Cold Bridging & Condensation

Several buildings are designed to have various sections of steelwork penetrating the insulation line.These, if not correctly designed can cause a cold bridge and in some cases serious condensation.

Typiclal situations are parapets and canopy type steelwork.

 

Parapets

The simple solution here is to insulate all three sides of the parapet.

Atempts to leave the parapet return ( vertical face on roof side ) and capping uninsulated will nearly always result in condensation issues. This is often mistaken for a gutter or roof leak and resultant in a contractor making several attempts to rectify without sucess. We have witnessed several attempts to execute a parapet like this, with the contractor spending an immense amount of time and sealant in trying to close the parapet at the gutter level. However, this requires materials to be carefully cut around stub columns, sag rods and diagonals and then to be fully sealed. Regretfully asthe building flexes with wind and snow loads, these members also move, breaking any seal that may have been made. Similarly the stub column itself may cause a cold bridge as it is passing from a cold un-insulated environment to the heated area of your building.

Even filling the void will not usually reduce this risk.

Refer especially to Page 10 of Technical paper 18 ( link at bottom of this page )

 

Canopies and similar

Similar situations occur with canopy rafters, brise soleil and some signage steelwork

 

xxxHowever, some penetrations in the
xxxwall situation may not be that
xxxdetrimental. This will depend on the xxxsize of the steel and the use of the xxxbuilding.

 

x

x

The Solution

Undertaking an " f factor " calculation will establish if such a detail is likely cause a problem. Most manufacturers will provide these, but some do not. We suggest that one only specifies products where the manufacturer offers this service ( though we can usually offer this service if required )

Below is a table showing the minium f factor as described in BSS250:2011 Table D.7.

All construction details, not just penetrations shoudl achive these minimum values, irrespective of thermal performance.

 

Humidity Class
Building type / use
Minimum f factor
1
Storage Unit
0.3
2
Offices , shops
0.5
3
Dwelling with Low occupancy
0.65
4
Dwellings with high occupancy, sports halls, Kitchens, canteens Buildings heated with unflued gas heaters
0.8
5
Special Buildings such as Laundry, swimming pools & breweries
0.9

 

 

Further Reading

MCRMA Technical paper 18

 

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