Brick+Stick frame wall build-up

[Annker]

Hi all,

I'm at building reg design phase for my planning permission granted victorian refurb and extension.

For context I am a carpenter and I have done a couple of stick frames previously but not in the past decade, therefore I would appreciate critique of my preliminary TF design before I sit down with my SE and Architect.  

 

From external to internal:

• 100mm brick external leaf 
• 50mm cavity 
• 25mm-50mm PIR insulation (Thickness selection TBC to achieve yet to be determined target U-value) 
• Breathable membrane 
• 9mm osb sheathing 
• 140mm timber studwork
• 140mm PIR insulation with studwork
• VCL
• 50mm x50mm battens (to provide service cavity)
• Plasterboard and Skim coat finish

 

Queries:

• Is a 50mm cavity sufficiently wide?
• Will placing PIR insulation cold-side of the breathable membrane create a interstitial condensation risk?
• I have seen the VCL positioned internal side of the service cavity, however I believe the VCL is better positioned external side of the service cavity

 

Any options or thought are appreciated       

 

 

[Russell griffiths]

9mm osb is terrible stuff, go thicker it will provide better racking stiffness. 

Why the insulation in the cavity?

ive stick built with a block skin, all the insulation on the inside in my opinion. 

 

Having stick built before i would use an airtight osb on the inside also, tape this and use it as your vcl

[Annker]

28 minutes ago, Russell griffiths said:

9mm osb is terrible stuff, go thicker it will provide better racking stiffness. 

Why the insulation in the cavity?

ive stick built with a block skin, all the insulation on the inside in my opinion. 

 

Having stick built before i would use an airtight osb on the inside also, tape this and use it as your vcl

Yeah I previously used 11mm osb but had read 9mm as being sufficient and commonly used, irrespective of that I probably will still use 11mm as the extra cost of 11mm vs 9mm on a TF of this scale is minimal.

 

Again, on my previously built TF I too kept all insulation within the studding & service cavity, however my research over the past day has pointed toward adding insulation external side of the breathable membrane and osb, commonly termed "cavity wrap"

The rationale is that insulation in this position is not interrupted by studwork therefore, continuous and goes some way to reducing cold bridging. It makes sense to me, although there must be a specific wall tie required to tie the TF to the brickwork, I'm yet to look or find it.

 

Why do you use osb internally, certainly it would create a very solid wall but it adds a significant chunk to labour and material cost, are there other benefits?    

Given the service cavity would internal side of that osb layer it doesn't provide patressing either, at least not in my build-up arrangement.

 

Edited November 16, 2023 by Annker

[Iceverge]

8 hours ago, Annker said:

From external to internal:

• 100mm brick external leaf 
• 50mm cavity 
• 25mm-50mm PIR insulation (Thickness selection TBC to achieve yet to be determined target U-value) 
• Breathable membrane 
• 9mm osb sheathing 
• 140mm timber studwork
• 140mm PIR insulation with studwork
• VCL
• 50mm x50mm battens (to provide service cavity)
• Plasterboard and Skim coat finish

 

Queries:

• Is a 50mm cavity sufficiently wide?
• Will placing PIR insulation cold-side of the breathable membrane create a interstitial condensation risk?
• I have seen the VCL positioned internal side of the service cavity, however I believe the VCL is better positioned external side of the service cavity

 

What's your target U value? 

 

I'm a fan of externally covering a timber frame in insulation.  It keeps the sheathing warm and really deals with any thermal bridging well. Similarly it makes it easy to "over insulate" window frames which are the worse performers thermally in a house. Do your windows open inwards or outwards?

 

I would omit the internal VCL. Along with the foil faced PIR you would end up with vapor two barriers which would be a recipe for trapping moisture.  EPS is a more breathable and cheaper material and I would prefer it for a timber frame on that account as external wall insulation.

 

PIR between timbers never sits well with me. Expensive, wasteful. Bad for thermal bypass, noise etc. 

 

50mm cavity is plenty.

 

Moisture gets into the structure through bad airtightness. It gets trapped because of bad breathability. 

If you do an excellent job in airtighess you'll be fine. Likewise leave the structure to breath(dry) to the inside and it'll be ok. 

 

VCL's are typically internal of the insulation to keep them always above the dew point so that if any vapor gets carried into the wall by drafts it wonts condensate. If you can assure that the structure will be very airtight and it can dry inwards (or outwards) if it does get damp then you'll be ok. 

 

How about

 

Brick

50mm cavity. 

 

Breather membrane. Lapped and taped to all opening's.

 

50mm EPS graphite enhanced ( GPS)  foam between all boards.

 

11mm OSB sheathing (metric sheets). All joints diligently taped as your airtightness layer. Seal to roof windows doors soil pipes foundations etc. 

 

140x38mm framing at 600mm cc.  With batt insulation in between.

 

15mm plasterboard or 2* 12.5mm

Skim

 

U Value about 0.18

 

 

External_EPS_on_TF_170100 (1).pdf

 

 

 

 

 

 

[Annker]

1 hour ago, Iceverge said:

 

What's your target U value? 

 

I'm a fan of externally covering a timber frame in insulation.  It keeps the sheathing warm and really deals with any thermal bridging well. Similarly it makes it easy to "over insulate" window frames which are the worse performers thermally in a house. Do your windows open inwards or outwards?

 

I would omit the internal VCL. Along with the foil faced PIR you would end up with vapor two barriers which would be a recipe for trapping moisture.  EPS is a more breathable and cheaper material and I would prefer it for a timber frame on that account as external wall insulation.

 

PIR between timbers never sits well with me. Expensive, wasteful. Bad for thermal bypass, noise etc. 

 

50mm cavity is plenty.

 

Moisture gets into the structure through bad airtightness. It gets trapped because of bad breathability. 

If you do an excellent job in airtighess you'll be fine. Likewise leave the structure to breath(dry) to the inside and it'll be ok. 

 

VCL's are typically internal of the insulation to keep them always above the dew point so that if any vapor gets carried into the wall by drafts it wonts condensate. If you can assure that the structure will be very airtight and it can dry inwards (or outwards) if it does get damp then you'll be ok. 

 

How about

 

Brick

50mm cavity. 

 

Breather membrane. Lapped and taped to all opening's.

 

50mm EPS graphite enhanced ( GPS)  foam between all boards.

 

11mm OSB sheathing (metric sheets). All joints diligently taped as your airtightness layer. Seal to roof windows doors soil pipes foundations etc. 

 

140x38mm framing at 600mm cc.  With batt insulation in between.

 

15mm plasterboard or 2* 12.5mm

Skim

 

U Value about 0.18

 

 

External_EPS_on_TF_170100 (1).pdf 369.76 kB · 0 downloads

 

 

 

 

 

 

Hi Iceverage,

ATM my target u-value is TBC.

Rightly or wrongly, I had in my head to get the structural arrangement figured out and then size insulation thicknesses/type accordingly to meet the confirmed u-value target.

 

My project is chiefly a renovation of a victorian semi-d with an 2 storey TF extension; the ground floor will be an office and a bedroom will be above on the 1st floor.

As such it is a relatively small extension, there are only 4no windows so I can easily design them to opening either inwards or outwards.

 

I have actually flicked over that pdf before, I think you had it attached to a previous comment in another thread.

I had discounted its design (omission of vcl) as it added more choice/questions/queries all which lead me away from making a decision.

I suffering badly from analysis paralysis, and I will never get the thing built if I continue this way!

 

I note that the internal finish is lime-gypsum plaster. Is that an actual product? Lime makes sense as it provide good breathable characteristics required in that design; in contrast to gypsum so that's a little confusing.

Also is there doesn't appear to be a service cavity?

Appreciate that there isn't a vcl to protect but I have always liked that the service cavity provides some manner of containment for cabling.

 

You make good points about PIR from an acoustics performance perspective, and that PIR foil faces introduce additional (potentially moisture trapping) layers. I have specific concern about that in my initial design.  

  

 

    

 

 

 

Edited November 17, 2023 by Annker
Spelling correction

[Annker]

@Iceverge I didn't initially realise that your pdf sample build-up was from an interactive tool and free to use. 

It's a great bit of software, I don't have German to understand some of the descriptions but thankfully numbers and values are same in both languages!    

 

I've played around with the buildup and came up with design that in separate components makes sense to me and I would be very comfortable delivering onsite but I'm not certain it works all together though.

 

• The 140mm mineral wool within the studwork makes up the bulk of the insulation, and, vs. PIR,  ticks the box for breathable and acoustic performance.
• Overboarding studwork is 60mm PIR in large format boards tends itself well to be the insulation choice to be fixed uninterrupted over the studwork, and it also deals with cold bridging spots. (Perhaps this will be reduce to 50mm or 40mm thickness)
• This PIR layer could be tape and may be sufficient to provide a VCL but for the minor additional cost I've chosen to spec a sheet VCL
• 40mm service cavity internal to the wall construction proper (will likely be formed with 45mm battens)
• 11mm OSB behind the plasterboard to give a frimer wall finish, especially as I have the studwork @600mm c/c

@Iceverge You assigned "Rear ventilated level (outside air)" to the cavity in your model; I have simply copied you in this.

How did you know to assign that particular selection to the cavity, its illustrates battens in the model which I don't believe should be there in reality.

 

 

Insulation_of_TF_1.pdf

Edited November 17, 2023 by Annker

[Iceverge]

10 minutes ago, Annker said:

@Iceverge I didn't initially realise that your pdf sample build-up was from an interactive tool and free to use. 

It's a great bit of software, I don't have German to understand some of the descriptions but thankfully numbers and values are same in both languages!    

 

I've played around with the buildup and came up with design that in separate components makes sense to me and I would be very comfortable delivering onsite but I'm not certain it works all together though.

 

• The 140mm mineral wool within the studwork makes up the bulk of the insulation, and, vs. PIR,  ticks the box for breathable and acoustic performance.
• Overboarding studwork is 60mm PIR in large format boards tends itself well to be the insulation choice to be fixed uninterrupted over the studwork, and it also deals with cold bridging spots.
• This PIR layer could be tape and may be sufficient to provide a VCL but for the minor additional cost I've chosen to spec a sheet VCL
• 40mm service cavity internal to the wall construction proper (will likely be formed with 45mm battens)
• 11mm OSB behind the plasterboard to give a frimer wall finish, especially as I have the studwork @600mm c/c

@Iceverge You assigned "Rear ventilated level (outside air)" to the cavity in your model; I have simply copied you in this.

How did you know to assign that particular selection to the cavity, its illustrates battens in the model which I dont believe should be there in reality.

 

 

Insulation_of_TF_1.pdf 352.2 kB · 0 downloads

 

Looks good. @Thorfun among others have the same buildup. It's quite common. I think MBC offer it as a factory spec. 

 

The OSB behind the plaster is a cheap enough addition for a much more robust wall. 

 

You could drop your service cavity to 20mm using 20*70mm strapping. 20*45mm battens tend to be a bit weedy and split. 

 

If you do this you can spec outwards opening windows and wrap the jambs etc and overlap the frames from the inside. 

 

One point of care with the airtightness will be the band joist area that will require a membrane wrapped around the end of the joists for the first floor connection. Likewise any buttressing internal wall. Nothing that can't be overcome however. 

 

 

Note the insulation outside the band joist too as this will be a weak spot otherwise. 

 

 

[Annker]

6 minutes ago, Iceverge said:

 

Looks good. @Thorfun among others have the same buildup. It's quite common. I think MBC offer it as a factory spec. 

 

The OSB behind the plaster is a cheap enough addition for a much more robust wall. 

 

You could drop your service cavity to 20mm using 20*70mm strapping. 20*45mm battens tend to be a bit weedy and split. 

 

If you do this you can spec outwards opening windows and wrap the jambs etc and overlap the frames from the inside. 

 

One point of care with the airtightness will be the band joist area that will require a membrane wrapped around the end of the joists for the first floor connection. Likewise any buttressing internal wall. Nothing that can't be overcome however. 

 

 

Note the insulation outside the band joist too as this will be a weak spot otherwise. 

 

 

Thanks again for the feedback.

Yes reducing the depth of the service cavity is certainly a strong possibility as the TF will comprise of an office and a bedroom, as such there will be fewer services to accommodate vs say a kitchen or bathrooms.

 

Noted also regarding continuity of membrane at joist level.

Although the set back of the band joist (aside, I would term rim joist) is not an arrangement I've seen previously.

I guess the sheathing continues over the insulation at that point to ensure both the GF and 1st floor TF panels are tied together. I think lining the respective studs through and strapping with 1000mm galvanised straps would be required also.

 

 

 

[Iceverge]

15 minutes ago, Annker said:

Although the set back of the band joist (aside, I would term rim joist) is not an arrangement I've seen previously.

I guess the sheathing continues over the insulation at that point to ensure both the GF and 1st floor TF panels are tied together. I think lining the respective studs through and strapping with 1000mm galvanised straps would be required also.

 

Yes, Certainly take structural precautions here as you'll loose a lot of strength. You could just insulate between the joists well as normal too. It wouldn't make much difference thermally. 

[Thorfun]

35 minutes ago, Iceverge said:

Looks good. @Thorfun among others have the same buildup. It's quite common. I think MBC offer it as a factory spec. 

yes indeed. and i was inspired by the buildup from MBC which was on their website.

 

37 minutes ago, Iceverge said:

The OSB behind the plaster is a cheap enough addition for a much more robust wall. 

yes but, for me, it was a cost i couldn't justify so late in the build as our budget has well and truly gone out of the window.

 

39 minutes ago, Iceverge said:

You could drop your service cavity to 20mm using 20*70mm strapping. 20*45mm battens tend to be a bit weedy and split. 

2"x1" battens can split but we piloted and countersunk every single batten. i bought a cheap table drill and we setup a system from chop saw to drill which worked quite nicely.

[Iceverge]

2 minutes ago, Thorfun said:

2"x1" battens can split but we piloted and countersunk every single batten

 

🤯

[Shaun McD]

On 17/11/2023 at 14:39, Annker said:

@Iceverge I didn't initially realise that your pdf sample build-up was from an interactive tool and free to use. 

It's a great bit of software, I don't have German to understand some of the descriptions but thankfully numbers and values are same in both languages!    

 

I've played around with the buildup and came up with design that in separate components makes sense to me and I would be very comfortable delivering onsite but I'm not certain it works all together though.

 

• The 140mm mineral wool within the studwork makes up the bulk of the insulation, and, vs. PIR,  ticks the box for breathable and acoustic performance.
• Overboarding studwork is 60mm PIR in large format boards tends itself well to be the insulation choice to be fixed uninterrupted over the studwork, and it also deals with cold bridging spots. (Perhaps this will be reduce to 50mm or 40mm thickness)
• This PIR layer could be tape and may be sufficient to provide a VCL but for the minor additional cost I've chosen to spec a sheet VCL
• 40mm service cavity internal to the wall construction proper (will likely be formed with 45mm battens)
• 11mm OSB behind the plasterboard to give a frimer wall finish, especially as I have the studwork @600mm c/c

@Iceverge You assigned "Rear ventilated level (outside air)" to the cavity in your model; I have simply copied you in this.

How did you know to assign that particular selection to the cavity, its illustrates battens in the model which I don't believe should be there in reality.

 

 

Insulation_of_TF_1.pdf 352.2 kB · 2 downloads

 

This is very similar to what we have chosen, we are using a breathable racking board, we have opted for a 70mm internal insulation board, and in place of the VCL, we are taping the joints. Another detail we have is to return a 25mm insulation board on window jambs to kill the cold bridge from window to frame. We have a calculated wall uvalue of 0.13.

Our frame is 400c so hoping that will help with wall firmess, but may still look to use OSB behind kitchen, and otherwise use 15mm or 2 x 12.5mm for utility/plant room.