Insulated concrete slab for workshop

[Henrik]

Not sure if this is in the right section, but didn't really feel like "project" or "tools and materials" either...

Anyway, I have a space of about 6.4 (width) and 6'ish length where I want to build a reasonable workshop. The walls will probably be double skinned breeze blocks (maybe with render or fake wood cement cladding), with sound absorbing insulation in between the skins.

I will build it slightly smaller than the available size, because I want to be able to fix issues around the building, so probably leaving 0.4-0.5m per side (left, right, back), so internal floor area will definitely be < 30m2.


However, when it comes to the slab, I want to insulate it as much as possible, but it seems difficult to find online how to actually do it, hence posting here.


My plan is to build something a bit like a raft:

- dig down footings about 30cm deep (reasonable ground), and use rebar cages here.
- in the middle of the slab, remove enough material to be able to add some hardcore + sand, and also 100mm celotex or similar, and then 100mm concrete with rebar mesh
- DPM on top of the celotex and also into the trench and up to the sides.
- poured in one go

The question really is - what do you do with the side of the footings? I don't particularly want a cold bridge between the wall and the slab, so to speak, and I'm assuming I can't just add celotex to the side of the trench and under it? where and how do I terminate the insulation?


Other alternative would be normal strip foundations, but then I'd have to get the concrete lorry in twice, and the single pour of the raft would get me "out of the ground" quicker, so to speak.

Access is difficult, but I have just about (3cm spare) space to get a micro digger in, and will get a concrete pumping firm to deliver the concrete.


Picture of site as it stands:

 

[gaz1]

i find it pointless insulating a slab unless your intending to fit concrete floor underfloor heating in it

as goes for choice of blocks pay the little extra and buy concrete blocks there just a little dearer and are better in the long run compare to skin flints buying breeze blocks

for the slab some info here

Garage rebuild

22 years after moving in and being told the garage was fallling down its time to rebuild it.The floor is cracking from front to rear and the walls are moving outwards so this will be new foundations upwards.House is a bog standard 1930's semi with a shared drive and I get on fine with my...

www.mig-welding.co.uk

 

[Henrik]

I want to insulate it, because I may at some point want to use it as an office, and tbh in Scandinavia we insulate the crap out of everything and it's just nicer to be in an insulated space IMO

Will have a look at that thread

 

[gaz1]

i know a few versions that can do it all depends on how much cost you want

scandinavia

now you can go down icfs

but theres a but id advise looking up icf problems uk sefers from other pests just as much as the usa and it has issues

im aware of another type which can give r24 concrete though

 

[Jonjws]

Evening all

My first post to the forum, standard floor slab regs at the moment are for domestic habituated floor slab. Excavation to sub soil depth, 150mm of compacted hardcore/ type 1, sand blinding, 1000 gauge dpm and 100mm of concrete on 100mm of rigid pir insulation ,the thicker the pir insulation the warmer the floor

 

[Henrik]

Evening all

My first post to the forum, standard floor slab regs at the moment are for domestic habituated floor slab. Excavation to sub soil depth, 150mm of compacted hardcore/ type 1, sand blinding, 1000 gauge dpm and 100mm of concrete on 100mm of rigid pir insulation ,the thicker the pir insulation the warmer the floor

aha, so basically I'd pour footings / normal strip foundations first, then build walls (at least up to floor level), and then pour the flor slab afterwards?

 

[Pete.]

50-100mm jablite polystyrene with DPM on top then just pour your slab with a layer of mesh.

 

[Gareth J]

Have a look at some detailing of passivhaus designs. While it wouldn't be reasonable to expect your shed to meet that spec, you're sensible to be concerned about cold bridging near walls. Would be good to fully isolate the slab one way or another and I suspect there'll be lots of options for doing so.

If there's ever a chance it gets heated, I'd double the insulation (maybe triple to 300mm!) And include ufh pipes. Even if you never use them. Relatively easy to upgrade wall/roof insulation.

In an unheated shed though, might an insulated slab actually get colder than one sat on the earth?

 

[brightspark]

50-100mm jablite polystyrene with DPM on top then just pour your slab with a layer of mesh.

yep did that with my sitting room floor apart from the mesh its the only floor in the house thats insulated and you can tell the diffrence in temperature between the ones that havent been done .used 100mm

 

[Roger440]

Evening all

My first post to the forum, standard floor slab regs at the moment are for domestic habituated floor slab. Excavation to sub soil depth, 150mm of compacted hardcore/ type 1, sand blinding, 1000 gauge dpm and 100mm of concrete on 100mm of rigid pir insulation ,the thicker the pir insulation the warmer the floor

Great in theory, but british weather being what it is, danger is the PIR gets wet. If it stays wet, it degrades.

Better with EPS. If rather more expensive.

As it was a raft, also did up the sides..

On top was an insulated steel building, so effectively a complete insulated shell.

I know Gaz says its pointless unless heated, but i disagree. Mine wasnt heated, but you could quite happily sit on the floor, even in winter, and you didnt get cold like you do on a regular garage floor.

It was very thermally stable, and dry as a consequence.

For those interested, here a build thread: https://www.garagejournal.com/forum/threads/my-gab-in-the-uk-thread.265998/

 

[Jonjws]

The pir sits on top of the dpm membrane so should stay dry, the membrane folds up the sides of he perimeter wall, installed loads of them and never had any problems with damp

 

[Roger440]

The pir sits on top of the dpm membrane so should stay dry, the membrane folds up the sides of he perimeter wall, installed loads of them and never had any problems with damp

Should. Unless it rains before the concrete goes down. Then its trapped forever.

In reality, they will often get wet this way. The concrete gets laid because no one really cares. By the time its a problem, its 20 years later.

Of course, if its inside a watertight building at the time of laying, then its all good.

 

[gaz1]

Better with EPS. If rather more expensive.

I know Gaz says its pointless unless heated, but i disagree. Mine wasnt heated, but you could quite happily sit on the floor, even in winter, and you didnt get cold like you do on a regular garage floor.

For those interested, here a build thread: https://www.garagejournal.com/forum/threads/my-gab-in-the-uk-thread.265998/

heres my knowledge of the stuff

eps is waterproof except its joints where you foam them to join them that is the weak point water can get through these points

now whilst you build a concrete slab on them that slab can and probably will sink over time due to weight upon them as your dealing with compression between the slab and the soil

so if the weight is more on the edges lets say double built brick wall you have more downwards force on those edges than in the middle thats not an equal push downwards on that slab

that slab can also shift rather well on those eps sheets more so in earthquake areas

however a brick wall built building with a floating insulated floor is the better type

 

[DAPPH]

Our conservatory built 22 years ago had to have the foundations dug in sandy free draining soil 1200 mm deep by 700 wide the floor depth was 800 mm
IIRC once the cavity wall was built to DPC level above ground the hole for the floor was filled with 200mm with crushed tamped down limestone , blinded with sand then the waterproof membrane was put down and almost 350 mm in bags & lose fill of round lightweight artificial aeriated insulation granules another sheet of DPC then a 150 mm concrete capping , floor screed for tiling was 50 mm thick


The floor heat retention and insulation was amazing

 

[DAPPH]

Should. Unless it rains before the concrete goes down. Then its trapped forever.

In reality, they will often get wet this way. The concrete gets laid because no one really cares. By the time its a problem, its 20 years later.

Of course, if its inside a watertight building at the time of laying, then its all good.

IIRC,
I asked much the same before the people who constructed the shell of the our DG glass conservatory with tinted glass roof to planning regs & BCO approval turned up.

They said that over the years any moisture in the slab would slowly evaporate ( it was laid in a rather wet March 2002 , a few days later the slab had at least three inches of standing water on it for three weeks or more . The basic construct took place with a good inch of standing water inside the construct.
Once the glazing & the two sets of double doors were in place, I used our Vax wet & dry to suck out the rest of the standing water and left the windows slightly open for three weeks, till the floor screed was laid and polished flat
The people who laid the imitation thermoplastic wood strips said much the same, that any moisture would eventually evaporate out without any problems due to ventilation, much the same as a new build house does over two to four years after being built.

When they came to do the floor laying, they put a sealed down " clamshell " moisture detector on the finished floor, with in six hours it had beads of water showing in the dome and a saturation reading for water. a week later it was showing 24 % moisture content when they returned ... still a lot of moisture coming out the slab as the exothermic chemical reaction in the concrete was still happening.

They simply made up and poured a clear acrylic sealer over the whole floor, used a spiky roller to disperse it level, four days later when fully set, they applied a very thin levelling compound to the acrylic then two days later laid the floor strips. We never saw any evidence of condensation or mould in the next two years we lived there.

 

[DAPPH]

Not sure if this is in the right section, but didn't really feel like "project" or "tools and materials" either...

Anyway, I have a space of about 6.4 (width) and 6'ish length where I want to build a reasonable workshop. The walls will probably be double skinned breeze blocks (maybe with render or fake wood cement cladding), with sound absorbing insulation in between the skins.

I will build it slightly smaller than the available size, because I want to be able to fix issues around the building, so probably leaving 0.4-0.5m per side (left, right, back), so internal floor area will definitely be < 30m2.


However, when it comes to the slab, I want to insulate it as much as possible, but it seems difficult to find online how to actually do it, hence posting here.


My plan is to build something a bit like a raft:

- dig down footings about 30cm deep (reasonable ground), and use rebar cages here.
- in the middle of the slab, remove enough material to be able to add some hardcore + sand, and also 100mm celotex or similar, and then 100mm concrete with rebar mesh
- DPM on top of the celotex and also into the trench and up to the sides.
- poured in one go

The question really is - what do you do with the side of the footings? I don't particularly want a cold bridge between the wall and the slab, so to speak, and I'm assuming I can't just add celotex to the side of the trench and under it? where and how do I terminate the insulation?


Other alternative would be normal strip foundations, but then I'd have to get the concrete lorry in twice, and the single pour of the raft would get me "out of the ground" quicker, so to speak.

Access is difficult, but I have just about (3cm spare) space to get a micro digger in, and will get a concrete pumping firm to deliver the concrete.


Picture of site as it stands:

View attachment 367257

That's very sensible using H section concrete posts .
Panels trimmed to fit in a jig that's put in a safe place under cover can be used to check any replacement panels which can be planed to size with an electric planer if needed once thy are well and truly dry .

My 15 yr old well past their life span 22 replacement panels had to be laid flat on pallets with breather batons between each panel for air flow for 10 weeks under a waterproof covering , till they had dried & shrunk enough to slide down the channels in the posts both in thickness and length as they were stored outside in the builders' merchants & were sopping wet & very heavy when they were delivered here.

Before installing them I spent several days spraying them with a Waterproofer & anti rot agent twice on both sides then hoisted each one up a thin 12 foot flagpole steel tube ( made up of interlocking sections of 700 mm legs off an old scrapped 12 foot diameter trampoline using a Haltrack style six to one pulley set and spreader bar with ratchet strap retainers pole on a spike welded into a 400 mmx 400 mm base then lowered the panels down the post channel by one person ( me .... a well disabled old fella ) .

 

[Pete.]

heres my knowledge of the stuff

eps is waterproof except its joints where you foam them to join them that is the weak point water can get through these points

now whilst you build a concrete slab on them that slab can and probably will sink over time due to weight upon them as your dealing with compression between the slab and the soil

so if the weight is more on the edges lets say double built brick wall you have more downwards force on those edges than in the middle thats not an equal push downwards on that slab

that slab can also shift rather well on those eps sheets more so in earthquake areas

however a brick wall built building with a floating insulated floor is the better type

If concrete sank into polystyrene over time then all the houses that have ever been done would now have huge gaps under the skirting boards and the door bottoms wouldn't seal.

This time last year I had to demolish a 400 ton 18metre square slab floating on 50mm of foam insulation (did it in 12 days BTW cut into 320 blocks and lifted out). It DID compress the foam but only from the initial weight not over time and polystyrene is many times more rigid than that acoustic foam that comes on a roll. Aside from that I've demolished a great many slabs on top of polystyrene. Driving a 5 ton demo robot across the concrete you would not know the foam was there.

 

[Gareth J]

EPS is an open structure. It's a bit vapour permeable. That doesn't mean it shouldn't be used, can even be beneficial. Just correcting a misconception somewhere above.

It also does have pretty decent comprehensive strength, particularly for its weight. Has been used as the bulk for motorway slip roads.

 

[Henrik]

Do you guys think something like the above would work?

It’s not a raft obviously, but maybe it’s easier for a hobbyist like me and my aging uncle to construct.

Would need DPC etc etc added, and the MOT layer probably a sand blinding under it too

 

[gaz1]

im going to give you this

an update with solar water heating of the garage