New Plasterboard Mould Problem?

[DeanAlan]

Morning folks,

 

Wanted to get an opinion on what looks like some mould spores on new plasterboard . Whole house is being re-boarded after massive renovation.

 

There are a couple of boards that have been put up and within a couple of weeks the seem to be showing a mould/damp problem.

 

However, the wall beneath has been exposed brick (line mortar) for over 6 months and bone dry. The is first floor so not rising damp. I actually blowerproofed the whole wall before hand and it was super dry. Not a hint of any moisture that I could see.

 

The mould areas seem to be where I think the dabs are.

 

No other issues in the boarding elsewhere.

 

The boarded were stored outside during heavy rain but keep below large DPM sheet and on pallets. Has some moisture got to them that way? If so, why just these boarded - maybe based on the location in the pack.

 

Any ideas on what this could be?

 

[Radian]

9 minutes ago, DeanAlan said:

The mould areas seem to be where I think the dabs are.

 

Cold bridging there. Assuming you're somewhere in the UK, we've got super saturated air coming up from the azores - hence the unseasonably mild weather. This is pushing RH levels way up and when the moisture laden air reaches a surface below the dew-point, it makes it wet. When it's wet, mould spores that are everywhere in the air start to grow.

[DeanAlan]

That that wall does need external wall insulation (as to others that aren't showing the problem).

We also have MVHR yet to be commissioned which will lower the relative humidity.

 

If it is cold bridging causing moisture to condense there, what can we do in the interim prior to EWI and MVHR commissioning? Both of those are 3 or so month out.

 

Cheers @Radian

 

PS...

 

Shocked!

 

Just had a closer look at some other walls. The one in the picture is now 3 weeks since being boarded, the others were done a week later and they are showing early signs of the same problem. 

 

There are three external wall of original property. Solid walls that need EWI put on them. They account for maybe 1/6th of the total wall area with the rest being a new (relatively massive) SIPS extension ground and partial first.

 

We've got skim coat to go on those and EWI but now wondering if we've made a big mistake and we're going to have to redo those walls and if so how!

 

Input appreciated.

 

Edited November 13, 2022 by DeanAlan

[Nickfromwales]

You won’t avoid the mould growth with D&D walls at this time of year, unless you are running dehumidifiers 24/7 with the house sealed up. 
It’s not terminal, just undesirable. 

[DeanAlan]

If they were wet plastered would that make a difference? 

I've also been reading that maybe I should use insulated plasterboard and that can be dot and dabbed and not suffer the same problem.

[Roger440]

You have a cold wall and warm plasterboard and an excellent thermal bridge. 

 

Until you solve that problem, its not likely to go away.

 

The real question is why you would d&d onto a solid uninsulated wall? There are loads of potential solutions.

[DeanAlan]

@Roger440

Should we have wet plastered?

D&D insulated plasterboard (35mm)?

Battened out and then boarded to the batons?

 

I D&D because the builder said so. Now - he's a regular builder and didn't get the potential issue - or - he knows we are going to EWI and thinks that will be fine.

 

While I'm on the topic, why would D&D have an issue when wet plaster might not? Wet plaster is in contact with the cold wall completely rather than in the dabs. It it that plasterboard is loved by mould spores and wet plaster material isn't? If so, why isn't plasterboard made of same stuff.

 

So many questions...

Appreciate any input. 

 

I don't think I've made any major mistakes on this project. This is a mistake but I hope a minor one that we can correct.

[Radian]

IMO D&D is an abomination. There. I said it. Been meaning to get that off my chest.

Now, if the boards were mounted on a continuous bed of low-expansion foam (continuous perimeter + infill) that would at least mitigate the cold bridging to an extent and would reduce the potential for infiltration from floor and ceiling but airtightness would still be compromised by the quality of blockwork jointing.

Had you gone for a fully sand cement plastered wall, you would be no better off with the mold which would take-up residence over a wider area starting in the corners and working out, until your EWI is installed at least.

[joe90]

3 minutes ago, Radian said:

IMO D&D is an abomination.

+1, I would not allow it in my build 👍

[Dave Jones]

we actually are in a similar position, solid 9" wall no insulation. Im going to put 100mm celotex internally then board on top. no dab using megastick with some thermally broken fixing to help the celtex stick.

 

its quite labour intensive having to cut 125mm around all the windows to get insulation to all the reveals. 

 

Only has to be done once though right.

[DeanAlan]

If we are agreed that in my case the planned EWI will be the major factor in keep the exterior sold brick walls warm (enough) then my primary concern is how do I get from here (no EWI for another 3 months, MVHR to be commissioned in similar time frame)?

 

Taking that pboard off and battening out or using insulated board will eat up space in the rooms - which I would like to avoid.

 

Options I see to get me through might include:

- Dehumidifier

- Direct warming (something like IR panels) of the offending walls internally

- Cranking up the UFH & making using summer fans to ensure good airflow across the offending walls

 

The aren't being skimmed yet - this gives me an opportunity to monitor the situation on the walls closely. I've wiped a couple of the patches down with mild bleach solution.

 

If I have to then I will remove them but want to know what to replace them with. Insulated board adds depth and (whilst more insulation is good) might be going above and beyond given the EWI is coming (which we have to do anyway to refurbish the outside to good quality). 

Moisture resistent board?

[Roger440]

8 hours ago, DeanAlan said:

@Roger440

Should we have wet plastered?

D&D insulated plasterboard (35mm)?

Battened out and then boarded to the batons?

 

I D&D because the builder said so. Now - he's a regular builder and didn't get the potential issue - or - he knows we are going to EWI and thinks that will be fine.

 

While I'm on the topic, why would D&D have an issue when wet plaster might not? Wet plaster is in contact with the cold wall completely rather than in the dabs. It it that plasterboard is loved by mould spores and wet plaster material isn't? If so, why isn't plasterboard made of same stuff.

 

So many questions...

Appreciate any input. 

 

I don't think I've made any major mistakes on this project. This is a mistake but I hope a minor one that we can correct.

 

What Radian said about D&D.

 

If you are doing EWI, then the problem will likely go away. In the meantime, a dessicant based dehumidifer is prob your best option.

 

Well, second best to removing it. How confident are you there will be absolutely no draughts or air movement beind the boards? If there is any at all, all youy EWI will be utterly pointless.

 

You mention lime mortar on the walls. Did you do this? Given its a solid wall, and therefore, im guessing no DPC, and assuming its downstairs, id plaster in lime. Not much point having lime motor if you are going to stick plasterboard over it! If its upstairs, just plaster it.

[Roger440]

4 hours ago, Dave Jones said:

we actually are in a similar position, solid 9" wall no insulation. Im going to put 100mm celotex internally then board on top. no dab using megastick with some thermally broken fixing to help the celtex stick.

 

its quite labour intensive having to cut 125mm around all the windows to get insulation to all the reveals. 

 

Only has to be done once though right.

 

Solid 9 inch wall suggests no DPC?

 

In which case, covering with impermeable celotex is asking for problems of the damp variety.

[Radian]

What I know about your issue is from my own experience. Your plasterboard is unfinished and mould just loves a damp paper surface. It's damp in local regions because those regions are cold bridged to the cold wall. This brings the surface temperature in those regions below the dew point.

I see three work arounds for now:

1. skim and paint the PB with a vinyl paint (washable). This will reduce the chance of mould getting a toehold.
2. dehumidify aiming for 50% RH
3. adjust the heating such that an IR thermometer pointed at the cold bridge area keeps the lowest temperature just above the dew-point established by feeding the ambient room temperature and RH into a dew point calculator such as this one

Say your ambient room temperature is 20^oC and the RH is 60%, then from that calculator you will see that the dew-point is 12^oC which may be close to the outside temperature bridging directly to your dabs. At RH 50%, this drops to 9.3^oC and condensation is much less likely to form but if you set the heating to 23^oC the DP goes back up to 12^oC and you're back in the danger zone again.

 

It's a bit of a dance between temperature and humidity, lower temperatures mean less humidity and a lower DP but you may not be comfortable.

[Dave Jones]

9 hours ago, Roger440 said:

 

Solid 9 inch wall suggests no DPC?

 

In which case, covering with impermeable celotex is asking for problems of the damp variety.

 

actually the opposite. DPC course is irrelevant.

 

 

[DeanAlan]

Thanks for all the feedback.

 

Solid brick wall from 1920's build - lime mortar. There is slate DPC. Outside also had roughcast (O M G) layer appplied decades ago.

 

I have filled any major gaps in bricks and then used blowerproof paint to make the wall as airtight as possible. I did want to passive purple spray but budget wouldn't allow.

 

@Radianthanks for the suggestion around monitoring the low temperature at the cold bridge and trying to warm to above dew point.

[mouldy2shoes]

I noted you stored the plasterboard outside albeit with a waterproof cover. When it rains, the relative humidity can easily hit 95% outside, even if it was lower than this, the cooling of the pile of plasterboard each evening would mean the RH within the paper on the plasterboard would be sufficient to start mould growth without it being readily visible. The moment the board was applied to the adhesive effectively provided sufficient moisture for the mould to prosper. In any case whilst you may have enjoyed mould free plasterboard for some considerable time, applying plasterboard to brick walls is a bad idea as no matter how dry a brick wall feels, bricks always hold moisture and one has to consider that behind the plasterboard one has provided a pocket of warm stale air. If you turned a refrigerator off even if empty and clean and left the door closed upon return months later you shall be met with a musty odour, so why should it be any different in the situation of dot and dab. Once you have visible black mould (likely the toxic starchybotrys chartarum as this loves paper) on your plasterboard it requires ripping out. Traditionally for several hundred years up until the 1950s houses were completely lime plastered, the lime is too alkaline whilst wet for any mould to prosper,  once dried it still isn't a very hospitable substrate for mould to thrive upon, I would suggest after ripping out the plasterboard, you either render the wall with lime cement or indeed lime plaster. Lime plaster will absorb moisture on humid days and release it on drier days, although many modern paints will prevent thus from happening and they themselves can allow mould to prosper underneath the paint surface. Consider either wallpaper or a limewash finish to that lime plastered wall.