Cold bridging at cavity reveals and floor slab detail

[ruggers]

After a review of my plans and discussion with the person completing them, I wanted to check a couple of areas that I think will cause cold bridging.

The construction is cavity wall, 100mm dense block inner leaf, 150mm partial fill cavity with 100mm PIR, and a mixture of face brick outer leaf and dense block with composite cladding screening. Weather zone is severe 4 for NW England so blown bead isn't an option. House will be parged, internal insulation and MVHR.

 

1. The architect has detailed for a a returning block on any opening reveals with a DPC separating the outer from the inner. Would this not be a huge cold bridging area, if so how do i get around it and where would the UPVC windows be positioned for a solid fixing? What are my best options to minimise cold bridging but to not over complicate the window fixing. I've seen someone suggest only a partial 75mm block/brick return of the 150mm cavity then use a push 75mm closer for the remaining part. This adds a thermal break and means the return blocks work out for 175 block & a standard brick above course. or just go full 150mm closer?

 

2. How do you extend the cavity wall insulation down to the bottom of the floor slab level when using a radon barrier which bridges the cavity. See image detail.

[jayc89]

XPS below the cavity tray, as you where above it

 

[tonyshouse]

I think radon goes through polythene, even think poly and so you need vents and radon sump under the floor to catch and expel it. 
 

I hate closing cavities with blockwork and that is a no no

proprietary cavity closers are expensive and unfit for purpose, I used sheet insulation. Aim for zero bridging and air tight. Don’t do dot and dab. 

 

I like fitting windows behind the masonry, ie in the cavity. See my site for pics and details. 

[tonyshouse]

Carry floor concrete to just under door frames, silly to stop it on top of inner skin

[ruggers]

On 04/04/2022 at 17:07, jayc89 said:

XPS below the cavity tray, as you where above it

 

Does it not need to be a cement & sandy gravel mix to infill below ground level for compressive strength? 

 

On 04/04/2022 at 18:59, tonyshouse said:

I think radon goes through polythene, even think poly and so you need vents and radon sump under the floor to catch and expel it. 
 

I hate closing cavities with blockwork and that is a no no

proprietary cavity closers are expensive and unfit for purpose, I used sheet insulation. Aim for zero bridging and air tight. Don’t do dot and dab. 

 

I like fitting windows behind the masonry, ie in the cavity. See my site for pics and details. 

The radon barrier is a red membrane but its still 1200 gauge, I can't see how it offers any benefit over a standard DPM other than satisfying building control. I've made some enquiries & you can test a built house for radon gas but not a plot of land. The radon charts for areas are based on stats not actual houses having it unless someones had a test done when built. So it seems you either hope for the best or add sumps and vent pipes with fans at cost. Beam & block would be easier to vent, can't see how you can stop it with a slab.

 

Cavity closing - I get that the closers can be over priced but why do you say ineffective? I was thinking a partial return of block would give a better fixing to the window if its fixed using the metal brackets that face back inwards, i presume you used these if your window sits back over the cavity? Did you run a vertical strip of DPC between your sheet closer & the inside of the outer leaf?

[tonyshouse]

Cavity closers cause cracks or if blobbed and dobbed over draughts , I didn’t do dpc at my reveals as I couldn’t see the difference between the reveal and the face of the wall, bricks can get wet .  I fixed my windows to the outside skin with the weight on steel pegs. 

[SuperPav]

I don't like the cavity closers as they're difficult to fix to. Also, in the past I've had just as much success of cutting and wedging a piece of 50mm Celotex between the leaves and foaming it to create a much more sealed cavity closer. You still can't fix to it so need to take the fixings back to the inner skin via straps. I've also found you need to use insulated plasterboard on the reveals, as it's more rigid (and you can only fix to the inner leaf really).

 

 

Below is what we did on the current build (essentially built a plywood box BEHIND the outer leaf with a 10mm gap).  Unfortunately this only works easily if you aren't tied to matching course heights between the outer and inner leaf (in our case the outer is cropped stone, inner is blockwork).

 

Our cavity is fully foam filled, but I don't see why this sort of setup wouldn't work with other insulation.

 

[Brickie]

As an aside-you are putting an admirable amount of thought & effort into detailing. With that in mind,I’d suggest ditching the PIR cavity insulation. Search the site here for others’ experiences. It’s too complex to summarise in a sentence why it’s such an impractical solution for masonry construction,but if pushed for a one liner I’d say the real world performance is NEVER anything like what it says on paper. 

[Iceverge]

 

 

+1 to avoiding cavity closers. 

OSB window boxs here. 

 

Easy to tape to the windows and fix plasterboard and window sills to. 

 

Very robust during the building process. EPS bonded beads filled all the areas behind 100%. 

[tonyshouse]

What happens to the walls? 

[Iceverge]

 

Sorry this didn't post for some reason. 

 

Wet plaster and skim over the airtight paint.  

[ruggers]

On 07/04/2022 at 09:44, tonyshouse said:

Cavity closers cause cracks or if blobbed and dobbed over draughts , I didn’t do dpc at my reveals as I couldn’t see the difference between the reveal and the face of the wall, bricks can get wet .  I fixed my windows to the outside skin with the weight on steel pegs. 

@tonyshouse I don't really understand what you mean by couldn't see the difference between the reveal and face of the wall? What were the steel pegs attached to or resting on, do they bridge the cavity? Reason i asked about vertical DPC down inside of outer leaf, was same detail as returning blocks but to prevent the wet outer leaf brick soaking into the PIR which it doesn't like.

 

 

On 07/04/2022 at 17:37, Iceverge said:

+1 to avoiding cavity closers. 

OSB window boxs here. 

 

Easy to tape to the windows and fix plasterboard and window sills to. 

 

Very robust during the building process. EPS bonded beads filled all the areas behind 100%. 

@IcevergeThanks for the image, It looks a good method as you say for taping the reveal to window frame, I presume you had to close this way when using blown bead. Were your inner & outer reveals flush or did you have to set the inside leaf back by the thickness of the OSB? Where does your window sit and fix to? I want to avoid fixing into the outer brick as they can be brittle. With your method i'm thinking the window straps would fix over the ply not behind so the finished reveals don't need ripped off to change window frames in the future.

[ruggers]

On 07/04/2022 at 16:08, Brickie said:

As an aside-you are putting an admirable amount of thought & effort into detailing. With that in mind,I’d suggest ditching the PIR cavity insulation. Search the site here for others’ experiences. It’s too complex to summarise in a sentence why it’s such an impractical solution for masonry construction,but if pushed for a one liner I’d say the real world performance is NEVER anything like what it says on paper. 

@BrickieI'd like to ditch the PIR after reading many posts now, but it's most commonly used locally apart from those going timber frame. As mentioned in my first post, we are in a severe wind/rain area on the NW coast which doesn't allow for blown bead unfortunately. I'm aware that PIR can off gas and degrade slightly in performance over the years, I'm hoping to either fit insulation myself & either tape joints or use low expansion foam during installation as best as I can. Builders definitely won't tape it and it's probably not practical since it rains a lot in England. A change of construction at this stage would involve too much time & cost unfortunately. I didn't think I'd need to be so involved but I've learned things from this forum thats been put on my design which aren't good.

 

On 07/04/2022 at 12:37, SuperPav said:

I don't like the cavity closers as they're difficult to fix to. Also, in the past I've had just as much success of cutting and wedging a piece of 50mm Celotex between the leaves and foaming it to create a much more sealed cavity closer. You still can't fix to it so need to take the fixings back to the inner skin via straps. I've also found you need to use insulated plasterboard on the reveals, as it's more rigid (and you can only fix to the inner leaf really).

@SuperPav Thanks for the sketch.  When using this method, do you wedge the cut insulation between the outer leaf and the cavity insulation, or between the outer leaf and the inner leaf? I've used closers on my current home years ago but the window was fixed to the outer, then i used 15mm plasterboard to close the reveal off. Cant remember if i used d&d or mechanical fixing into the inner leaf with some adhesive behind. 

 

My course heights need to be the same throughout but inner reveals could be set back the thickness of ply board if needed.

[Iceverge]

5 hours ago, ruggers said:

On 07/04/2022 at 17:37, Iceverge said:

 

@IcevergeThanks for the image, It looks a good method as you say for taping the reveal to window frame, I presume you had to close this way when using blown bead. Were your inner & outer reveals flush or did you have to set the inside leaf back by the thickness of the OSB? Where does your window sit and fix to? I want to avoid fixing into the outer brick as they can be brittle. With your method i'm thinking the window straps would fix over the ply not behind so the finished reveals don't need ripped off to change window frames in the future

 

 

Didn't have to, mostly people just put some pir as a cavity closer. I'll link a good method with plasterboard and membrane. 

 

https://www.edmondodonoghue.com/build-it-better-window-door-detailing/

 

We set the inner leaf back 100-150mm for splayed reveals. 

 

Window rests on the external concrete sill and is bolted with L brackets to the external leaf.

 

Not planning on ever changing the windows but if we did we would need to re do the window boxes. They're just butting up against the inner frame of the window and are entirely self supporting. The window is pinched between them and the outer leaf. 

[Iceverge]

On 04/04/2022 at 16:47, ruggers said:

Weather zone is severe 4 for NW England so blown bead isn't an option.

 

 

You can fill your cavity according to Bregs. 

[tonyshouse]

Steel pegs in the outside skin. Window in the cavity touching on the back of the outside skin, nothing bridges my cavity, not even lintels 🙂

[ruggers]

20 hours ago, Iceverge said:

Window rests on the external concrete sill and is bolted with L brackets to the external leaf.

 

the window boxes. They're just butting up against the inner frame of the window and are entirely self supporting. The window is pinched between them and the outer leaf. 

Thanks for the link, interesting read, the very last part - 3……Fit plastic covered insulation rebates to the vertical reveals ? I'd have thought that problem applies to blown bead too, not just PIR. I didn't think it would be such an issue if the windows foam filled and taped between frame and reveal.

 

I can't picture L brackets on the external leaf, thought the fixings just went through the frame for external leaf fixing. 

Was building control involved in your build/renovation? My provisional plans were saying all opening reveals require some type of fire board installed, not plasterboard type.

 

15 hours ago, tonyshouse said:

Steel pegs in the outside skin. Window in the cavity touching on the back of the outside skin, nothing bridges my cavity, not even lintels 🙂

Tony, did you use separate lintels over your openings like concrete ones if they don't bridge? How are the pegs fixed to the outside skin for the window frame to rest on?

[ruggers]

I will look into the blown bead again, I spoke to a firm called Koresystem who supply blown bead who provided a lot of good info. but they weren't pushing it so much once they realised the exposure zone. It's looks a great way to do it, but it makes me nervous, if it goes wrong it's disastrous. wrong as in causing damp or water to track towards the inside somehow. There must be a reason it's not recommended in zone 4 and if it comes down to the type of pointing, flush joints zone 4 but recessed is zone 1 that doesn't sound right the mortar being the decision maker. 

There also seems to be a lot of cowboys installing it who just wanted the government grants for retrofits so it would need to be a reputable company. 

[Iceverge]

https://www.nhbcfoundation.org/wp-content/uploads/2016/05/RR10-Full-fill-cavity-wall-insulation.pdf

 

Worth a read. 

 

Kore is a reputable manufacturer. They make the MBC insulated raft. We bought our floor EPS from them. 

 

Install is very straightforward with a wide cavity. Problems more likely with narrow cavities. Make sure the installer uses enough PVA glue.

 

I'll have a look for drawings/pics of our window install. 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Iceverge]

Here is the plan view of what we actually built

 

 

 

 

 

Here is the section. I've omitted the external render and external airtight paint here to show the overlap of the window bottom on the sill.

 

 

 

 

 

The windows were installed bolted to the outer leaf first. No connection to the internal leaf. I later cut out and threw away the vertical DPM as I have no idea what purpose it serves in a wide full fill cavity

 

 

 

 

To make the OSB window boxes  we first made the base and the top. and bolted them with concrete screws to the lintel and blockwork. Note the angled brackets used to fix the side panels afterwards.

 

                   

 

 

 

 

 

 

 

The sides were fixed using galvanised angle brackets afterwards.  In doing do the whole OSB box became quite rigid despite only being connected to the inner leaf. It was simply floating against the window.  As we have quite deep windows I wanted to make sure that a child (or adult!!) jumping on the interior window sill would not collapse it. This was a fear if it was only cantilevered from the inner leaf. 

 

Sha

Delete

 

 

 

 

 

 

From there the Airtightness process was straightforward.

A/T tape from the window to the OSB.

A/T paint from the OSB to the blocks.

Wet plaster over the A/T paint.

 

Plasterboard to the OSB and skim. 

 

 

I used A/T paint to seal the external to the external render for wind tightness. White paint on white windows is a bit of a mess but nobody notices unless they're fussy. 

 

 

Overall we had an excellent blowerdoor (0.31ACH) and the splayed windows are nice. The Sills are solid and perform thermally very well.  I had a THERM model somewhere but I can't find it. A 25mm hole drilled in the OSB head and sill allowed EPS blown beads to completely pack out below and above the window.  I drilled test holes to check in what I considered the hardest to get to locations and I was impressed. 

 

PROBLEMS encountered. 

 

1. The window fitters measured the windows when I wasn't there. I wanted them to get the outer blocks to overlap the frame by 10-20 mm. It would have made the frames appeared slimmer and we'd have had more glass. Moreover we'd have been able to use compriband or similar to seal the outer blocks to the frames. As it was the blocks and the frames met with two opposing 90deg corners ( angle to angle) whick meant that gap was very annoying to fill. In the end a mixture of expanding foam(yuck), plaster and silicone got it done but it was very tedious.  I had to seal the frames to the outer leaf for our wind tight layer with airtight paint which was a bodge. It would be awful on coloured frames. 

 

 

2. Plastic DPC/DPM used as cavity trays and vertical DPM's. What a terrible way of building in the 21st century.  They catch all the mortar droppings (sometimes bridging the cavity), often get torn. Prevent a good seal from the window frames to the outer blocks. Generally blow about and slap you in the face in the wind. Total RUBBISH.  If I had my way again I'd put an angled PVC profile just over the window head and leave it at that. I cannot see how vertical DPM's prevent water making its way inside in a wide cavity with the inner totally isolated from the outer. 

 

3. The OSB window boxes were very time consuming to make. Maybe premaking them and building them into the blocks would be better. A solution to prevent them degrading in the weather would need to be found. 

 

4. A few hours spent pretaping the windows before install would have saved many hours later on. 

 

5. When screwing the angled OSB reveals to the blocks I was a little over zealous and put a bend into a couple of them.  

 

6. As the OSB was only "floating" against the window some of the gaps around different windows are inconsistent. I don't think anyone else has noticed but me however. 

 

 

 

I'll add another post about what I'd do differently the next time as this one is getting a little long.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Iceverge]

Improved UPVC window installation. I didn't bother drawing the horizontal section as most of the changes are the same. 

 

 

 

 

 

 

 

Points to Note

 

1. The useless cavity tray is gone. A small UPVC profile pre screwed to the frame will now direct any drips/condensation that run down the inside of the outer leaf sideways and harmlessly away. 

 

2. Compriband is pre installed to the frames allowing tidy and quick expansion to seal the window to the outer blockwork at all sides. 

 

3. 12.5mm J beads are screwed to all 4 sides of the window. This will allow an airtight 12.5mm OSB to be slotted in and then shot nailed to the inner leaf for speed. The block layer should leave the  upper lintel maybe 30-50 mm higher than the outer one and packers can make up the difference. Similar story with the sill. 

 

4. Pre install the A/T tape to the windows to all for very speedy taping later to the OSB. 

 

5. The OSB to Block junction may be better dealt with with fleece A/T tape and plastered over. 

 

 

You would need a little time with the window frames before the fitters returned to pre install all the bits and bobs.  Also cowboy window installers would be no good. The J beads should be screwed securely into the window frames, especially at the base.  Some accuracy would be needed from the mason but not enormous. The windows would need to be accurately measured. No rocket science but everyone would need to understand. 

 

 

Its a no mess install, no expanding foam or A/T paint or silicone.  Very quick to windtight and airtight. All materials readily available and pretty cheap. 

 

If only we could all build 100 houses. We'd get it right eventually! 

 

Next task, Incorporate external foam filled PVC electric roller shutters which I would very much like.   

 

 

 

 

 

 

 

 

[tonyshouse]

I used separate lintels inside and outside, yes 

[Iceverge]

To finish this off. 

 

I modelled my revised detail in THERM. Overall a negative external thermal bridge so satisfies the passivhaus criteria AFAIK. This only applies for insulated frames/ UPVC. 

 

For simplicity I just used a window that was all "frame". U value of 1.2W/m2K

 

 

 

 

 

 

 

 

 

[ADLIan]

Sorry but that THERM model is wrong. The lintel, jambs and cill should be modelled separately effectively ignoring the window. I've not used THERM for some time but in the past it did not give the psi-value directly - the output needed more number crunching to arrive at the psi-value. Whilst the psi-value may be low in this instance I doubt that it would be negative.

[joe90]

I,m with @tonyshouse and @Iceverge, I am in a fairly exposed location and used rockwall batts full fill(they have a BBA certificate for full fill) I also had check reveals (inner skin opening bigger than outer) to give angled reveals. Brick arches in outer skin and separate lintel internally. I made my own closures like @Iceverge shows above, window sitting on brick sills (just) and held to outer skin with stainless brackets and screws.

the only thing I would do differently if I did it again is use plaster stop bead next to the window to give a movement gap that can be filled with flexible mastic (several here have had cracking at this point like me).