external insulation

[billybuntus]

I'm sorry before I start. I'm sure its been covered a hundred times.

The rear of our house is stepped. The ground floor is stone and at some point a single skin (brick) 1st floor has been plonked on the top (all rendered).

We have a permanent damp ground floor wall where I believe all the water from the first floor hits the sloped step and drives into the stone (even though its rendered). I'll not be taking off the render and leaving it bare to breath as the brickwork and stone'work' is awful.

The difference is probably around 60mm

I'm looking at some stage in the future to build out the 1st floor by insulating then over boarding (if needed?) the whole rear of the house and rendering.

I understand the concept of the insulation boards (either green wood variety or kingspan) but how do I fix them to the building. Do I need to provide ventilation etc? I'm sure I'll be greeted with hate email but it will be a concrete based render (probably webber monocouche to match the gable).

The roof above is flat so perfect to build out from and I only have one gable (the other side will meet a parapet wall).

Any advice will be gratefully received.

Thanks

 

[robgil]

Nothing wrong with cement render if the building doesnt object. :wink: I wish I was lucky enough to be able to use it on my cottage but it would do more damage than good.
If I were you I would sort the damp issues out first, then think about insulating.

 

[billybuntus]

If I were you I would sort the damp issues out first, then think about insulating.

That's the thing. I think the insulation and render will sort out the penetrating damp Hence my need for further information on the subject.

 

[middi]

Fixing kingspan or celotex boarding to the exterior of a building is not difficult, you just lay the boards directly on to building face , with the aid of suitable pressure treated battens ( say, 50 x 25 mm) screw onto the wall whilst trapping the insulation boards tight against the face of the building. Use aluminium tape to seal edges of ins.boards.
Stretch breatheable felt tight across the battens ( hold this in position with staples) then use eml ( expanded metal lath) nailed or screwed onto battens for rendering onto. You can use whatever type of render you like on the building you describe, makes not much difference.
The only air gap needed is the one between the external face of the insulation board and the breatheable membrane, This ( 25mm) gap is provided by the 25mm thickness of the batten. This gap must be there otherwise the battens will rot prematurely, the breatheable paper is mainly there to stop the render going right through the eml when rendering ( this is why it needs to be stretched tight).
If you have a stepped ledge on the same elevation you need to use a water shedding arrangement ( ie. leaded step) to guide water away from the lower protruding rendered elevation.
also make sure that the water running down the walls does not gain entry at dpc ( damp proof course) level at the base of the walls
( being stone, if old, there may not be any dpc in place, in which case carry the render as low as you can down the wall an keep you fingers crossed that ground water does not capillary up the wall from the ground and into the wall, still, if it does then it is a project for another time

Middi

 

[billybuntus]

Fixing kingspan or celotex boarding to the exterior of a building is not difficult, you just lay the boards directly on to building face , with the aid of suitable pressure treated battens ( say, 50 x 25 mm) screw onto the wall whilst trapping the insulation boards tight against the face of the building. Use aluminium tape to seal edges of ins.boards.
Stretch breatheable felt tight across the battens ( hold this in position with staples) then use eml ( expanded metal lath) nailed or screwed onto battens for rendering onto. You can use whatever type of render you like on the building you describe, makes not much difference.
The only air gap needed is the one between the external face of the insulation board and the breatheable membrane, This ( 25mm) gap is provided by the 25mm thickness of the batten. This gap must be there otherwise the battens will rot prematurely, the breatheable paper is mainly there to stop the render going right through the eml when rendering ( this is why it needs to be stretched tight).
If you have a stepped ledge on the same elevation you need to use a water shedding arrangement ( ie. leaded step) to guide water away from the lower protruding rendered elevation.
also make sure that the water running down the walls does not gain entry at dpc ( damp proof course) level at the base of the walls
( being stone, if old, there may not be any dpc in place, in which case carry the render as low as you can down the wall an keep you fingers crossed that ground water does not capillary up the wall from the ground and into the wall, still, if it does then it is a project for another time

Middi

Thank you

That's exactly the information I was after. Does the gap need to be vented or is it literally just to keep the timber from coming into contact with the render?

 

[middi]

No, there is no need for vents. If you use lime render it would be much better as the breatheability of the treated battens will be more efiicient and therefore last a lot longer.

Middi

 

[billybuntus]

Sounds promising. Do the modern renderers tend to do a standard lime finish i.e. power floatable?

 

[middi]

Any " standard" plasterer should be be able to apply " lime rich " renders, power float or otherwise.

If they sound confused, they are the incorrect people for the job.

Middi

 

[billybuntus]

Thank you. Your knowledge is very much appreciated.

 

[Zebra]

I believe all the water from the first floor hits the sloped step and drives into the stone (even though its rendered).

Just a thought, might it be worth testing this theory before you spend lots of time and money doing the insulation, and then have to try to do further remedial works once the insulation is in place, therefore having to work round it? The question of "where is all this dampness coming from" is sometimes not easy to pin down (as I know to my cost!) There may be more than one cause, like a combination of penetrating and rising damp for example, and sometimes you think you've nailed it, only to find you still have a problem.
If you were to take off the render, never mind if it looks awful for the time being, and temporarily fix a sort of flashing into the lower courses of the single skin upper storey brickwork, which deflects water out over the stone wall and not into it. You could even make this out of a length of dpc, you wouldn't have to go to the expense of lead, it's just an experiment. You'd have to leave it a while and see if the lower storey dries out. If it does, that was your cause. If it doesn't, perhaps you have another problem to address, perhaps the lack of a dpc in the lower storey. Don't know if this is helpful at all, just an amateur idea....

 

[billybuntus]

I like the theory. I'm a little way off this work (probably early next year) but it would certainly be worth a trial/ I'll report back on the results.

 

[Zebra]

Oops. Deleting this post as posted it twice..