Moisture between plasterboard and Vapour barrier above ceiling

[JohnnyB]

Yesterday I was drilling holes in the ceiling for the MVHR vent points and found there is quite a bit of moisture trapped just below the vapour barrier, between the plasterboard and VB.  I realise this is what the VB is supposed to do but I'm wondering if seeing moisture trapped and the back of the PB being damp is normal a few weeks after the plaster was put on and the ceiling looks dry inside?  The plasterers have only just finished but the upstairs ceilings were plastered a few weeks ago.

 

I was intending to put the first layer of loft roll down this weekend and put the MVHR pipes in to help dry the house out but now I have seen the moisture I'm wondering if this is normal and will dry out from inside the house as the plaster dries fully, or if I need to leave the loft roll wrapped up for now and possibly split the VB in areas to let it dry out and then tape up the splits with airtight tape once it is dry?

 

I have used Steico Multimembra5 above most of the ceilings which I believe should let some moisture through slowly, but above the bathrooms I used a green plastic VB from the local builders merchant.

 

The plaster on the ceilings is onecoat at about 8mm thick, not multifinish, and that must mean more moisture in the plaster than normal.

[JohnMo]

If you have just plastered that carries huge amounts of water. Plus if you don't have effective ventilation...

 

Maybe get a dehumidifier until your ventilation is up and running (after decorating)

 

37 minutes ago, JohnnyB said:

 possibly split the VB in areas to let it dry out and then tape up the splits with airtight tape once it is dry?

Your vapour barrier is there to stop water vapour going into the structure and it rotting, why would you encourage water vapour to go there? NO.

 

I would continue with the insulation and MVHR install.

 

 

[Crofter]

You're best to crack on with the loft insulation, this will help prevent the water from condensing where it meets the cold air of the loft.

[JohnnyB]

OK thanks, I'll carry on with the insulation and get the MVHR running ASAP.  I was wondering if others have experienced the same.

 

I've already got a dehumidfier running upstairs 24/7 and the loft has enough ventilation to dry out any moisture going up there.

 

[JohnMo]

1 hour ago, JohnnyB said:

get the MVHR running ASAP

Don't rush to get running, especially while your still working on house, it will drag in all the dust etc - you don't want that.

[Mike]

6 hours ago, JohnnyB said:

I'll carry on with the insulation and get the MVHR running ASAP.  I was wondering if others have experienced the same.

Yes, I've seen that happen and adding insulation will resolve it.

 

Be very cautious about running the MVHR - as @JohnMo says, you don't want to be drawing dust into the system. However it is possible to have it running when there's no work going on if you take some precautions & accept the risk, as I have. While work is in progress and until dust settles, keep it switched off; during work put plastic shower caps over the terminals; add sock filters to the extracts (a good idea anyway).

 

Also consider upgrading the MVHR return air filter to better protect the heat exchanger - from (normally) ISO 16890 Course (old G4) - to ISO 16890 ePM1 (old F7 & as normally used on the air intake), which filters out at least 50% of very fine dust and 85% of medium-sized dust (up to 55% of sawdust, for example, is too fine to be trapped by a course filter). I've not done this myself as I suspect that these smaller sizes (<10 microns) would be fine enough pass through the heat exchanger without building up significantly, but I've not looked for any research on the topic.

[JohnMo]

31 minutes ago, Mike said:

Also consider upgrading the MVHR return air filter to better protect the heat exchanger - from (normally) ISO 16890 Course (old G4) - to ISO 16890 ePM1 (old F7 & as normally used on the air intake), which filters out at least 50% of very fine dust and 85% of medium-sized dust (up to 55% of sawdust, for example, is too fine to be trapped by a course filter). I've not done this myself as I suspect that these smaller sizes (<10 microns) would be fine enough pass through the heat exchanger without building up significantly, but I've not looked for any research on the topic.

I use cone filters in each extract terminal, you would be shocked how much dust they stop getting in to the system.