Condensation on RSJ

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Part of a build I have been working on this past 12 months.

Rear extension of a bungalow.

Pitched roof
Vaulted ceiling
9x2 rafters
Breathable roof membrane
Steel size is 9m @ 203x203x46kg and is sitting on the inner leaf of the walls.

Windows/doors/bifold were installed just under a week ago to make the area water tight. I have since noticed the RSJ is covered with condensation - especially after this cold snap we have just had.

I am yet to install insulation in the rafters. Will be using rigid board and approx 150mm between (leaving air gap of up to 75mm) and 25-50mm over the rafters and RSJ.

Floor is yet to have screed down and walls are also not yet plastered (built from blocks, thermalite internal and standard dense concrete external with a 100mm insulated cavity)

Question,
Will the 25-50mm insulation over the RSJ prevent condensation build up? Or do I somehow need to insulate the top of the RSJ? Which I am not sure is possible now!
(My architect is not available at the moment so trying to get more info before I am back in touch with him.)

Thanks in advance.
 
Last edited:
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So to get this clear, the steel beam will be entirely on the warm side of the insulation, to the tune of 25mm celotex? In which case it sounds reasonable to me, but the architect would calculate.
You really need the beam to either be on the warm side entirely (preferable) or on the cold side. It gets more tricky if there's a wall outer leaf built on top, but it sounds like you only have rafters on it.
 
I'm having the same issue. The steel is fine (no condensation) along the length with insulation on top but at the end where it rests on the wall was poorly insulated and got wet giving mould on the ceiling underneath. I've cleaned the mould and added more insulation and will see what happens
 
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Is this beam acting as a lintel, ridge beam or purlin? Ie where is it?

Is there no timber bearing plate on top of it, for the rafters to sit on? :rolleyes:
 
Not clear where the beam that you're referring to is or what its supporting - can you post a picture?
 
Beam is supporting a new pitched/vaulted roof/ceiling.

Extension to the rear of a bungalow.
One side the beam sits on a internal cavity wall.
The other side sits on a internal wall.

On top of the beam Aris rail have been installed and roof tiles on top.
 
Part of a build I have been working on this past 12 months.

Rear extension of a bungalow.

Pitched roof
Vaulted ceiling
9x2 rafters
Breathable roof membrane
Steel size is 9m @ 203x203x46kg and is sitting on the inner leaf of the walls.

Windows/doors/bifold were installed just under a week ago to make the area water tight. I have since noticed the RSJ is covered with condensation - especially after this cold snap we have just had.

I am yet to install insulation in the rafters. Will be using rigid board and approx 150mm between (leaving air gap of up to 75mm) and 25-50mm over the rafters and RSJ.

Floor is yet to have screed down and walls are also not yet plastered (built from blocks, thermalite internal and standard dense concrete external with a 100mm insulated cavity)

Question,
Will the 25-50mm insulation over the RSJ prevent condensation build up? Or do I somehow need to insulate the top of the RSJ? Which I am not sure is possible now!
(My architect is not available at the moment so trying to get more info before I am back in touch with him.)

Thanks in advance.
We had similar concerns with a recent extension along with a cranked beam that was deeper then the rafters it was positioned alongside....


If you look at the bottom image you will see I have done all I can to surround it with insulation, even though there is no room beneath. We managed to get some 25mm insulation above and between the roof tile battens. We also glued a piece of DPC to the underside of the beam and draped it down the wall a wee bit. Everything was foil taped or foamed to within an inch of its life.
 

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