I may be misunderstanding your query, if so apologies. I think you are asking what value to use for the thermal properties for air conductivity for a 40mm cavity?

The value entered in the material settings is for the conductivity of the material, whereas the calculation should use a predetermined resistance value of approx 0.18. To fudge the calc, you need to enter a conductivity value that generates the correct resistance (0.18) for the given cavity value of 40mm.

For 40mm, if you use a conductivity value of 0.222, the calculated resistance used will be approx correct (0.18), it will, however go up the spout if you change the cavity size, this is only a work around.

Martin

[bimandict]

Hi,

Thank you very much, no misunderstandings.

I helps a great deal to get confirmation even if its not an exact Revit science so to speak.

Up to now I was just putting in what to me seemed like random numbers. Now I have something to work with and an idea of the accuracy of the data.

Thank you.

[info]

Hi, this thread has helped me enormously. I've  got walls to work to allow the u-value to be checked in schedules and point out anything that will cause issues (people changing the wall build up without approval). I then tried a roof....and its off, waaaaay off. Im checking against BRE u-value calc and cross checking with a kingspan calc to sense check. I just can't get it to work, particularly for pitched roofs. I've calculated the air resistance from BRE, i.e create a material for the specific thickness either vented or unvented.

Should I give up and just manually enter a parameter?

Also, is there any scope to work out walls below ground? Floors to unheated crawl spaces?