# How can I create a construction with a user input specific u-value?

I am trying to model an additional 0.15W/m2K of heat loss through external walls and roof as a rough approximation of heat loss through thermal bridging in various different models. This is to mimic what is required in the Standard Assessment Procedure in the UK, but to create a parallel dynamic energy model. My models are being built in OpenStudio with SketchUp for the geometry.

The ideal solution would be a measure that increased the u-value of an existing construction by a user-defined u-value (in this case 0.15W/m2K). The alternative would be to have a measure, or some way of creating or editing constructions so that it produces a construction that has a specific overall u-value (including the internal and external surface resistances).

I tried to use this measure but the measure seems to keep failing, giving the error: "wrong number of arguments (given 1, expected 0).

I hope this makes sense, any suggestions would be great.

Thanks, Jack

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@jaklloyd does this post answer your question? There is a link in the last comment to a spreadsheet tool that you should find useful to know how to "derate" the insulation layer of an assembly (reduce its R-value) to account for thermal bridging. There is also a related post here.

( 2020-11-08 12:39:35 -0600 )edit

Hi Aaron, thanks, I had seen that post and also this one which I think is the post with the spreadsheet tool mentioned in the comments.

My problem though is not how calculate by how much to de-rate u-values, but once I know my target u-value, how I would simply achieve this value for a construction in OpenStudio.

( 2020-11-08 14:25:31 -0600 )edit

For example I have a wall solid brick wall construction with a U-value of 2.214 W/m2K and I want to de-rate it by 0.15W/m2K so that it has the final U-value of 2.64W/m2K.
I know that I need a total R of 0.42301 to achieve this, however E+ will automatically assign internal and external resistances and so I don't know how much R my construction actually needs to change to achieve an overall construction with a U-value of 2.64W/m2K.

My question is really whether there is a way to modify a construction so that it matches a user-defined U-value?

( 2020-11-08 14:33:39 -0600 )edit

@jaklloyd take a look at this post, I believe it answers your question.

( 2020-11-09 18:57:01 -0600 )edit

@ericmartinpe thanks very much, that's very helpful and gives me a way forward! It does seem like it will be very time consuming to do manually for every construction though. Is there not a way in which you can enter the desired u-value or R value and let OS do the calculations?

( 2020-11-11 04:50:59 -0600 )edit