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- This topic has 5 replies, 3 voices, and was last updated 11 years, 8 months ago by Anonymous.
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28 June 2012 at 8:53 am #31699Anonymous
I'm currently completing my first PHPP model having recently attended the Carbonlite course in Plymouth.
I have an external wall construction (design by other Architect) that comprises of the following layers:
Rsi
12.5mm plasterboard
38mm service void
15mm OSB3
140mm timber frame with Pavaflex between studs
120mm Pavatherm board
50mm cavity
100mm recon stone
RseMy question is, how does one include for the wall ties between the timber frame and recon stone? I have followed the procedure given in ISO 6946:2007, D.3.2 and have come up with a correction of 0.003w/m²K. In PHPP, can I simply calculate the percentage area of ties per m² and add this as a briging layer in PHPP? The U-value calculated in PHPP without the ties is 0.135w/m²K, so add the correction and this becomes 0.138w/m²K is that correct? How do I get PHPP to correct the U-value accordingly?
My peference would be not to have any stainless steel ties, but I am working for a contractor who has inherited the design and the external wall finish is a planning requirement.
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28 June 2012 at 12:00 pm #38588Anonymous
Apologies, but I've just realised that as the correction is <3%, the correction need not be applied under ISO 6946. Does this also apply to PH – will this be acceptable to the certifier, or shoud I include the correction to be on the safe side?
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1 July 2012 at 8:35 pm #38589
Leigh,
I assume that the cavity is ventilated to address moisture risks?
Speak to your certifier! If they are not avaialable then work on the worst case scenario. (Though I would expect that the EN standard will prevail.)Mark
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3 July 2012 at 10:03 am #38590Anonymous
Cavity will be un-vented to minimise the risk of thermal bypass.
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7 July 2012 at 9:44 pm #38591
With a moisture permeable construction such as the one that you propose are you sure that you are prioritising in the right order?
http://www.greenbuildingadvisor.com/blogs/dept/musings/belgian-passivhaus-rendered-uninhabitable-bad-indoor-airAs for the PHPP enter the x-value (chi-value) in the thermal bridging sheet.
Mark
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16 July 2012 at 6:55 am #38592Anonymous
Maybe this is still useful, I'm making a new effort to get on the forum! These point repeating thermal bridges can make a pretty big difference to the U-value, especially if your architect has used manufacturers claimed psi values in the initial PHPP model like a recent one I've had – makes for thicker walls than he/she was expecting.
Once you've got the calculated delta U using the proportion of area as in 6946 you can add it as a separate chi value like Mark said, or you could work out the effective psi for the material it is going through and use this value in the U-value sheet, making a note in the description that you've adjusted it – makes for a neater PHPP.
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