Timberframe cladding - Cost of cavity barriers and insect mesh

[AliG]

Hi,

 

Looking at he crazy pricing we have for our build at the moment, a last minute increase has come as they have added in cavity barriers and insect mesh.

 

We have a 195sq metre build with an MBC frame clad mainly in rendered cement board but also around 70sq metres of larch cladding.

 

At the last moment costs have come in at £5000 for cavity barriers and £2500 for insect mesh.

 

On top of this there is an extra cost of £1140 to use black breather membrane behind the cladding and then an estimate of almost £200 a square metre for the cladding including the necessity to use black battens.

 

The cavity is ventilated at the top and bottom.

 

A few thoughts spring to mind.

 

1. The architect has specified cavity barriers at the corners. However, why not use solid battens at the corners. I don't think there is any necessity for ventilation around the corners. Solid battens would meet the regs for a cavity barrier.

 

2. The windows are shown as having ventilation all around. I am not sure thatches detail is necessary. The plan is to use Russell Timber alu clad triple glazing. If you take the render board right up to the window, presumably you don't need insect mesh or cavity barriers around the openings.

 

3. Presumably T&G larch cladding would be much cheaper. It would also eliminate a lot of the insect mesh and the need for black battens. It seems a bit of a nonsense to me to put cavity barriers behind open cladding but that is what the regs call for.

 

Has anyone else run into these issues. The fire regs are new and stricter in Scotland. Cavity barriers in two storey houses seem entirely unnecessary to me.

 

[SuperJohnG]

I've literally just done this. 

 

SIPS kit with knauf aquapanel (200m2) mounted on battens main part of the house with a 80m2 leg that is getting clad in fibre cement (was origonaly siberian larch but changed mind). All mounted on battens with a ventilation gap. 

 

£2500 is a lot. I used battens at corners as normal firestops, and normal firestops around windows the same. You just ventilate around windows cutting battens back to get airflow.

 

You will need special Intumescent fire barriers which are not the normal red type from tenmat. They do a specialist one that comes in 1m sections and is 3 times the price, but it is the right one and goes horizontally.

 

So in short

 

1 - agree

2 - bang on. 

3 - use t &g or flip to fibre cement and then you only need them top and bottom. Cedral sell them at a reasonable price in the solid versions, avoid the mesh rolls. I found it a PITA in most areas. 

 

Do you have an insulated foundation? 

 

I'm also Scotland so everything I have done will work for you. 

 

[saveasteading]

3 hours ago, AliG said:

£5000 for cavity barriers and £2500 for insect mesh.

That sounds far too much.

The fancy cavity barrier product costs about £3/m.

Must be a very big house or a contractor who is either nervous of it or charging nicely for extras.

[AliG]

Thanks guys.

 

2 hours ago, saveasteading said:

The fancy cavity barrier product costs about £3/m.

Because of the ventilated cavity the architect has specced the Tenmat intumescent, expanding cavity barriers which cost around £10 a metre. I note looking online and from another thread that the Envirograf ones may be more like £7 a metre which would help.

 

3 hours ago, SuperJohnG said:

normal firestops around windows the same. You just ventilate around windows cutting battens back to get airflow.

By normal fire stops do you mean the firesocks or do you mean timber? If you cut battens back to get airflow around the windows would you not need intumescent seals there?

 

3 hours ago, SuperJohnG said:

avoid the mesh rolls

I was looking at the drawings and wondering how easy it would be to attach. What would you suggest instead?

 

2 hours ago, saveasteading said:

Must be a very big house or a contractor who is either nervous of it or charging nicely for extras.

 

This was my initial though, but having looked the contractor just asked what he was to do re cavity barriers then priced up what the architect specified. I think it is a case of the architect not realising cost implications when specifying things.

[jamieled]

Is your mesh for ventilation gaps or is it going behind all the cladding? We used mesh rolls for board on board vertical cladding. Robinson wire cloth. Easy enough to staple to the sheathing and battens at the top and bottom. Also used at eaves/soffit. You can get quite wide rolls which are easier to use.

 

We also used Envirograf cv intumescent barrier. Can't remember the exact cost at the moment but it would have been closer to £7/m than £3. Obviously its a bit dependent on the size of the gap you need to fill as they have different products dependent on the spec. Envirograf were helpful. I have bits left over if an offcut might help you decide.

Edited December 20, 2021 by jamieled
Spelling

[redtop]

2500 for mesh! Seems a massive over price

[AliG]

8 minutes ago, redtop said:

2500 for mesh! Seems a massive over price

I have queried that.

 

Looking at the architect’s drawings they have it around all openings and at the top and bottom of the cladding. I wonder if the builder has also priced to put it behind the larch. Doesn’t seem to me to be any point of having it around the larch if it is open between the strips.

 

TBF the architect only produced the drawings a few days ago and I have queried the estimate.

 

 

[saveasteading]

Can I suggest that you go back to basics?  The mesh is to keep out insects and other creatures. Therefore it only goes where required...mostly (or only) top and bottom.

In reality there will be all sorts of gaps at every joint in the timber, but I don't think we worry about that. No such concern at cement board.

 

Also the purpose of the fire stops is obv to stop fire spread between different areas of the building. Unless the rules have changed recently that does not require intumescent material everywhere. It is confused by the need for retaining ventilation but that is upwards only.

 

Black Battens? Is this because the larch cladding is completely open and more like a trellis? In that case I don't think it needs either the fire block or the insect mesh.

 

The building regulations are more  a principle than doctrine in many cases. Therefore you simply have to justify your proposals.

This means more work for your architect to save your money, and so you might have to press hard for it.

However this is not cutting edge research and a 'expert' should be pleased to improve the design. When you explain the cost perhaps the architect will jump to it.

 

I would like to read up more on this at some stage so it might as well be now. Can anyone point me straight to the clauses on the subject?

 

 

 

[jamieled]

@saveasteading Standard 2.4 of the domestic handbook.

 

Building standards technical handbook 2019: domestic - gov.scot (www.gov.scot)

 

[saveasteading]

Thanks Jamieled. I had just read that section and thought I was missing something.

[jamieled]

It seems to be a bit open to BCO interpretation (in my opinion!). I used the intumescent barriers where we have ventilated openings (top and bottom of cladding and mid-height) but having tested them using direct heat I'm not entirely confident they'd do what they're supposed to in a fire.

[saveasteading]

9. Horizontal cavity barrier [3]	Short	None	30	None	E 30	From the underside
10. Vertical cavity barrier [3]	Short	None	30	None	E 30	Each side separately

 

That is from appendix 2A.

 

This doesn't look very onerous.  I think untreated timber (eg 2x2) would satisfy this as long as it filled the cavity, but would need proving, as there is a reluctance to believe it.  Metal flashing certainly enough or a slice of rockwool.  

 

Testing with a blow torch or by putting on a fire is not a realistic test as a building fire takes time to catch and will vent out of the top.

[AliG]

There is also this guidance from the structural timberframe association.

 

https://www.structuraltimber.co.uk/assets/Links/STA_Cavity_Barriers_Technical_Document_FINAL.pdf

 

As of last year you need cavity barriers around all openings, between floors, every 10m if a wall is longer than 10m and absurd it seems at the top of the cavity. even if in our case there is no roof above this.

 

We are using cement render board and larch cladding that needs to be ventilated behind so solid timber/seals cannot be used as they block the ventilation.

Hence the pricey intumescent seals.

 

I have just confirmed though with MBC that there is no need to ventilate around window reveals which was probably around half the cost of all of this.

 

I have also confirmed that we can use solid battens at the corners as the ventilation is vertical not horizontal. They was another 25% of the cost.

 

So these two details should result in a large saving.

 

[saveasteading]

1 hour ago, AliG said:

at the top of the cavity

completes the ventilation from the bottom.

 

1 hour ago, AliG said:

so solid timber/seals cannot be used as they block the ventilation.

But can vertically I assume.

 

Keep getting these changes and your silly price of £7,500 (and the rest) should be very much less. Try giving them  a target?

[saveasteading]

That reference looks good. Will have  a proper read but have immediately seen this, which confirms my thoughts earlier.

 

DEEMED TO SATISFY BARRIERS UNDER THE ADB 2019 AND TECHNICAL HAND BOOK 2019

a Steel, a minimum of 0.5mm thick b Timber, a minimum width of 38mm thick into the cavity c Polythene-sleeved mineral wool, or mineral wool slab, under compression when installed in the cavity d Calcium silicate, cement-based or gypsum-based boards, a minimum of 12mm thick

[saveasteading]

Just an aside. I think that intumescent products are often specified when they are not necessary. This is due to caution by the specifier and encouragement from the publicity of the manufacturers. Of course they are not going to publicise that 38mm timber, or 0.5mm metal, or rolled up rockwool will suffice in some details.

[Gordo]

Intumescent cavity barriers should only be used horizontally. Vertical cavity barriers are usually only required at 10m centres for timber frame in (N. Ireland anyway). Ventilation is generally only recommended at bottom of cavity now. 

[SuperJohnG]

I spent considerable amounts of time trying to rationalise the rules as both the handbook and the images in it are not very clear. 

 

I'd visited others and found they had cavity barriers actually at the base of the cladding, between floors and also at the top of the ventilated gap- which seemed insane.

 

In agreement with my BCO I have one single strip around my main house between ground and first floor, which would stop a fire in the ground floor reaching upstairs through the cavity theoretically  but should also be stopped by the window firestops from coming in the cavity. 

 

I didnt need at the bottom as it's a slab and the  didn't need at the top as it's a SIPS kit and I have fully vaulted ceilings so no loft space.

 

I did add it at a point where I added loft space over the kids rooms as technically it's a new compartment. I didnt add anywhere the top of the cavity I don't need it as I dont have standard eaves where it could get into that loft space as its a SIPS overhang and solid. 

 

I didnt add anywhere around on my single story vaulted space as its double height but the same space so no compartment to transfer into. 

 

Of course all around the house horizontal travel is stopped by battens and where the battery  wasn't straight I used Firefoam to fill gaps. 

 

I mounted the intumescent barrier straight on the kit where the void was small enough, but some areas the void was above 44mm and that was its rated width so I mounted on 22mm sarking to close the gap to within tolerance but maintaining the ventilation gap. 

 

some pics to help. You have to look close to see the barrier as it's silver. It's not in some pics, just battens.

[Gordo]

13 hours ago, SuperJohnG said:

I spent considerable amounts of time trying to rationalise the rules as both the handbook and the images in it are not very clear. 

 

I'd visited others and found they had cavity barriers actually at the base of the cladding, between floors and also at the top of the ventilated gap- which seemed insane.

 

In agreement with my BCO I have one single strip around my main house between ground and first floor, which would stop a fire in the ground floor reaching upstairs through the cavity theoretically  but should also be stopped by the window firestops from coming in the cavity. 

 

I didnt need at the bottom as it's a slab and the  didn't need at the top as it's a SIPS kit and I have fully vaulted ceilings so no loft space.

 

I did add it at a point where I added loft space over the kids rooms as technically it's a new compartment. I didnt add anywhere the top of the cavity I don't need it as I dont have standard eaves where it could get into that loft space as its a SIPS overhang and solid. 

 

I didnt add anywhere around on my single story vaulted space as its double height but the same space so no compartment to transfer into. 

 

Of course all around the house horizontal travel is stopped by battens and where the battery  wasn't straight I used Firefoam to fill gaps. 

 

I mounted the intumescent barrier straight on the kit where the void was small enough, but some areas the void was above 44mm and that was its rated width so I mounted on 22mm sarking to close the gap to within tolerance but maintaining the ventilation gap. 

 

some pics to help. You have to look close to see the barrier as it's silver. It's not in some pics, just battens.

Cavity barriers should be fitted to top of cavity (quite important this as any fire would otherwise enter roof voids and may spread smoke and fire rapidly). No need for CB at floor level or bottom of cavity in a dwelling house.
 

I’m not too sure intumescent CB are suitable for timber frame construction. I know this is popular in England but i believe they are not accepted in N. Ireland.

Edited December 21, 2021 by Gordo

[SuperJohnG]

10 hours ago, Gordo said:

Cavity barriers should be fitted to top of cavity (quite important this as any fire would otherwise enter roof voids and may spread smoke and fire rapidly)

You maybe didn't read my post properly, there are no roof spaces in a vaulted SIPS build, and where a loft space was added I added a barrier on the outside but not required again at top as it cannot enter a roof space as there is no eaves access it is a solid panel.

 

For typical timber frame with trusses you are correct. 

 

10 hours ago, Gordo said:

I’m not too sure intumescent CB are suitable for timber frame construction.

They are (I also spent considerable time talking to tenmats technical dept). For reference for others it is a tenmat FF102/50 for ventilated cavity timber frame systems or with outer block its a tenmat FF107 or equivalent. 

Edited December 21, 2021 by SuperJohnG

[Gordo]

25 minutes ago, SuperJohnG said:

You maybe didn't read my post properly, there are no roof spaces in a vaulted SIPS build, and where a loft space was added I added a barrier on the outside but not required again at top as it cannot enter a roof space as there is no eaves access it is a solid panel.

 

For typical timber frame with trusses you are correct. 

 

They are (I also spent considerable time talking to tenmats technical dept). For reference for others it is a tenmat FF102/50 for ventilated cavity timber frame systems or with outer block its a tenmat FF107 or equivalent. 

Good for you. As long as you satisfy yourself

Edited December 21, 2021 by Gordo

[SuperJohnG]

1 hour ago, Gordo said:

Good for you. As long as you satisfy yourself

I think it's important to understand the standard and ensure you comply, get agreement with the BCO also. But importantly for me regardless of cost its one if the singular areas of importance and everyone should ensure they have a pragrammtic approach to fire safety to ensure it works. 

[eandg]

Currently wrestling with detailing cavity barriers for (vertical) timber cladding. Am I right that vertically (at corners) a 2*2 is fine, horizontally (between floors) it should be some form of intumescant strips (i.e. given the cavity barriers I've erroneously bought will block ventilation event if I could fit battens around) and around windows it should be?

[SuperJohnG]

Yes if you have a ventilated cavity you need to buy the correct intumescent straps. Sig (in eurocentral) sell them, they ate tenmat and expensive- but they are the right thing. 

 

Firestops in the form of wood around windows is fine. Ensure you cut the battens above the windows short so the air can get it. 

 

 

 

 

 

 

 

 

Edited March 4, 2023 by SuperJohnG
Adding images

[markharro]

On 20/12/2021 at 03:21, AliG said:

I note looking online and from another thread that the Envirograf ones may be more like £7 a metre which would help.

Hi @AliG I am just at the stage now of having to check this tedious fire barrier stuff - like you it seems we are having open timber cladding and it seems totally pointless. Our vertical battens are 45mm deep and horizontals 22mm giving 67mm in total. Is that similar to you? What product did you end up using - was it the Envirograf ?

 

Also how did you deal with the Envirograf being blue I think and the Tenmat red when, you like, we are looking for something black to camouflage with our black membrane and battens etc?

 

thanks.

 

PS did you ask Building Control if you could just dispense with the stuff for the wall sections where you are using open cladding? Same with the insect mesh - both seem pointless and just more money down the drain.