Internal Wall Stain - Condensation?

heating12numpty12 · 4 Jan 2017

I would appreciate your thoughts. Please see attached photograph, sorry, but is not a finished build picture.

I have an extension with a central box gutter, on 2 occasions in the last week we have noticed a small (100mm dia) stain on the inside plasterboard (always the same place). This appears to only occur the next day following very cold nights of -5/6 centigrade.

The roof structure from memory is tile, batten, breathable membrane, roof structure filled with 150mm on Celotex insulation, plasterboard.

I do not think this is a leak as a result of rain, but condensation occurring in the roof space. Is that possible? What is the likely cause? How would it be resolved?

Many thanks

Pigeon85 · 4 Jan 2017

Probably a cold spot, where abouts in relation is it to the outside?

heating12numpty12 · 6 Jan 2017

Pigeon85 said:
Probably a cold spot, where abouts in relation is it to the outside?

Sorry for the delay in responding and thank you for the reply.

The damp patch is in just one space. The picture is of the inside and underneath the box gutter.

We have had -4 C for the last 3 nights with no damp, but today Friday, the weather has warmed up and the damp patch has appeared this morning.

Corazones · 6 Jan 2017

Pigeon85 said:
Probably a cold spot, where abouts in relation is it to the outside?

I agree, particularly if there's been no rain recently; perhaps there's a joint and slight gap in the Celotex at this point? If you want to be really sure it's not a leak, try running a hosepipe down the gulley for a few minutes.

heating12numpty12 · 6 Jan 2017

I think I agree it's condensation.

Is it possible for a small gap in the Celotex to produce enough water/moisture to stain a 150mm diameter damp patch?

How do I resolve the issue?

^woody^ · 6 Jan 2017

Put 50p in the meter, turn the lights on, and take a better picture. Thanks

heating12numpty12 · 6 Jan 2017

Hopefully this is a better close up picture for you. Currently around 150mm diameter.

^woody^ · 6 Jan 2017

That is indicative of a stain from penetrating water, not condensation or a cold spot.

heating12numpty12 · 6 Jan 2017

^woody^ said:
That is indicative of a stain from penetrating water, not condensation or a cold spot.

Thank you. As mentioned previously the area has been dry for the last 3 days/nights as we have had -4C overnight temperature (no rain). Frost has also remained on the roof for all of these days (north facing aspect). This morning (Friday) temperatures rose and around 11.00 a small 10 pence size wet patch appeared, within a few hours it was the size of 150mm dia. I tried to record accurately the events and prove what the problem is.

Does this not conclude a condensation problem?

^woody^ · 6 Jan 2017

heating12numpty12 said:
Does this not conclude a condensation problem?

Not necessarily.

With dampness, there is a hierarchy of probability, and the damp patterns tend to indicate that one cause is more probable than another. But it's not always clear-cut

That type of stain is more common from a single concentrated and sustained period of damp, which tends to be from penetrating damp. The appearance after a cold night could be from frost or ice thawing, which occurs on roofs when moisture is between layers of felt, or ice has slightly expanded laps or joints, and then it melts in a single spot and trickles downwards. Hosepipe tests don't always find this.

It is probable that condensation formed in the roof and then trickled down. But the appearance in the morning, rather than during the night or other times of the day (when there would still have been a large temperature difference to cause interstitial condensation) would make me suspect external thawing to be the cause, and penetrating damp.

But either way it need to be investigated as if it has happened once, it will happen again. A look externally would be the first step, by almeans try a hose pipe, and if nothing is obvious then open up the pitched roof section

Pigeon85 · 7 Jan 2017

^woody^ said:
That is indicative of a stain from penetrating water, not condensation or a cold spot.

Just adding that the paint work bubbling is a problem caused by moisture trying to escape.

heating12numpty12 · 10 Jan 2017

Hi

I have now lifted some tiles to see what's going on. On the underside of the breathable membrane I found droplets of moisture.

Appreciate your thoughts now that I have established the actual problem.

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Internal Wall Stain - Condensation?

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