Internal stud at 600cc when the roof trusses are 400cc

[saveasteading]

Up til now I had thought how easy it will be to stud internally up to the exiting roof trusses. An approximate cut length and a screw into the side and it is done.

 

However, as it just occurred to me, 600cc is plenty for internal use, and it reduces material and labour costs, and also reduces the cold bridge of the timber and improves insulation

 

How best to do this? 

So far I have only thought of fixing studs at 1200cc to the rafters then bridging between them close to the top (at a useful position), then fixing an in-between stud.

More work, more wood.

 

These studs will vary in height from 2.5m to 3.6m. This is the highest section.

Studs probably 140 deep, to receive PIR, and for stiffness at this height.

 

This is for the building warrant application, so isn't a terminal decision, but it would be nice not to need to change it.

 

Edited October 25, 2021 by saveasteading
correction

[Iceverge]

What a lovely project. It's nice to see old buildings being looked after. 

 

I would probably stick at 400mm centers as it is much easier to fix. Just set your sole plate. Cut your verticals roughly to length. Tie a string in the right place, nail the top horizontally to the rafter, get a mate with a level to toe nail the bottom. Job done. 

 

To save cost, drop from 140*45 studs to 88*38 CLS. Doing anything too clever with 600mm centres will probably only increase labour I suspect. 

 

I have potentially a similar project somewhere in my distant future. I would like to set my stud wall at least 50mm back from the outside stone wall, put a an airtight layer inboard of the studs and blow eps beads between the studs and between the studs  the wall. 

 

Then counterbatten 45*45 with 50mm batt insulation and plasterboard. 

 

Can anyone see any issue with this? 

Edited October 25, 2021 by Iceverge

[saveasteading]

I agree re the labour on the timber, and another element of wood to add back in to the equation, but perhaps a detail will present itself.

On the other hand, cutting PIR into the voids is also a horrible job, and that labour is reduced too.

 

I didn't mention that the rafters are very approximately 18" c/c. Can be anywhere from 380 to 420 gaps. I thought these old builders were more precise than that. Perhaps with 600cc the c/c will average out. Every piece of PIR in the roof will have to be measured.....grrrrrr.

 

As posted elsewhere, we have rerun this wall liner with 100mm studs and it is good for insulation, so that will save us a lot of timber.

Also I have found that standard CLS (or near) can be much cheaper if bought by complete pallet, effectively straight from the docks with no handling by the merchant.

I'd like to use local timber, but the mills say that Russian/Finnish will be cheaper for CLS. I'm not so sure.

published local price for C16 treated is 

100mm x 50mm x 3.6m

£9.18 Excluding VAT

 

that is per one, collected, so there will be discounts.

 

We will 100 x 50 (or 38)  at 600cc and reconsider later, especially for the longer sections.

Then if we revert to 400cc there will be no prob from the BCO.

[Iceverge]

9 hours ago, saveasteading said:

Every piece of PIR in the roof will have to be measured.....grrrrrr.

 

Batt insulation, sprayfoam or blown up in. Followed by a membrane and insulated service cavity

 

Life is too short. 

[saveasteading]

24 minutes ago, Iceverge said:

Batt insulation, sprayfoam or blown up in

Batt insulation is half the insulation performance and isn't enough

Sprayfoam, no never.

blown in....polystyrene balls or newspaper....no thanks.

 

One chance to get this right.

[Iceverge]

What U value do you need? 

 

How deep are your rafters?  

 

 

[saveasteading]

150 rafters, with 20mm ventilated sarking then slates.

There is no prescribed U value as it is a conversion. 'As good as reasonably practicable' which includes cost.

Aiming for new-build standards where possible then will use pragmatism.

Can't offset it all in walls or roof due to the geometry (narrow and sometimes low)

[Gus Potter]

1 hour ago, saveasteading said:

150 rafters, with 20mm ventilated sarking then slates.

There is no prescribed U value as it is a conversion. 'As good as reasonably practicable' which includes cost.

Aiming for new-build standards where possible then will use pragmatism.

Can't offset it all in walls or roof due to the geometry (narrow and sometimes low)

This as "reasonably practicable" crops up from time to time and is mentioned in the Scottish regs for conversions.

 

A few decades ago we just used to line out these steadings in Scotland with 50 x 50 timber with brackets back to the stone walls, a bit of 50mm cellotex between and off we went. Now I suppose I'm a bit more cautious. The insulation requirements are more onerous, we have less drafts and now we worry about dew points and so on as we are insulating these places to death. These days I want to know bit more before hanging my coat on a peg. I have mentioned Scotland as the climate is much different and challenging for a designer.

 

Saveasteading has posted some photos of a cracking old steading that could be frankly stunning. Hopefully more to come. Looks like there could be the odd structural issue but nothing drastic.

 

For me steading conversions in 2021 are a different animal from the ones I did 30 years ago..

 

@saveasteading maybe this is the time for a review of what you have. There comes a time in a project on say a steading conversion when you need to make some decisions on how you approach the design in detail, you start getting bogged down in the regulations, insulation for example.. as far as reasonably practicable...maybe it's time for a wholesale review of where you are at. You invest so much time on just getting the plot that for a while you can take your eye off the ball.

 

To make your case for "as far as reasonably practicable" I would start with the founds and the ground under. You almost have to tell a story.. just like I'm doing here in some ways.

 

The soil and water table. Is the soil say clay, that can transmit moisture or is it gravel. Say you have gravel /sand soils. Is the gravel / sand igneous, if so the porosity of each particle will be very low. In other words. I could have a pail of glass marbles. Each marble is water tight. Put water in the bottom of the pail and it won't get to the top. But add to the marbles some clays, silts, organic material and the water can rise a little. This is called permability. Describe the soil, behavoir and work up.

 

Say you have demonstrated that ground water / significant moisture can't get near the floor level and that you have a gravel soil that does not promote frost (ice) crystal grouth. That solves the damp proofing and frost protection issue. For all..yes you can have a foundation formation level less than 450mm below finished ground level.

 

The next thing to would be to ask.. how do we heat the place? Are we going for underfloor heating? We need to comply with the regs. To do so we are having a slab with perimeter insulation?, it is all air tight?. Now we have a stone wall just behind the insulation above the floor level envelop that is freezing cold. You invite trouble in the long term as it's an often unventilated space and is right in the cold spot? That leads to water vapour that can't get out and manifests as condensation..often called "rising damp"

 

I have chucked a spanner in the works here to demonstrate a point. It's all about understanding how an old structure works, what happens when you insulate it, understand how modern approaches of say insulated slabs for underfloor heating work. Appreciate where too much insulation could actually cause a problem. Once you get a handle on this and get your head around it you'll probably find that BC just love you! and you'll just not sail through the warrant process but also know you really understand what you are doing.

 

Take a step back here, work from the ground up and be sure of your ground (excuse the pun) Understand the consequences of where you place insulation and why you are doing so. Do this and you will be able to satisfy the "as far as reasonable practicable" requirement. That in actual fact should be the least of your worries. You need to satisfy yourself that you are right!

 

In summary I would want to review / look at what you have. Take a holistic view, yes you can often only do this if you have time on your side. Barn conversions suffer from the lack of width, a few extra inches can make a world of difference.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[saveasteading]

My habit is to do holistic designs, ie everything considered before we even submit for Building Warrant/ Regulations. The alternative is a linear process where everything is made to work with what was before it. Hence I am hoping not to have any regrets about previous decisions, and to keep the cost to our demanding budget.

The cost difference for the same quality can be huge, from experience. Even worse if it is out of close control.

 

Hence the original question. Yes we can change our minds, and tweak as we go,  but I would rather not.

 

Re Gus's points.

We are lucky with the ground. Or rather, we knew what the ground was before committing to the purchase. It is pure sand, other than some cobbles, probably for 10m. Research shows that this was dropped by glaciers, some time ago, hence the sand is single sized, as are the cobbles, and the drainage is excellent.

 

Heating. underfloor, from Air Source unless anything changes soon. Have discounted boreholes as they are expensive to install and can go wrong.

Could still be persuaded to go with slinky in the ground, as the ground is easy to dig, and there is enough land. This is not ground source really as the heat comes from the sun and air, and is seasonal, as is ASHP.  Composite would be good but probably too complex.

 

Insulation: if we put lots in the walls, the rooms become too narrow. Lots in the roof, and some head-rooms become too low. Lots in the floor and we bump heads on existing lintels.

Dig out the floor and risk disturbing the shallow footings......and the floors are at varying levels anyway......and so we juggle all of these.

 

A section of the building has moved....consequently the rafters have rotten ends, and the walls are in a poor state. I wanted to hoist and repair with splicing but am over-ruled but, fundamentally, no local joiners were interested anyway.  So we will remove that bit (20%) and rebuild all in timber. When built nobody will know the difference.

 

This is all so much more difficult than new build.

 

[Gordo]

38mm pre insulated plasterboard VCL on 50 x 50mm treated studs at 600mm crs. 75mm gap to walls with stainless steel brackets from studs to walls mid span. 100mm drytherm 32 semi ridged mineral wool between studs. Leaving 25mm gap to walls. This will give a U value about 0.25 roughly. The gap should be left open to vent condensation into ventilated roof void. Best to fit a horizontal intumescent cavity barrier at top of cavity (fixed to horizontal timer with 50mm gap) just in case there’s a fire this will allow for ventilation.

 

The studs aren’t structural just need firm grounds support timber  at tops fixed to US rafters.

 

do not pump the cavity with any form of insulation period.

 

there are indeed minimum U values requirements for renovated external walls

Edited October 27, 2021 by Gordo

[saveasteading]

Thanks Gordo

 

How interesting. A very different approach.

 

My dislike of insulated plasterboard is the cost. That is near to £50/sheet which is an expensive way to buy polystyrene.

 

50 x 50 would certainly be  a lot cheaper, but the timbers are seldom straight and I would be concerned about alignment, even with a fixing strap, which I don't fancy screwing into granite too often. May consider for shorter heights..

(I have had 50x50 which had a 90 degree rotation in the length, as supplied.)

 

Does 100mm dritherm hold between 50mm studs? It is half the price of PIR but half the insulation, so an interesting balance.

 

Needs a vapour barrier behind the studs, which will clash with the dritherm, and an airtight barrier on the room side.

25mm gap yes.

Do you mean intumescent to stop ventilation in case of fire, rather than 'allow'? 

Anyway there is only the polystyrene to burn, as wood seldom does without a lot of heat, so I don't see this as necessary

 

There are targets for insulation in conversions, but not regulations.

 

Conversion of Traditional Buildings Whilst achieving the U-values recommended in clause 6.2.6 and 6.2.7 should remain the aim, a flexible approach to improvement should be taken, based upon investigation of the traditional construction, form and character of the building

 

 and   ......other than where proposed works are wholly categorised as a conversion, where the standard in question may be met as far as is reasonably practicable

 

However we are aiming for something better than the recommendations, hence the juggling.

I do appreciate your comments, but need further convincing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Gordo]

10 hours ago, saveasteading said:

Thanks Gordo

 

How interesting. A very different approach.

 

My dislike of insulated plasterboard is the cost. That is near to £50/sheet which is an expensive way to buy polystyrene.

 

50 x 50 would certainly be  a lot cheaper, but the timbers are seldom straight and I would be concerned about alignment, even with a fixing strap, which I don't fancy screwing into granite too often. May consider for shorter heights..

(I have had 50x50 which had a 90 degree rotation in the length, as supplied.)

 

Does 100mm dritherm hold between 50mm studs? It is half the price of PIR but half the insulation, so an interesting balance.

 

Needs a vapour barrier behind the studs, which will clash with the dritherm, and an airtight barrier on the room side.

25mm gap yes.

Do you mean intumescent to stop ventilation in case of fire, rather than 'allow'? 

Anyway there is only the polystyrene to burn, as wood seldom does without a lot of heat, so I don't see this as necessary

 

There are targets for insulation in conversions, but not regulations.

 

Conversion of Traditional Buildings Whilst achieving the U-values recommended in clause 6.2.6 and 6.2.7 should remain the aim, a flexible approach to improvement should be taken, based upon investigation of the traditional construction, form and character of the building

 

 and   ......other than where proposed works are wholly categorised as a conversion, where the standard in question may be met as far as is reasonably practicable

 

However we are aiming for something better than the recommendations, hence the juggling.

I do appreciate your comments, but need further convincing.

 

 

Quote

 

 

Quote

 

Pre-insulated plasterboard usually use polyurethane insulation. don’t do this if you don’t think it is good value for performance. But it does make quite a difference in eliminating cold bridging at studs etc.

 

If you can’t get decent quality timber which sound strange (likely poor storage in your area pre delivery) you can substitute with more stable 50mm metal stud (which is economic and easy worked with) or even 100mm timber studs which is more expensive but I have seen quite a few 50mm stud walls over years, but not so popular nowadays. 
 

While drytherm may be half the performance it is 100mm thick! You can increase this to 150mm if you like. What U value do you aim for? It will compression fit hold fast between studs. If you don’t trust it then use some plastic netting like used on suspended floors or PVC coated tie wire runs. The big advantage of Dry therm is how easy it cut with a blade and how well it compress fits to fill gaps at studs. The VCL goes on warm side of studs obviously. This will not clash with drytherm and will act as air seal also.

 

All cavities require cavity barriers to limit the spread of fire. The intumescent cavity barrier will allow ventilation but in the event of a fire will expand and seal the cavity. Unless you use A1 non-combustible products they will burn and fire will spread. It’s you and your family's life if you want to take chances.

 

I said there were U value requirements I didn’t say U values were regulations. I am not intimidate with Scottish B Regs but like all regions assume there are reasonableness get outs clauses but unusually this is only on grounds of excessive payback period or excessive encroachment on room size. You’ll have to argue this with your BCO. But do you really want to cheap out on insulation?

BTW the existing A frame roof structure looks a little ropey. Are your walls still plumb or have they been pushed out any? Is it a tin roof? Be very careful about adding ANY additional loading without thinking about improving the roof structure 

Edited October 28, 2021 by Gordo

[Gordo]

I would suggest you build stud off the concrete screed, leaving a small gap between screed & walls at perimeter with DPM below wrapped up and over screed (under the sole plate of stud) this will allow a drainage trough just in case. If at all possible (I know it’s probably too muck an ask) provide a few low level drainage/vents at low level with some insect screening.

Edited October 28, 2021 by Gordo

[Iceverge]

On 26/10/2021 at 23:18, saveasteading said:

Sprayfoam, no never.

blown in....polystyrene balls or newspaper....no thanks.

 

How come?

 

I would be more anti PIR in a historic building. It's lack of breathability would be a worry. 

 

 

[saveasteading]

I don't like spray foam anywhere. Once in place it doesn't come out again. Also uncontrollable, as we need a ventilation gap and the only way would be to make a box and fill it

And it doesn't breathe.

Blown in also needs to be contained.

 

I am not a fan of PIR but this will be within a box contained by VB on the outside, and polythene on the inside, so breathability isn't an issue.

I originally favoured wall batts, but they have half the insulation, and we don't want to lose floor space  or insulation.

 

breathability will be a 25mm gap between the masonry and the vapour barrier, up to the roof where the same applies. by all accounts we don't have to worry about draughts through  the wall, as there will be plenty. But the BCO may want more holes, because he wants holes.

[Gordo]

49 minutes ago, saveasteading said:I am not a fan of PIR

I am not a fan of PIR

- totally agree. Mineral wool like drytherm is very breathable and no toxic off gassing.
 

breathability will be a 25mm gap between the masonry and the vapour barrier,

- make sure the vapour barrier goes on the warm side of insulation not the cold side. Breather paper can be used on the cold side if desired.