I’d like the forum’s help with a thought experiment (involving some maths).

For context, we’ve identified a thermal bridge at the corners of a proposed office building. The envelope subcontractor is proposing to change the psi-value test criteria to -2° rather than -5° to scrape a pass, on the grounds that it doesn’t get that cold very often.

I’d like to demonstrate the amount of water vapour that is likely to be produced by the occupants, as a ball park figure, to illustrate why we’d be better off eliminating the thermal bridge or using a VCL than relying on balmy winters.

I scraped the following figures from the web:

• normal respiratory rate for an adult at rest is ~15 breaths per minute.
• 0.5 litres per breath, so 7.5 litres/minute or 450 litres exhaled/hour
• when you exhale, you release up to 17.5 milliliters of water/hour (assuming exhaled air is 37°C @100% RH)

The highest occupancy room is around 120 persons in a room that is 350m² and 3.5m high. So the volume is 1225m³ or 1,225,000 litres.

Average ventilation rate from AHUs is 12 l/s/person.

Internal temperature 20°, RH assumed at 50%, Dewpoint is 9.3°

So each hour, the 120 people are generating 2.1 litres of extra water vapour (120 * 17.5ml). But that is going into a space that receives (12*120*3600) 5.2M litres (?!) of fresh air in the same time period.

Does that mean I’m worrying about nothing? Or will there be other factors that I haven’t considered? Bear in mind this is just a ‘back-of-the-envelope’ assessment.

Posted By: tonyRH is, presumably the building will be ventilated

I always use a vb , but it won’t prevent Condensation if the vb itself goes below dew point condensation will form on it.

Thanks Tony,

Yes the room is mechanically ventilated, 12 litres per second per person.

In this case the facade is curtain wall spandrel (metal tray filled with rockwool) with a plasterboard lining in front. So the envelope itself is well sealed, the debate is just around whether to include a vapour barrier within the plasterboard zone.

According to the analysis, the metal tray part is fine, the cold spot is occurring on the curtain wall profile itself. One option is to try and open up the plasterboard zone so that it can reach the same temperature (and ventilation) as the room - apparently if the lining is ignored i.e. 'thermally invisible', then the problem goes away.

I'd still like to look at filling the profile with rockwool or lining it on the inside with slim insulation batts, but this will all require re-running the thermal calcs. I wanted to try and figure out the magnitude of the issue first.