Weather Seal at Check Reveal

[Mr Blobby]

We have check reveals on our block cavity build with a layer of DPC at the reveal.

Like this:

 

 

 

We will be installing the windows up against the check reveal of course.  So up against the DPC.

And then rendering outside to the window.

We will be foaming between the edge of the window and the PIR closing the reval cavity.

 

I am told the DPC will provide weather proofing and compriband is not needed at the front edge of the window. 

 

Is that correct, compriband not need  in this type of masonry check reveal install?

Should I get some decent foam like FM330 to seal and insulate the side of the windows?

 

(we will of course be taping the windows internally for the airtightness)

[joe90]

I used expanding foam tape between window and brickwork with check reveals. 

Edited March 22 by joe90

[Redbeard]

The dpc might provide a degree of weather sealing depending on whether it sits smoothly or gets trapped and twisted when the windows go in. It certainly won't do anything much for air-tightness. Compriband and/or FM330 or similar, and a/t tape.

[joe90]

2 minutes ago, Redbeard said:

It certainly won't do anything much for air-tightness

Surely the air tightness layer is from the inside of the window into the reveal etc.

26 minutes ago, Mr Blobby said:

we will of course be taping the windows internally for the airtightness)

👍

[Redbeard]

Fair point @joe90. I always try to go for excellent air-tightness inside *and* out - 2 bites of the cherry. When I was doing IWI NBT used to say the a/t layer was the final plaster layer. OK, but you deliberately sit the wood-fibre on a parge coat designed to make the wall air-tight, so why move the a/t layer at jcts? Hence I do both.

 

(You have highlighted that I did not read the OP carefully enough, though. ;-(

 

Re the original Q, with dpc only I'd worry about capillary action if you were relying on it for weather-tightness.

[joe90]

6 minutes ago, Redbeard said:

I always try to go for excellent air-tightness inside *and* out

Mm, if there is two layers are we not risking trapping any possible moisture? There is talk about “breathability” so I wonder if not having a second external airtight layer is better (tin hat on) 🤞

[Redbeard]

Point tooken, to a degree, but water vapour permeability is often talked of as  'breathability', so the lay-up I described has 2 a/t layers and a 'breathable' construction (Brick/lime/WF/lime). Unless there is an inherent damp issue then in this example I don't feel I'd be risking trapping any moisture (but also I acknowledge myriad ways of cat-skinning!).

[Dave Jones]

wheres the cavity closer ?

[Mr Blobby]

20 hours ago, joe90 said:

I used expanding foam tape between window and brickwork with check reveals. 

 

And I thought about doing the same, to put a strip of compriband on the front of the window frame against the check. 

I did suggest this but was poo-pooed by builder and architect.  An English thing they said.  No compriband here in Northern Ireland.

 

If I were to put a strip of compriband on the front then I assume this would mean removing the DPC, otherwise there would be a crap seal.  My builder would freak out at the suggestion of no dpc at the reval.  As he says, the DPC makes everything waterproof!

 

It seems common practice here to render to the window frame so mayb that's why no other weather sealing is done.

But then I think the render will crack at the window frame and not be very waterproof any more.

 

6 hours ago, Dave Jones said:

wheres the cavity closer ?

 

AFAIK a block return and PIR closing the cavity is what is always done here in Northern Ireland.

At the top of the window from the window frame to the inner leaf we plan to install a peice of supalux board.

Is this a bad idea?

 

20 hours ago, Redbeard said:

The dpc might provide a degree of weather sealing depending on whether it sits smoothly or gets trapped and twisted when the windows go in. It certainly won't do anything much for air-tightness. Compriband and/or FM330 or similar, and a/t tape.

 

We'll be foaming round the side between the window frame and the block return and PIR in the reveals but I guess this should be kept dry hence my concern about the weather prrofing at the front.

Edited March 23 by Mr Blobby
contriband?

[Iceverge]

23 hours ago, Mr Blobby said:

am told the DPC will provide weather proofing and compriband is not needed at the front edge of the window. 

 

DPC.......I hate them. I cut our vertical ones out and threw them away. With UPVC windows nobody could explain what they were achieving. I kept the one at the window head. 

 

2 hours ago, Mr Blobby said:

did suggest this but was poo-pooed by builder and architect

 

.....Sighs.... compriband is the solution here.

 

The windows and wall are dissimilar materials and it's worthwhile having a material that will remain flexible to accommodate differential movement. Otherwise you'll soon have a crack where wind and rain can get in at the outside of the windows. Mastic should only be a final mainly aesthetic covering as it will fail at some stage. 

 

22 hours ago, joe90 said:

Mm, if there is two layers are we not risking trapping any possible moisture? There is talk about “breathability” so I wonder if not having a second external airtight layer is better (tin hat on) 🤞

 

It's very important to have a good "windtight" layer as well as an "airtight" layer in all construction. Otherwise your insulation becomes like a wooly jumper on a windy day. Useless! Put a light anorak over the top and seal all penetrations. In this case the windtight layer is the external render. 

 

"Breathability" is a bit nebulous. Everything is breathable to a greater or lesser extent. Vapour permeability is a better term in my opinion. Concrete blocks and cement render are vapour permeable to some extent. Otherwise they'd never dry out. The important thing is that they can dry faster than they get wet.

 

With a good internal airtight layer the amount of wetting will be absolutely minimal and there will be no moisture accumulation in the cavity. 

 

On our house I sealed all penetrations, windows and doors to the external render with airtight paint to this end. 

[joe90]

17 minutes ago, Iceverge said:

The important thing is that they can dry faster than they get wet.

👍

[Mr Blobby]

4 hours ago, Iceverge said:

compriband is the solution here.

 

 

Even with a check reveal (compriband on front face of window frame) and external render to the window frame compriband is still the solution?

 

Would compriband push the window inward slightly?

 

We would render over the compriband to the frame?  Or stop bead near to frame I guess.

Edited March 23 by Mr Blobby

[Iceverge]

Put the compriband on and put the window in place. Yes it will push it a few mm inwards. 

 

Then stick one of these to the window frame externally and render to it. 

 

 

You'll get very tidy and equal reveal lines. 

[craig]

Compriband is an airtight/watertight seal.
 

Putting it on the face of the window (alu clad), immediately takes out the airtight aspect (90% of the time, as it is system dependent).

 

Compriband should be on the frame, against the reveal (it is normally on side of frame but can be on the front of frame against the check reveal).

 

Externally, I would always recommend the use of a render stop bead. 

Edited March 24 by craig

[craig]

On 23/03/2024 at 16:38, Iceverge said:

With UPVC windows nobody could explain what they were achieving. I kept the one at the window head. 

Nothing, exact same as aluminium.

[Mike]

6 hours ago, craig said:

Compriband is an airtight/watertight seal

It's good to 600pa, if correctly installed, which is roughly equivalent to a steady wind speed of 70mph. Consequently you may want some kind tray behind it if any water leakage could reach any moisture-sensitive materials, plus a secondary air seal (e.g. FM330 foam / tape).

 

 

[craig]

It’s also a good sound insulator. 

[Iceverge]

13 minutes ago, craig said:

It’s also a good sound insulator. 

 

Our window man later told me they did this for a client once. Had I known I would have jumped at the chance to take him up on it.