Humid rockwool under the roof slant

[jido]

I removed insulating wool that was between rafters for a few days.

The upper side of the wool became wet, specially on the low end and closer to the window. There seems to be condensation along the membrane.

Is that normal?

Will it damage the rafters?

 

[tomfe]

Just to check the insulation in your rafters or your ceiling joists?
Any moisture in contact with timber will rot it.

 

[jido]

Just to check the insulation in your rafters or your ceiling joists?
Any moisture in contact with timber will rot it.

They are rafters.

How can I prevent condensation behind the insulation?

 

[KenGMac]

jido, good evening.

On the information posted it appears that your loft is not ventilated, that is either eaves vents, or ventilated tiles on the roof slope and probabily ridge vents?

What is probably happening in your roof space is that warm moist air is percolating into the loft space where it is cold, moist air condenses in cold spaces here the moist air is dumping vapour / water on to the underside of the insulation.

If there is no or inadequate cross ventilation within the roof space the insulation will continue to become wet [and useless]

Can you see any other indications of water droplets / dampness on any other roof surfaces inside the loft space?

As an aside if you look at the "pinned Posts" in floors loft Etc" on this board you will find a load of information about Condensation in loft Spaces.

Ken.

 

[jido]

Hi KenGMac,

Thank you for your response.
All right I understand that the moist air and difference in temperature can cause condensation.

I should specify though, this is not in the loft space. The roof slant belongs to a bedroom in the house extension. So I do not see a provision for adequate ventilation.

Is my issue that the membrane is not letting vapour through?

I would expect that heat from the bedroom would help vapour escape through the roof otherwise. This was with just 100mm Rockwool, no plastering. When the ceiling is complete the insulation will be more and the difference in temperature greater (but there will be foil-backed PIR to block most of the vapour).

 

[tomfe]

You need to leave a 25mm gap behind your insulation as the roof still needs to ventilate.
Also if you have pushed it right up against the tiles you'll stop the membrane/felt from working as there will be no sag in it.

 

[jido]

I see, so the membrane does need to sag.

I will just remove the rockwool and insert foam board insulation between joists with a 50mm air gap above.

Hopefully that will allow the roof fabric to work correctly.

 

[Ian H]

Does it need the sag to stop any water running down where the lats are nailed to the rafters?

 

[John D v2.0]

It needs to sag to let water on the outside run between the membrane and the tile battens. This only needs to be slight eg 10mm.

 

[Ian H]

Ah!! So without the sag the batten could trap the water.