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Is there hope for this window opening?


WWilts

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Trying to find detail drawing of a window reveal. To get it as airtight and thermally sound as possible (on a budget).

 

Not encouraged by parts of the brickwork around the opening. Builder says that's the inside, there will be an insulated closer and nobody will see the broken bits. Trying to understand how a reveal works for deep set windows, so that I can talk sense in our conversations. On what does a deep set window sit?

Skins from out to in:
100mm brick
110mm cavity (bonded EPS beads insulation)
100mm aircrete

 

 

PS for context: the external face of the brickwork looks good generally. Concave tooled joints. This bit was done by the labourer practising his skills.

 

Window reveal.jpg

passiv haus window detail 5 cavity wall.jpg

Edited by WWilts
added pic with question
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That’s the Denby dale detail from green building store. 300mm cavity.

 

They made boxes for the windows out of marine ply. I think some delaminated so they refined the detail by treating the edges with something for future designs. 

 

I made and used 18mm OSB ones after the windows had been installed, 250mm cavity. 

 

I suggest you don’t need them, however you will need something to rest the bottom of the windows on. Are you using precast sills?

 

When the window installers are done you can tape the windows to the interior wall and plaster over.

https://www.siga.swiss/ie_en/products/fentrim/fentrim-20 or similar.

 

Then screw a J bead to the frame and install plasterboard as your reveals. The EPS beads will push the tape to the plasterboard giving you fully insulated reveals. 

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17 minutes ago, Iceverge said:

you will need something to rest the bottom of the windows on. Are you using precast sills?

Just the brick, block and anything the builder might use as cavity closer.

Would like to try and get the windows overlapping the 110mm insulation layer as much as possible. But can't see how to support the window if that is the case.

Tks for links, will have a good look.

Edited by WWilts
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You can’t close the cavity with brick or block or you create a cold bridge. Also, blown bead needs a properly sealed closer in place to stop the beads coming out at the window frames - it’s easier to use a plastic/ foam closer over the 3 sides and then fit the window and the internal cill to that. 

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10 hours ago, WWilts said:

Just the brick, block and anything the builder might use as cavity closer.

Would like to try and get the windows overlapping the 110mm insulation layer as much as possible. But can't see how to support the window if that is the case.

Tks for links, will have a good look.

 

If you don’t support the bottom of the window it will sag over time and your windows won’t operate well.

 

We used precast sills built into the external skin with 20mm pertruding into the to cavity for the windows to rest on. I have seen and heard of many methods but letting it float over the cavity held up by the straps won’t be a great long term idea. 

 

By the way the middle of the insulation layer is the best place thermally however as long as the windows are somewhere in the insulation layer you’ll be ok.

 

have you decided on what type of windows you are going to use?

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If I was you I’d insist in it being neater or it finish a-least past the edge of internal brick edge we had to grind a few to fit them in, if it is not neat and flush you could have issues with cavity closer sealing or even fitting in it it protrudes to much into cavity.

Regards. James

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38 minutes ago, James94 said:

cavity closer sealing

Trying to understand how cavities are closed. So that I can talk sense in conversations with builder.

The "plan view" pic I drew to show what the builder described. Not sure if that is realistic, will look ok (concealing the brick reveal), or even if that is really what he described.

Will the bonded EPS beads suffice to close the cavity?

 

 

Insulated window reveal jamb.jpg

Cavity closer.png

Edited by WWilts
added question
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6 hours ago, PeterW said:

You can’t close the cavity with brick or block or you create a cold bridge. Also, blown bead needs a properly sealed closer in place to stop the beads coming out at the window frames - it’s easier to use a plastic/ foam closer over the 3 sides and then fit the window and the internal cill to that. 

 

cavity closers are plastic with insulation stuck on them.

 

Much better to use subframes from the outset so the brickies can build them square.

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18 minutes ago, Dave Jones said:

 

cavity closers are plastic with insulation stuck on them.

 

Much better to use subframes from the outset so the brickies can build them square.


Yep but you can’t blow bead in then as the subs will stick. Closer meets the building regs requirements for insulation and DPC

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48 minutes ago, Iceverge said:

Have you read the blog post I put up? It's quite good. 

 

Here is a sketch of the detail. Cheap, easy to build, airtight, thermally sound. 

Great, tks. Did read the blog post but a lot went above my head. The pics assumed knowledge that I don't have.

Your drawing makes things clear for me.

Before I saw it I was sketching my own understanding. The builder will have a go at whatever drawing I give him, within reason. Don't know if my drawing makes sense or is buildable.

Window reveal v 1.0.jpg

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That marine ply won't hold up a large window. The Denby Dale one was able to because they made a box. The sides supported the bottom. 

 

I think you're overthinking this. With a 110mm cavity you're short of bregs not to mind PH. With low insulation levels thermal bridging isn't as much of an issue.  I would pursue whatever kind of window installation the builder knows and is comfortable with and will definitely keep out water. 

 

The only low hanging fruit on your build is airtightness. Concentrate on this and your house will be warm. 

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43 minutes ago, Iceverge said:

I would pursue whatever kind of window installation the builder knows and is comfortable with and will definitely keep out water. 

Makes sense. Can push on things that will really make a difference. Besides, seems wise to have people doing their best rather than sulking because they were pushed on iffy things.

 

Incidentally, can get to wall U value under 0.23 with the 110mm cavity blown EPS beads, aircrete blocks (even 7N), plasterboard on dabs & brick outer. 

There is however one 2.6m wide window that is important for heat loss. That's where the broken brick was (original pic). Will see if there are obvious opportunities for improvement in the builder's methods, perhaps a little more attention to airtight taping etc. esp around that window. The broken brick was already flagged as a concern.

I'd already sketched the attached modification of my original drawing. Both are now an academic exercise. But all this has certainly helped me understand what a window reveal actually is and how it can work if done well. Might gain a little airtightness and a tad less thermal bridging as a result. 

 

Window reveal v 2.0.jpg

Edited by WWilts
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