Surface Condensation underneath sash window sill

[Ginga]

Hi,

Hope someone can shed a little light for me.

We have had wooden sash windows installed. In one particular bedroom ( whose extrernal wall is south facing and hence bears the brunt of the "weather" ) I have yet to make good the internal wall in the bedroom and between the underside of of the sash window and internal brickwork is a gap of approx 8mm, as follows:

Untitled

• Ginga
• 11 Apr 2010
• 1

When I've looked into this gap, on the underside of the sash window, I've noticed water droplets have appeared. At first I thought the external mastic seal had failed, letting in rain, but I now know that the water droplets are due to condensation. I've proved this by inserting a cut up plastic folder into the gap, left it overnight ( when it didn't rain ) and sure enough the brick facing side of the plastic is covered with water droplets. I can only presume that the underside of the window is cooling quicker than the bricks ( which heat up throughout the day), and then condensation forms overninght.

Question is, what should I do about this? Fill the gap with mortar? put a DPC on the brick and then mortar?

The wall is no a cavity wall ( Victorian House ) and the whole wall will be taken back to brick and re-plastered soon.

Any help appreciated!

 

[JohnD]

...underneath sash window sill...

...the brick facing side of the plastic is covered with water droplets...

This is a leak, not condensation. Condensation would appear on the room side of the plastic, not the wall side. You could try sticking a broad strip of sellotape over the gap to see what happens, though. If it does turn out to be caused by warm moist air hitting the cold frame, you could use expanding foam to fill the gap with an insulating material.

Also the position under the sill is characteristic of water penetration. It may be trickling down the frame so the point of entry might not be where the wet patch is.

 

[Ginga]

Hi John,

Thanks for the response .

From your reply, I don't think I've explained myself properly

The plastic sheet I insetred wasn't covering the gap. It went in horizontally and was sandwiched between the brick and the wooden underside of the window ( so the plastic was laying on top of the bricks.).

It was the brick side that had the condensation on, not the window side, which I thought indicated that hot air was rising from the bricks , hitting the colder plastic surface and then condensing?

It didn't rain last night which kind of confirmed my suspicion.

Does this sound feasible?

 

[JohnD]

can't tell. the water will have dripped onto the bricks and made them wet. you will have to shield the bricks from the drips for long enough for them to dry out.

wooden window frames are not going to be as cold as the glass. Does the glass get a lot of condensation? It is very unusual for a wooden frame to get condensation.

New brickwork is always damp, from the wet mortar and also from bricks being left in the rain by the builder. Is your brickwork old or new?

 

[Ginga]

No condensation on glass, brickwork is as old as the house ( 105 years ). No new work has been carried out other than the fitting of the windows.

I can also see down the side of the window ( fitters have left me to fill the gaps with foam and put architrave round) and can't see any evidence of drips running downwards and then underneath the sill.

Its a puzzler. Can't help thinking that because this back wall gets a lot of sun, the bricks warm up and let the heat out slowly through the night

Maybe I'll leave the plastic resting on the bricks for a period of time ( hopefully it'll rain! ) and then if I never see droplets appear on the window underside then I guess that would prove it is condensation.

If it is john, would you recokmmend just filling with mortar or putting some DPC in between?

 

[JohnD]

No condensation on glass,

then it isn't condensation, as the glass will be much colder than the frame.

but you can try putting a dpc under and around the window frame to separate it from the brickwork and watch what happens

I still think water penetration. if there is a delay in its appearing, there might be some cavity holding the water. Might be at the head or the lintel perhaps.

presumably there is no overflow pipe or dripping gutter over the window?

 

[Ginga]

No, no overflow pipe or anything.

The external wall is rendered.

Think thats what I'll do, put a DPC around the sill and frame and see what happens. I originally had the same thought about the glass and why doesn't that have condensation on it? I thought that as the glass surface does not sit directly above the bricks ( and is not in a parallel plane, like the sill) then the glass surface would not come into contact with the warm air rising off the bricks.

I'll let you know the outcome John. Thanks for you input its great to bounce the problem off someone and get there input.
apprciate it