Damp spot in fresh plaster - update

NoisyGiraffe

Update

We dried out the plaster, and the dampness is coming back through again. 

It hasn't rained here all week but has been very cold and frosty.

I'm still thinking that the cavity above the window is blocked, I want to investigate this and clear it if necessary, but I want to avoid having to knock off this plaster and redo part of the room. Is this possible? Could it be done from the room above instead? 

stuart45

Are you sure you have a cavity wall on top of the bay window?

NoisyGiraffe

stuart45 said:

Are you sure you have a cavity wall on top of the bay window?

Hadn't even considered this, I just assumed! Underneath the bay has a cavity (that was full of debris and got cleared 2 years ago because it was causing damp) i can't see an obvious difference in the thickness of the wall below and above the window. Outside is brick between the top and bottoms bays. Is there anything I haven't thought of? Photos of inside and outside... 

stuart45

Looking at the exterior it probably is a cavity wall. Didn't see all the masonry holding the lintels up from the interior pics.
Looks like it's had CWI, so that you make cleaning it out a bit harder.
Have you had a look at the top of the bay to see if any moisture can run down the brickwork between the frames and into the wall
from the cornice.

NoisyGiraffe

stuart45 said:

Looking at the exterior it probably is a cavity wall. Didn't see all the masonry holding the lintels up from the interior pics.
Looks like it's had CWI, so that you make cleaning it out a bit harder.
Have you had a look at the top of the bay to see if any moisture can run down the brickwork between the frames and into the wall
from the cornice.

Yes there's the wool type CWI, put in before we bought it. 

By cornice do you mean externally/the fancy bits at the top of the sandstone? We refurbished the sandstone bays upstairs and downstairs this summer so everything has been filled before priming and painting including building up the bottoms of the cills and making sure there's no gaps anywhere (we know this is temporary and some replacements will be needed in the future). All the windows were re-siliconed at the same time. We've been told the roof of the bay is relatively new and in good condition, but I haven't seen it myself so I can't say much more. Gutters and fascias were all done in spring too. 

stuart45

Yes, that's the cornice. How were you thinking of cleaning the cavity?

NoisyGiraffe

stuart45 said:

Yes, that's the cornice. How were you thinking of cleaning the cavity?

Thanks, I wasn't sure if I'd gotten confused! 

I'm not completely sure! Last time we had somebody do it for us, but he's since retired. My understanding is to remove a couple of bricks and scoop out whatever is in there, put the bricks back and move on to the next spot. Last time it was done from underneath the living room floorboards. I'm wondering if this time we could knock off the plaster from the upstairs bay and reach down to check/clear whatever is in the cavity that way. I'm not sure if it's feasible? Or whether we should just be removing the plaster from the problem area itself and removing bricks there? Took us ages to get our plasterer round in the first place so I'd rather not have to wait to have him back to redo a section, if at all possible! 

stuart45

Just above the windows will be the lintels. You could lift the floorboards upstairs and take out the bricks between the joists.
Not sure how easy this will all be as the cavity may only be 2 inches, and if full of insulation you won't see much.

NoisyGiraffe

stuart45 said:

Just above the windows will be the lintels. You could lift the floorboards upstairs and take out the bricks between the joists.
Not sure how easy this will all be as the cavity may only be 2 inches, and if full of insulation you won't see much.

Ahh. I hadn't thought about how narrow the cavity could be, I don't even know what could be used to do that.

Looks like I need to find someone who can do this for me, and prepare to replaster afterwards! 

Thanks for your help! 

stuart45

If water is entering the cavity and running down to the lintel, it will need an escape route through weep holes. Cleaning out any debris on it's own may not be enough.