Sealing new window opening to rendered exterior

[malcolm_macaulay]

Hello,

I’m looking for some advice on sealing a new window opening against a rendered exterior. I put the question here rather than the window section as I think it pertains more to building than a window in particular.

The wall construction is lightweight concrete block, then blue polystyrene (about 1 ½ inch thick), then a rendered exterior on an aluminium lath (I think that’s the right word - kind of an aluminium netting). The flat was build around 1989.

Here are a couple of pictures:

The exterior rendering is hard white ‘plaster’ looking stuff. It’s waterproof. I haven’t broken through the rendered cladding yet as I want to get as much of the window frame finished, then order the window, then break through and finish just before the window arrives.

I plan to line the opening with something like Hardibacker (cement based backing board – what you use for tiled shower enclosures etc), but am unsure how to seal the edge where this meets the outside cladding.

On the existing windows, it looks as though the edge has been covered with an aluminium angle, which is embedded into the exterior rendering. The window frame appears to overlap this aluminium to give a waterproof exterior.

Re-rendering the exterior isn’t possible (or desirable ), so I am wondering if an aluminium angle is what I need, and if so, how to bond and seal this to the rendered exterior?

Any pointers or advice greatly appreciated.

I posted this question earlier on the eBuild forum, but have not had any replies.

Kind regards

Malcolm Macaulay[/img]

 

[noseall]

can you post a picture of another window (outside view) to give some idea as to the exact fitting position of existing windows?

it is quite an unusual wall make-up you have there. there are clip type fittings that can be pre-screwed to the frame sides that allows the window to sit a lot further forward of the actual fixing point.

whatever you do, don't allow any internal materials span to the outside. the window frame should be the division between internal and external.

have a look at thermabate or similar for a fixing solution.

as for dealing with returning the render back to the window, then i'm stumped. you appear to have a specialist stucco rendering that will be difficult to match with conventional materials. i need to chew on this one.

 

[malcolm_macaulay]

Hi Noseall,

Thanks for your reply which am digesting now... I will get a photo of the exterior right now and post it.

One moment please.

cheers

Malcolm

 

[malcolm_macaulay]

Hi Noseall,

Thanks for the reply. Here are a few more pictures:

Above: A general picture of similar windows in other part of the building. The windows are almost flush with the exterior (looks minimalistic but you can't have the windows open at all in the rain).

Above: Exterior of existing window. The grey bit with specks of paint appears to be an aluminium angle which is embedded in the exterior render and is overlapped by the window frame. Adjacent to that is the bead of silicone which seals the window frame to the aluminium angle.

Above: Bit of a confusing photo. Looking at the top corner from outside. Notice that there is essentially no pelmet/ledge above the window. Same aluminium edge as in the previous photo.

Above: Edge of rendered wall where it meets an opening (enclosed 'smokers deck'). When I first saw this, because the edge is square and painted, I assume the building was clad with some type of 'fibro-lite' type cladding. Now after opening the window hole I understand it is rendered and the edges (windows and other openings) have aluminium edge beading.

Above: Added as an edit later. This shows the inside of the window.

it is quite an unusual wall make-up you have there. there are clip type fittings that can be pre-screwed to the frame sides that allows the window to sit a lot further forward of the actual fixing point.

OK, yes the window frame are very far out in the opening. There is 25 mm from the face of the exterior to the exterior of the frame and then the frame is about 55 mm wide (i.e. the inside line of the window frame is only 80 mm form the exterior face.

Do you mean the window is installed on these extended fixings, and then interior plastering is done? I assumed that window opening would be all finished before the window was installed. Perhaps not.

Looking at the wall I have cut, it would appear that the fixings on the window frame would have to be drilled hard against the inside edge of the frame, otherwise they would be going to into polystyrene only. And even at that they would be quite close to the edge of the lightweight concrete blocks. What you say about the extended brackets might be the case.

whatever you do, don't allow any internal materials span to the outside. the window frame should be the division between internal and external.

Ok, that kind of makes sense. Presumably this is for insulation purposes, so the heat is not conducted through from inside to outside and condensation/mold does not form on the inside?

I had a glazier around a few months back replacing some failed double glazing units and took the opportunity to ask him what state the window opening would need to be in when they came to install the window. He said he just needed a sealed opening and he would stick the window in. I took this to mean that the sheet material I used to line the window opening (I was going to use Hardibacker) would extend from the interior to the exterior and would seal the wall and insulation cavity. From what you say I think he was probably just giving me his perspective without really understanding how this is to be achieved.

have a look at thermabate or similar for a fixing solution.

I had a quick look and will study further. This seems to tie in with the above - i.e. the polystyrene must be bridged by an insulating and sealing barrier.

as for dealing with returning the render back to the window, then i'm stumped. you appear to have a specialist stucco rendering that will be difficult to match with conventional materials. i need to chew on this one.

I was thinking I could put aluminium angle with covered the exterior edge of the opening and then somehow bond this to the rendered exterior, perhaps rebating into the exterior render. Naive?

OK, thanks for your help so far and hopefully the above gives you more information.

Kind regards

Malcolm Macaulay

 

[noseall]

the plot thickens

have any of your neighbours tried to retro-fit a window into this rendered facade? did you, or indeed do you need permission for this type of work?

are those windows anodized aluminium?

do you intend matching the window style?

the problem is, the majority of the facade in the picture seems to be a specialist, bespoke, architectural build and not something you can deal with from b&q. the trims used around the window openings will be specialist jobbies that are designed to be rendered-in, which will make retro-fitting all the more difficult.

have you thought about consulting the original builders?

how have you supported the masonry above the window?

 

[malcolm_macaulay]

Hi Noseall,

Thanks again for your quick reply.

have any of your neighbours tried to retro-fit a window into this rendered facade? did you, or indeed do you need permission for this type of work?

As far as I can see no new windows have ever been added since the whole place was built circa 1989. I did get planning permission from both the council (Tower Hamlets) and the building freehold company.

are those windows anodized aluminium?

I'm not sure about anodized. They are aluminium and look factory painted, but which may be what you call anodized. I think of anodized as colour 'cooked' into the aluminium, which is not what this looks like - it chips off.

do you intend matching the window style?

As close as possible. I got a quote from a glazing company and they we confident they could get a similar window. Fortunately the new window is on a face where there are no other windows with which to compare and no one who sees that face can see the other windows (at least close up).

the problem is, the majority of the facade in the picture seems to be a specialist, bespoke, architectural build and not something you can deal with from b&q. the trims used around the window openings will be specialist jobbies that are designed to be rendered-in, which will make retro-fitting all the more difficult.

I see what you mean. I avoid B&Q like the plaque as my life is too short to be so frustrated . When you say "trims used around the window openings" do you mean that the aluminium I see is a single piece made (welded?) in the shape and size of the window opening, then embedded into the render?

have you thought about consulting the original builders?

I had not. I understand they went bust, but if I need to I can defer to professionals. I am quite resourceful and (I think) competent but I am not familiar with these building techniques. I have just read the installation guide for the Thermabate as you suggest and that seems to make sense to me.

Now that I understand the need for Thermabate, I think the only problem is how to seal the rendered exterior back so the window frame can overlap.

I had thought that I could cut the opening in the existing rendering, then rebate a little on the exterior, then bond my new aluminium angle edging in. With what I would bond I do not know.

how have you supported the masonry above the window?

Oh yes. I know that much at least The building is reinforced concrete (you can see in the pictures if look very closely), so there is a concrete lintel which run the length of the building. You can actually see in the pics where i encountered the concrete pillar on the left side of the opening. The lightweight concrete blocks are not structural and in fact there was only a little mortar between them and the lintel . I would not have attempted this if a lintel was required.

I appreciate you help with this. If it would be easier for you I can call you.

Kind regards

Malcolm Macaulay

 

[noseall]

at least i know the source of the trims, beads and metal lathing.http://amico-lath.com/lath/paper_back_metal_lath.htm

 

[malcolm_macaulay]

at least i know the source of the trims, beads and metal lathing.http://amico-lath.com/lath/paper_back_metal_lath.htm

Thanks for that link - there are good pictures which detail how it is done. My builders merchant seems to have a nice range of this trim, lath etc.

Now that you have pointed out I need the Thermabate, I think the only outstanding knowledge is how/what compound to use to bond/fill the cut edge of the existing render to the aluminium trim.

Thanks again for your help.

Kind regards

Malcolm Macaulay