RSJ in loft causing condensation underneath

ferry

Years back we had a stack removed up to the loft and an RSJ was placed under the remaining stack in the loft. Weve noticed that when the weather gets colder we get a condensation line forming in the bedroom ceiling directly under it.
Any idea in how to stop this coming through?

thanks as usual

Kynthia

An RSJ wouldn't cause condensation, too much moisture in the air would cause that. Presumably you're confident that there's no leaks or water getting into the area? Therefore does your roof space have adequate ventilation as it's common for house buying surveys to point this out? This link might explain some ventilation optiins: http://www.fixmyroof.co.uk/roof-vents/#do%20i

TheCyclingProgrammer

The problem you're having is known as cold bridging. You need to rectify this before you start having mould problems. The RSJ to be insulated on the cold side so prevent it from becoming cold enough to cause the warm air below it to condense.

Is your loft well insulated?

TheCyclingProgrammer

Kynthia wrote: »

An RSJ wouldn't cause condensation, too much moisture in the air would cause that. Presumably you're confident that there's no leaks or water getting into the area? Therefore does your roof space have adequate ventilation as it's common for house buying surveys to point this out?

I disagree that an RSJ couldn't cause this problem. All homes have moisture in the air to some extent but not usually enough to condense unless exposed to a cold enough surface for the air to condense on.

In this case the RSJ could be causing a bridge in the loft to the ceiling if it has not been properly insulated causing a cold spot. The moisture in the air will condense on the underside of the RSJ if its cold enough and that condensation will then fall on to the ceiling below.

brightontraveller

If you’ve eliminated possibility of a leak /water ingression other common causes are high humidity, lack of insulation around beam, it fully or partly breaks the insulation envelope of the house e.g one end in the cavity or part of beam is above loft insulation other below ( no insulation), draught hitting beam, warm air cold beam not good....

Remove draughts , reducing humidity , insulating will normally rectify, If it persist after you may need to look at thermal imaging to identify the cause

Furts

If I am understanding correctly (and I may not so apologies) a typical steel will have plasterboard for fire protection on the underside. This will need something to fix it to, so there will be timber noggins in the web, underneath and similar.

A solution to your problem would be Celotex, or similar, fixed to the underside of the steel, then plasterboard over for a finishing detail. The Celotex should be at least 25mm - but this would be scimpy so try to aim for 50mm or more.

TheCyclingProgrammer

Furts wrote: »

A solution to your problem would be Celotex, or similar, fixed to the underside of the steel, then plasterboard over for a finishing detail. The Celotex should be at least 25mm - but this would be scimpy so try to aim for 50mm or more.

Are you proposing Celotex to act as a vapour barrier? If so, why not just a normal vapour barrier? Shouldn't any insulation be above the RSJ so the RSJ is located on the warm side, not the cold side?

brightontraveller

TheCyclingProgrammer wrote: »

Are you proposing Celotex to act as a vapour barrier? If so, why not just a normal vapour barrier? Shouldn't any insulation be above the RSJ so the RSJ is located on the warm side, not the cold side?

If its warm air hitting a cold surface a scenario of

(A)Beams supported/ touching cavity / solid wall etc and that contact is making beam “cold” then insulating above beam will have no affect it will still be cold because contact is still there solution insulate below you do start to get into materials thermal conductivity, transference etc.


(B)If its solely due to cold air in loft cooling beam ( little unusual as beams often boxed around the plaster board itself etc is normally sufficient to minimise heat to beam , you could insulate above but run risk of (A)being cause

stone7

Give the RSJ a few coats of SBR and cement.

Furts

TheCyclingProgrammer wrote: »

Are you proposing Celotex to act as a vapour barrier? If so, why not just a normal vapour barrier? Shouldn't any insulation be above the RSJ so the RSJ is located on the warm side, not the cold side?

I have not seen the detail, but imagine the steel is cold and acting as a cold bridge. Is there access above? My approach is to suggest isolating this, just like the edge of a floor slab, or a window reveal. Sure you can put in a barrier, but that should be a default action - insulated ceilings should have one.

Davesnave

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