Ubakhus moisture calcs?

[Alan Ambrose]

Anyone have any thoughts on this? Ubakhus seems to have found a moisture problem beneath the outer ply. Is this a real problem? Seems to me that whatever the most outside skin is, it would throw up this problem, no? Is the internal membrane doing anything useful given that it's backing some fairly air-impermeable ply?

 

Is this the key sentence? During the winter season of 90 days, a total of 0,021 kg of condensation water per square meter is generated. This quantity dries in summer in 5 days (Drying season according to DIN 4108-3:2014-11). In accordance with DIN 4108-3, the maximum allowable amount of condensate is 1,0 kg/m² if all affected layers are capillary water absorbable, else 0.5 kg/m².

 

That implies it's fine?

 

 

 

 

[ADLIan]

A few comments:

• The DIN standard is specifically for Germany and perhaps linked with compliance with their Building Standards so may not be relevant here
• You need to use BS 5250
• Above BS does not accurately model ventilated airspaces. Convention is to do 2 calcs, one including the ‘ventilated’ airspace and another ignoring all elements external to the ventilated airspace (zero moisture vapour resistance). Actual performance is somewhere between these extremes. Allows a comparison to be made to select safest option

[Alan Ambrose]

Thanks, but both BS 5250 and NHBC seem to just have prescriptions for the common stack-ups. Go, slightly off-piste...and you're on your own.

 

And I was trying to understand the building physics rather than find a somewhat-relevant cookbook in the relevant standard.

 

 

For instance NHBC says:

 

NHBC: 7.2.15 Ventilation, vapour control and insulation - NHBC Standards 2025 NHBC Standards 2025

 

Where a Type LR underlay is fully supported on sarking sheets or boards which offer a high resistance to the passage of air or water vapour, such as plywood, OSB, chipboard, or tongue and grooved sarking boards, the Type LR underlay and sarking should be treated as a Type HR underlay, for roof ventilation purposes.

 

 

... and that's kind of what I've got.

[Gus Potter]

9 hours ago, ADLIan said:

few comments:

• The DIN standard is specifically for Germany and perhaps linked with compliance with their Building Standards so may not be relevant here
• You need to use BS 5250
• Above BS does not accurately model ventilated airspaces. Convention is to do 2 calcs, one including the ‘ventilated’ airspace and another ignoring all elements external to the ventilated airspace (zero moisture vapour resistance). Actual performance is somewhere between these extremes. Allows a comparison to be made to select safest option

This is good comment. Quite often I'll design using the British Standards and some of these are quite old. A good example here is when designing a steel portal frame to BS 5950. To comply with BC regs etc you can use older standards but must be aware where the Eurocodes are more strict and take this into account. To simplify, you can use old design codes provided you know where the modern codes have shown the deficiency in places.

 

In this case @Alan Ambrose we are at the leading edge of design, so we must look to the latest guidance, take that into account and form a view. The DIN codes also apply to bolts / fixings and other things, often seen in the statutory CE compliance regs.

 

To my mind the internal vapour barrier (plastic) is essential to stop moisture entering the roof structure in the first place Then, if we get condensation within the roof structure we want to vent that upwards and outwards. The question is, is the layer of ply on the top of the posi joists able to let seasonable condensation out. 

 

@Nickfromwales might be able to help here. 

 

My own view is to say, lets look at where we may get a high moisture content.. bathrooms / kitchens and do our MHVR to make sure we don't get elevated moisture where we may have vaulted roofs and the damp air stagnates. This is pragmatic design. In other words you remove the potential problem at source. 

 

@Alan Ambrose asks a good question. OSB and ply are not permeable when compared with traditional timber sarking with gaps between the boards. 

12 hours ago, Alan Ambrose said:

Is this a real problem?

Good question. Am I missing something as well as Alan? 

 

Help!

 

 

 

 

 

[Nickfromwales]

11 hours ago, Gus Potter said:

This is good comment. Quite often I'll design using the British Standards and some of these are quite old. A good example here is when designing a steel portal frame to BS 5950. To comply with BC regs etc you can use older standards but must be aware where the Eurocodes are more strict and take this into account. To simplify, you can use old design codes provided you know where the modern codes have shown the deficiency in places.

 

In this case @Alan Ambrose we are at the leading edge of design, so we must look to the latest guidance, take that into account and form a view. The DIN codes also apply to bolts / fixings and other things, often seen in the statutory CE compliance regs.

 

To my mind the internal vapour barrier (plastic) is essential to stop moisture entering the roof structure in the first place Then, if we get condensation within the roof structure we want to vent that upwards and outwards. The question is, is the layer of ply on the top of the posi joists able to let seasonable condensation out. 

 

@Nickfromwales might be able to help here. 

 

My own view is to say, lets look at where we may get a high moisture content.. bathrooms / kitchens and do our MHVR to make sure we don't get elevated moisture where we may have vaulted roofs and the damp air stagnates. This is pragmatic design. In other words you remove the potential problem at source. 

 

@Alan Ambrose asks a good question. OSB and ply are not permeable when compared with traditional timber sarking with gaps between the boards. 

Good question. Am I missing something as well as Alan? 

 

Help!

 

 

 

 

 

May just need counter battening as well as fundamental battens atop the sarking, to increase the air gap / flow.