Minimum height of insulation build up on concrete slab

[seano]

I'm new here and loving the knowledge of users.
Were looking at converting a portal barn that already has PP. It has an offset ridge and so one roof slope cones down quite low at the eaves. We want to retain the existing concrete slab and insulate over it but want to find the thinnest material build up that will still meet regs for U values.

So our bottom layer is 100-150 slab and top will be 65mm screed with UF heating. What can we put between these two?

Thanks

Edited February 20, 2023 by seano

[IanR]

13 minutes ago, seano said:

I'm new here and loving the knowledge of users.
Were looking at converting a portal barn that already has PP. It has an offset ridge and so one roof slope cones down quite low at the eaves. We want to retain the existing concrete slab and insulate over it but want to find the thinnest material build up that will still meet regs for U values.

So our bottom layer is 100-150 slab and top will be 65mm screed with UF heating. What can we put between these two?

Thanks

 

Welcome @seano

 

Ideally you'll want 200mm of PIR insulation so that the UFH is efficient. Regs will allow you to drop lower, but I'll let others say by how much as I've not looked at this since the U Values were recently changed.

 

But, a few questions first.

 

Do you own the barn? have you had a good look at the levels of the floor and the condition of the bottoms of the portal frame? Agricultural barns/sheds will typically have a fall on their slabs. If it was built for animals then it could be quite significant for draining slurry. My own had a 200mm fall from one end to the other.

 

Often the base of the portal uprights, which are likely bolted to a foundation pad, are then concreted over when the slab gets poured. It can trap water, or worse, slurry, that can corrode steel columns, or the reinforcement of concrete columns.

 

It may be worth considering taking the existing slab out, and digging a bit lower to install an new insulated raft foundation.

[Nickfromwales]

2 minutes ago, IanR said:

It may be worth considering taking the existing slab out, and digging a bit lower to install an new insulated raft foundation.

There's no "may" about it, and it would be the best money spent.

 

Ola @seano, welcome to the madhouse  

28 minutes ago, seano said:

We want to retain the existing concrete slab and insulate over it but want to find the thinnest material build up that will still meet regs for U values.

If I can be blunt, no, you do not want to.

It doesn't sound like you'd have room to ad the minimum 140mm of phenolic rigid insulation which would be the very minimum insulation I'd ever install for a client if they wanted UFH. If the SE says this can all be dug out, then please do so! You'll need to examine the foundations first and strategize how this could be done ( by leaving sufficient undisturbed ground around footings / pads etc, but it would be well worth the effort. Plus, you'd end up with a new, flat, even slab to have underfoot. FYI, the UFH pipes would then reside in the concrete and all the insulation would go under the, then, constructional slab.   

[Iceverge]

Is it permitted to level it and start from scratch with a new TF that would match exactly the old building? 

 

[IanR]

49 minutes ago, Iceverge said:

Is it permitted to level it and start from scratch with a new TF that would match exactly the old building? 

 

 

2 hours ago, seano said:

Were looking at converting a portal barn that already has PP.

 

Assuming the permission is for a Change of Use Conversion, then no. But some LPA's will allow a CoU to be used as a fall-back for a New Build application, and that has to be the better option for all concerned if the primary frame of the existing building has no architectural or historic merit.

[seano]

Mmm, much food for thought here. We haven’t exchanged on it yet. PP will definitely not allow a new build as its in AONB and was apparently a tough process to get consent for a dwelling.

Im wondering what would be left to retain if the slab came out and the column pads need underpinning etc. Beginning to feel like too much effort/cost for little reward.

But out of interest, what might it cost to break out and replace 2600sq ft slab?

 

[Iceverge]

14 hours ago, IanR said:

Assuming the permission is for a Change of Use Conversion, then no.

 

I get how it's important to have the same look of building as essentially the countryside will remain unchanged aesthetically but from a building science its raving lunicy. 

 

Agri buildings are always very budget constrained and mostly planned with a lifespan of only a few decades. They don't have the structural margins required of domestic houses and are often made from carcinogenic materials like asbestos and creosote. 

 

Add to this the possibility of zoological diseases from old mould spores and animal waste I would favour taking some accurate measurements, before bulldozing the entire thing. 

 

"National Heritage" is not something that will suffer if someone takes a 1980's cow shed and replaces it with an exact replica. 

 

 

 

 

 

[Dave Jones]

you want 300mm of XPS, its a simple job to get the floor up (drains ???) and do the job properly.

[LSB]

I would only add, if you can get planning permission, over the border from you in Suffolk, that's just not possible in the countryside, believe me we've tried

 

[Nickfromwales]

1 hour ago, Dave Jones said:

you want 300mm of XPS, its a simple job to get the floor up (drains ???) and do the job properly.

200mm of XPS or 300mm of EPS ( much cheaper but you need to cost for the extra excavations / muck-away then ) so I'd go with 200mm of rigid which will be around Passiv standards. 300mm unnecessary.

[Nickfromwales]

240m2 / 2.88 ( an 8x4 sheet ) = 83 sheets for minimum advised thickness of 150mm of insulation.

83 x £75 = £5,810 for 150mm of kingspan / cellotex ( you may get cheaper using seconds / damaged boards ).

 

Then you'll need prob 120mm of RC35 concrete, laid in sections with expansion joints, with reinforcing / anti-crack mesh, and then UFH pipes get zip=tied to the mesh prior to the pour.

You'll need sand blinding, then 25mm of EPS as a sacrificial layer, then the DPM, plus some perimeter insulation skirting ( 30mm PIR ). 

 

Not inconsequential amounts of money / labour as that's a big old floor!

[Nickfromwales]

Whats your VAT status?

[Dave Jones]

52 minutes ago, Nickfromwales said:

240m2 / 2.88 ( an 8x4 sheet ) = 83 sheets for minimum advised thickness of 150mm of insulation.

83 x £75 = £5,810 for 150mm of kingspan / cellotex ( you may get cheaper using seconds / damaged boards ).

 

Then you'll need prob 120mm of RC35 concrete, laid in sections with expansion joints, with reinforcing / anti-crack mesh, and then UFH pipes get zip=tied to the mesh prior to the pour.

You'll need sand blinding, then 25mm of EPS as a sacrificial layer, then the DPM, plus some perimeter insulation skirting ( 30mm PIR ). 

 

Not inconsequential amounts of money / labour as that's a big old floor!

 

your right of course regarding muckaways but 300mm of XPS is still 40% cheaper than 150mm celotex for a much better U value.

[Nickfromwales]

17 minutes ago, Dave Jones said:

but 300mm of XPS is still 40% cheaper than 150mm celotex for a much better U value.

Where have you been getting prices?

[Dave Jones]

https://flooringwarehousedirect.co.uk/product/jablite-polystyrene-insulation-board-150mm-eps70-2400mm-x-1200mm/

 

Travis are cheaper for us though at £6.25 /m2

Edited February 21, 2023 by Dave Jones

[dpmiller]

XPS and EPS are a wee bit different y'know...

[Dave Jones]

50 minutes ago, dpmiller said:

XPS and EPS are a wee bit different y'know...

 

i was referring to and linked EPS, unfortunately i have dyslexia so autocorrect doesn't always catch everything.

 

Here is the standard buildup giving a U value of 0.103 against a min spec of 0.13

 

Edited February 21, 2023 by Dave Jones

[Chanmenie]

So 2 layers to give 300mm at £30 a sheet is £60 plus the extra muck away won’t be much if any cheaper than 200mm PIR.

I used 2 layers of 100mm Celotex so 88 sheets £4500 

[Nickfromwales]

3 hours ago, Dave Jones said:

i have dyslexia so autocorrect doesn't always catch everything

My wife is the same. I go to the shops and open her text for the shopping list and just stand there asking myself WTF half the things are on the list. When she calls out what they are, it's spelt how she thinks the words sound. Then I say "oh, yea".

[Nickfromwales]

2 hours ago, Chanmenie said:

So 2 layers to give 300mm at £30 a sheet is £60 plus the extra muck away won’t be much if any cheaper than 200mm PIR.

I used 2 layers of 100mm Celotex so 88 sheets £4500 

Can you share where you purchased for the OP, please?  And how long ago, of course, as the world's gone entirely tits-up over the last few years.  

[IanR]

13 hours ago, seano said:

Im wondering what would be left to retain if the slab came out and the column pads need underpinning etc. Beginning to feel like too much effort/cost for little reward.

 

I'm not suggesting the pads will need under-pinning, just that the condition of the uprights is checked where they join their pads. As long as there are no signs of movement, the pads should be capable of continuing to hold the frame up, as long as you don't plan to increase the loads through the uprights with a new mezzanine or substantially heavier roof.

 

Your conundrum is how to support the insulation for the roof without overloading the existing structure. There are insulated profile sheeting panels from Kingspan that are light weight and maybe able to go straight on to the existing (or replacement) purlins, but they do come with compromises.

 

When I did mine I tried to avoid the compromises and built a new timber structure inside the existing portal frame that was filled with blown cellulose fibre insulation. The timber frame structure took the new loads straight down to the new insulated raft foundation and not through the portal frame. The portal frame remains in place holding up a light weight rain screen (standing seam aluminium roof in my case). 

 

It would have been cheaper to have removed the portal frame, but 8 years ago the LPA would not entertain a knock down and rebuild, so I had to retain the superfluous steel portal frame.

 

13 hours ago, seano said:

But out of interest, what might it cost to break out and replace 2600sq ft slab?

 

Surprisingly cheap, although I wouldn't be sure how much a today's prices. Two people with a 5T excavator, pecker and a dumper will have it out and levelled in under two weeks. If you've got some hardstanding to do then crush the concrete on site to use later. There will still be perhaps 100T to muck-away (or loose in a corner of a field if you have land) as you won't be able to reuse the hardcore that the existing slab is sitting on.

 

Taking the existing slab out then allows you to put in an engineered insulated raft that can be spec'd to take whatever loads you want to put through it, drop the finished floor level if need be (accepting that you may need to detail the raft around the existing pad foundations) and putting the new drainage in will be an easier task.

Edited February 22, 2023 by IanR

[Iceverge]

8 hours ago, IanR said:

would have been cheaper to have removed the portal frame, but 8 years ago the LPA would not entertain a knock down and rebuild, so I had to retain the superfluous steel portal frame.

 

This is completely bonkers.

 

It's not as if it's ye olde wall Henry VIII boinked an fluttering milkmaid against. 

 

Someone needs to challenge this legally.  

[seano]

Yep, that all makes sense.
I'm talking with the SE that did the calcs for planning, arriving at the view the the existing structure is capable of conversion top resi without much of a problem/ 

[Chanmenie]

9 hours ago, Nickfromwales said:

Can you share where you purchased for the OP, please?  And how long ago, of course, as the world's gone entirely tits-up over the last few years.  

June last year, a company in Norfolk 

https://easterninsulationsupplies.co.uk/

[seano]

8 hours ago, IanR said:

Your conundrum is how to support the insulation for the roof without overloading the existing structure. There are insulated profile sheeting panels from Kingspan that are light weight and maybe able to go straight on to the existing (or replacement) purlins, but they do come with compromises.

 

 What do you mean by compromises Ian?