Closing wide cavities - timber clad building. help!

[sean1933]

This is a decision which has been put off far too long... one of those that I cannot decide upon a best route so ignored it until I can no longer... I fully accept this should have been decided upon before brickwork went up but I'm doing most of the build myself and working full time so decisions like this get bumped. Roof slates are going on now so I'm left with open cavities and am unsure of the best approach.

 

Plan is for mid-cavity window/door positions. Buildup is 200mm cavity blown EPS bead, block & block in most places which will be felted and timber clad (possibly open jointed rainscreen... but thats up to the wife). There is face brickwork in some places but this is relatively limited, I'll come to that later. Our gable ends are vaulted ceilings with a 5m ceiling so are double stud timber, rather than blockwork. Some may ask why not all timber frame given the cladding - we're near a busyish road and I hate noise. 

 

We have a mix of large heavy sliding doors on the ground floor (sitting on compactfoam), and floor to ceiling windows/balcony doors on both floors. In addition there are a number of through eave windows, which is hurting my brain slightly when I think about structural rigidity of closures because of our choice of set back positioning. I have read the many threads on this topic but still remain full of indecision as I note most if not all of those discussions surround face-brickwork or rendered finish. My natural go to was the Golcar approach 18mm ply box but with already formed openings I wanted to minimise making them smaller, plus blown bead and the heavy doors/through eave windows complicates my thought process. I'm therefore greatly interested in the views of others.

 

My starting point question (given how MBC form their twin stud frames) is... is using ply fully across the cavity really that bad thermally? Moisture shouldn't be considered an issue as the entire building (save for small facework brick areas) is to be wrapped in membrane just as a timber frame would be. 

 

Current options/thought processes:

- To minimise opening reduction, simply close the cavity with 12mm ply (upstairs windows/doors to sit on 18mm/22mm ply across the cavity). Is 12mm rigid enough if crossing cavity? My main concern is, I assume this isnt thick enough to fix into...? Assuming not, glue another 12mm strip on the back (so sitting within cavity where windows will fix, or use tiebacks to internal blockwork? Note I don't think think through eave windows can use tiebacks - see HERE for usual fitting approach. 

- Suck up the reduction in size and use 18mm ply across the cavity.

- 18mm ply box set back from outside face - close the face with celotex et al. Concern I have here is through eave windows obviously don't have a top so not technically a box - greatly reducing strength. Also weight/height of windows/doors (@ gables will be 4.5m high triple glazing) means I'd probably want support across the cavity.

 

Note where there is brick facework if closing the full cavity with ply my intention would be to vertical DPC between ply/brick. Breather membrane/tape over the top face. Cover with powdercoated aluminium strip much like Golcar.

 

Help!

 

[sean1933]

I was trying to use the same approach at all openings, but an alternative option would be to just close entirely with celotex and tie windows/doors back to inside blockwork. Using only one of the above ply approaches where through eave windows are concerned. Thoughts?

 

I must say my preferred option at the moment is just closing entirely with 12mm ply - its simple, quick and helps airtightness. Question is does it work?

Edited September 29, 2021 by sean1933

[nod]

You probably should and still could Go old school and brick across the cavity With a dpc at the inner edge 

It will create a thermal bridge But to close off 200 mil and fix the frames midway It May be a compromise you have to make 

[sean1933]

Thanks Nod. Not sure if my original rant has come across right but I'm trying to broadly follow the thermally efficient approach of midspan window/door placement as per green building store, albeit as I have large upstairs glazing and through eaves their box approach doesn't seem to entirely work. 

 

Rather than bridging the cavity with brick I assumed the fall back last resort option for me would be move to the standard position of outside skin fixing... although this seems unnecessary given the options available. Is there any reason why you do not like the idea of timber closing the cavity? Noting it is all to be wrapped in breather membrane anyway. Or are your concerns restricted to where there is fairfaced brick externally?

[redtop]

We tied back to the timber frame but extended the bottom so the windows sat on solid wood Helps with thermal bridging but is a pain to get airtight. I would ply box if doing it again

 

Edited September 29, 2021 by redtop

[Mr Punter]

Why not just use standard 200mm cavity closers?  There is a choice of PIR and mineral wool insulation ones.  They close the cavity, stopping your blown insulation escaping, prevent thermal bridging and stop any rainwater tracking past.  Some stainless straps to the inner leaf of block to secure the windows.

[sean1933]

2 minutes ago, Mr Punter said:

Why not just use standard 200mm cavity closers?  There is a choice of PIR and mineral wool insulation ones.  They close the cavity, stopping your blown insulation escaping, prevent thermal bridging and stop any rainwater tracking past.  Some stainless straps to the inner leaf of block to secure the windows.

 

They are expensive so alternatively could just use cut rigid insulation I suppose ala Tony's approach ... my main concern here is getting a good solid fixing for large glazing (gables are 4.5m vertically, lounge sliders are 5m wide) - which neither can be directly fixed to. Not to mention through eave glazing appears to require direct fixture, ie no tiebacks. 

 

I am sure my inexperience is over complicating this - fact is I dont know if just tying back to inside skin is acceptable on big glazing. Or instead of using metal ties whether a half way (ie not to outside skin) strip of 12mm ply would do the same job but provide a better airtightness approach.

 

9 minutes ago, redtop said:

We tied back to the timber frame but extended the bottom so the windows sat on solid wood Helps with thermal bridging but is a pain to get airtight. I would ply box if doing it again

 

 

Great - bridging the cavity at the base is what I expected to have to do to support the load. Did you tie back for relatively normal  size windows, or did you use this approach for large glazing/big doors?

[Mr Punter]

20 minutes ago, sean1933 said:

They are expensive so alternatively could just use cut rigid insulation I suppose ala Tony's approach

 

The fire rated ones are £23.30 each, the non fire rated are £11.60 ex VAT.  Cut them on a sliding chop saw.

 

Get some nice meaty galvanised fixing lugs.  With some windows these twist fit into the frame.  Ask your window people what they recommend.  God knows how to fit those cranked eaves windows.  Make sure you have allowed for the gutter and downpipes either side.