Insulated Concrete Slab Garden Office - Questions

[Ticky]

Hi @Temp

 

I'm not going with that design now. I'm following the one both @IanR and @Iceverge landed on. See ian's post above.

 

I'm going to draw out my full design this weekend and I'll share on here

 

Edited April 21, 2023 by Ticky

[Ticky]

On 21/04/2023 at 11:59, Mike said:

MOT Type 1 is a specific Government standard for the sub-base of roads, with multiple requirements that must be met. To be precise, the specification is technically now 'Specification for Highway Works (or SHW) Clause 803 Type 1' - the specification was originally written by the Ministry of Transport, hence the snappier 'MOT' is still widely used.

 

Crushed hardcore is just that - it doesn't necessarily meet any specification at all. It could be close in composition to MOT Type 1, or not.

 

MOT Type 1 is frequently specified because there is no doubt that it will perform well long term. If you choose something different, you need to satisfy yourself that it's up to the job.

 

 

Thanks @Mike - so it sounds like crushed hardcore could be pretty much anything that's around 40mm in size.

Where would I buy pea shingle that's guaranteed to have no fines? No one seems to mention it in the descriptions and I don't wanna buy the wrong gear

 

I'm based in Manchester, so any pointers on where to source bulk bags of the above would be great @Iceverge @IanR @Temp

 

Thanks again for everyone's help so far. 

Edited April 24, 2023 by Ticky
updates

[saveasteading]

1 hour ago, Ticky said:

Pea shingle that's guaranteed to have no fines? 

It should all be no fines. 

I did buy a bag recently that was a bit mucky (some sand ) and would have rejected it if it had mattered.

[Mike]

7 hours ago, saveasteading said:

8 hours ago, Ticky said:

Pea shingle that's guaranteed to have no fines? 

It should all be no fines. 

Yes, pea-shingle should be just shingle - no sand or other fines.

[saveasteading]

A self-builder wants to fill an infinite hole with spheres to maximise the void space for drainage. All the same size.

Should they use spheres the size of peas, golf balls or cricket balls?

If you know, perhaps smugly don't answer yet.

[Ticky]

So, how does this look?

Any tips on the size of the piping used for the drain?

[Iceverge]

Looks fine. 

 

Mesh will be easier than Rebar I would say. 

[Ticky]

26 minutes ago, Iceverge said:

Looks fine. 

 

Mesh will be easier than Rebar I would say. 

Thanks, I actually thought they were the same thing.

What should I be looking at is it the A142 type? with mesh spacers to hold them up?

[Ticky]

Also, not sure I got the question about levelling/compacting the pea shingle. This is the bit that I’m most nervous about as most other concrete slab foundations look like they sit on type1 and sand. Paranoid as I don’t want to mess this bit up obviously. 

[IanR]

No need to compact the pea shingle, just rake it level. I'd say you don't need 150mm of hardcore, 100mm - 120mm is enough with the type 1 you have on top (save the dig).

 

A142 mesh is fine for horizontal, but I'd use 6mm or 8mm rebar vertically in the ring beam at 300 centres, tied to the mesh, then a long horizontal rebar 25mm from the bottom of the ring beam tied to the verticals. You could pierce the DPM and push the verticals into the EPS, and stick some Duct tape around the holes you've made. It would help stabilise the rebar for the pour. 

[Ticky]

34 minutes ago, IanR said:

No need to compact the pea shingle, just rake it level

Why is that? Is that because it should 'self level' in a way?

35 minutes ago, IanR said:

use 6mm or 8mm rebar vertically in the ring beam at 300 centres, tied to the mesh, then a long horizontal rebar 25mm from the bottom of the ring beam tied to the verticals.

I'm struggling to visualise that.... would that be 1 row of verticals? and then 4 long horizontals going round the perimeter to make a rectangle?

[saveasteading]

The layer of gravel confuses me. Am I forgetting some need for it?

Gravel moves all over the place so can't be walked on.

Rebar is jargon for steel reinforcing bars. Mesh is made of bars at preset centres for convenience in slabs. If the bars are different sizes, remember that the big ones may have to go in a certain direction.

 

My other concern is that the floor plate sits on a narrow strip of concrete.

Also the fixing is very near the edge and likely to fail. And take it deeper....the first 25mm doesn't provide worthwhile resistance.

With that tiny amount of side insulation, I'm not sure that the 200mm under the slab is necessary.

 

It's just all a bit weird. Sorry I don't have time to draw something else up.

On the other hand, I assume it won't be used an awful lot, and nobody is sleeping in it, so it is somewhere between a garden shed and a house.

 

[Nickfromwales]

On 15/04/2023 at 18:24, JohnMo said:

I just went dMEV, with presence sensor.  No-one in the garden room, ventilation is off. Someone in room for 90 seconds or more, fans runs for the time the person is there and an additional 30 mins.  Simple cost effective, ventilates only when required. No heat recovery but hay ho.

 

Plus points, only ventilated when needed.

Fan not running 24/7

No or little ventilation heat loss when ventilation isn't needed.

Very low electric consumption.

 

Cons

When someone is in room vent fan is running, with no heat recovery 

OK, more thoughts and planning are afoot!

 

I've decided on the "twin 3x2" separated stud walls and a decent insulated slab. I'll go 200mm EPS, but will focus on sound insulation* as well as thermal insulation. Pre-insulated metal profile sheet roofing, and a layer of acoustic batts ( 100mm ) will get bonded onto those, ideally, but then the question of getting plasterboard or white PVC cladding boards installed to the ceiling. May screw through the metal profile sheets from outside, with the proper screws, to grab a perpendicular batten at 400mm o/c and use that for mechanical fixings under the sheets.

 

*Reasons; DJ decks to go into this room for me to mess about on and relive my youth, in-between dealing with clients and being a grown-up, ergo I will need to keep the sounds ( not noise btw ) inside this room; so there's little to no sound pollution from it to piss the neighbours off ( well the ones who aren't bell-ends anyhoo!! ).

[Nickfromwales]

12 minutes ago, saveasteading said:

My other concern is that the floor plate sits on a narrow strip of concrete.

? It sits on the edge of the concrete raft? It site over a narrow 50mm of perimeter insulation.

12 minutes ago, saveasteading said:

With that tiny amount of side insulation, I'm not sure that the 200mm under the slab is necessary.

Lets remember this isn't a heated slab   Or is it? I'm putting a split A/C for heat / cool, plus an in-screed heater wire for when I just want heat but peace and quiet so I can speak with clients. Screed heater will be set to keep the room at 19.5 and A/C will be for auxilliary heat / cool aka "comfort". Was considering a Chinesium spa / hot tub heat pump to go into wet UFH, but you've gotta stop somewhere!!

 

I'd also just drill Hilti bolts in after the pour, vs try and set threaded bar in whilst pouring / levelling / finishing the slab. Don't see the attraction there tbh, unless this was having dozens and dozens of fixings and the price was set to go silly.

[saveasteading]

17 minutes ago, Nickfromwales said:

threaded bar in whilst pouring

I prefer threaded bar in epoxy, at least a week after pouring.

[Iceverge]

4 hours ago, Ticky said:

Thanks, I actually thought they were the same thing.

What should I be looking at is it the A142 type? with mesh spacers to hold them up?

 

A142 might be a small bit weedy. 

 

Maybe a sheet of A252 would be better 

 

I would happily place it in the slab as shown. One or two pieces of rebar around the perimeter would make a good job out of it. As shown by the dots below.  Frankly it's a lightweight garden room. It ain't goin anywhere MR. 

 

[Iceverge]

35 minutes ago, Nickfromwales said:

OK, more thoughts and planning are afoot!

 

I've decided on the "twin 3x2" separated stud walls and a decent insulated slab. I'll go 200mm EPS, but will focus on sound insulation* as well as thermal insulation. Pre-insulated metal profile sheet roofing, and a layer of acoustic batts ( 100mm ) will get bonded onto those, ideally, but then the question of getting plasterboard or white PVC cladding boards installed to the ceiling. May screw through the metal profile sheets from outside, with the proper screws, to grab a perpendicular batten at 400mm o/c and use that for mechanical fixings under the sheets.

 

*Reasons; DJ decks to go into this room for me to mess about on and relive my youth, in-between dealing with clients and being a grown-up, ergo I will need to keep the sounds ( not noise btw ) inside this room; so there's little to no sound pollution from it to piss the neighbours off ( well the ones who aren't bell-ends anyhoo!! ).

 

I think someone just wants a twinwall frame.......😂. 

 

Are you planning to hang the outer stud over the insulated upstand AKA MBC? 

 

TBH for a single story the extra stud outside the framing is a bit wasted in my opinion as you don't have to solve the first floor ceiling band area. 

 

If you are committed to the twinwall I would frame the outside wall with 95*44 or maybe 88 or 63mm CLS. 3X2's can be bandy pieces of sh*t. Stand it on a concrete ring beam or strip foundation. 

 

Pour your floor with say 100mm upstands. 

 

Then put a rockwool style batt between the framing. Joining up to the floor insulation. Then some rockwool batts "freestanding" inside. Then an internal wall with a VCL on the back stood up against the wall to make an insulated service void. 

 

This video shows it better. 

 

 

 

 

 

For better sound qualities line the inside with woodwool like Herkalith.

 

 

 

[LA3222]

Is it just me or is drifting into the realms of OTT? The slab is for a single storey garden room, not a great big multistorey house - all this talk of ring beams and the like is a lot of unnecessary time/cost. Yes to how its being insulated etc, no issue with the thickening per se....just stick A142 across the whole thing in the top layer, I really wouldn't bother adding layers in the 'ring beam' - this is basically a big shed🤷‍♂️ 

[Nickfromwales]

It's not going on Grand Designs lol, looks quite nice thought!

 

The 2x 3x2 walls are for separation for sound and it would be fully filled with acoustic batts ( 75mm between first studs > 50mm continuous layer in gap between > 75mm between second studs - so 190mm of acoustic ). Speakers will be on expensive heavy metal speaker stands with ground plane spikes on rubber pads.

 

I'd sister the 3x2's internally, and install at 400 o/c, and have a single synthetic 'strap', eg to tie the internal and external stud walls together whilst still maintaining acoustic qualities.

 

Won't be flimsy at all, 'cos I'm making it and I "have the technology"  

 

I'm not in love with twin-wall, just seems a very good solution considering I'll be "in the mix" and will need the thickness and acoustics to work well. I'll be able to stick-build the whole thing in less than 4 days including the roof on my Jack Jones. Labour cost = 2 weekends.

 

12 minutes ago, LA3222 said:

Is it just me or is drifting into the realms of OTT? The slab is for a single storey garden room, not a great big multistorey house - all this talk of ring beams and the like is a lot of unnecessary time/cost. Yes to how its being insulated etc, no issue with the thickening per se....just stick A142 across the whole thing in the top layer, I really wouldn't bother adding layers in the 'ring beam' - this is basically a big shed🤷‍♂️ 

I 1,000,000% will NOT be putting a ring beam in. That's one thing I've been reading and cannot agree with. This is a single storey wood structure, and the loads will be the square root of F - all . FYI, I'll be cutting the fingers off the edges of the mesh sheets to give me a linear re-bar along the perimeter of the slab, 4 sides around, and if that breaks then I'll buy everyone a drink.

Also, I'll be using EPS 100 everywhere and simply use a run of 89x44 as the soleplate atop the 100mm EPS100 upstand. That will still leave me a 25mm gap between the two soleplates at ground level.

End of Aloo Chat.

 

I'm off to start my cutting lists and order me some materials. "The time has come for action, people!!!!". 

[IanR]

4 hours ago, Ticky said:

Why is that? Is that because it should 'self level' in a way?

 

You are compacting the other layers to get rid of any voids, but pea shingle won't have any. If you did compact you'd push the pea shingle down into the Type 1 and then it would not do its job, which is to be free draining so that it clears any water to the French drain. It's not self levelling, so you are going to have to set yourself up some levels and then rake it to those levels.

 

4 hours ago, Ticky said:

I'm struggling to visualise that.... would that be 1 row of verticals? and then 4 long horizontals going round the perimeter to make a rectangle?

 

Yes, I believe you've understood my description. Hopefully this sketch doesn't make it worse. Black is mesh, blue is rebar:

 

 

1 hour ago, LA3222 said:

Is it just me or is drifting into the realms of OTT? 

 

It's not just you, my first post in this thread, the third post of the thread started with:

 

Quote

Here's my go at an over-engineered insulated raft, for a Garden Office.

 

When I posted my original sketch.

 

But, while we now have a view on the wall construction, we don't know the roof structure or finishes, it could be tile. In the future maybe there could be some PV. Maybe the floor finishes will be unforgiving to any movement, such as a ceramic or porcelain tile straight on to the slab. As part of an insulated slab (the title of the thread) the integrated ring beam is a very simple solution to de-risk the floor in my view.

 

4 hours ago, saveasteading said:

With that tiny amount of side insulation, I'm not sure that the 200mm under the slab is necessary

 

I believe we established it's an unheated floor, or at least I made that assumption in an earlier post. It would be nice to see more, but for an occasional use room with an unheated floor, I think what is shown is better than most.

Edited April 30, 2023 by IanR

[Iceverge]

4 hours ago, Nickfromwales said:

It's not going on Grand Designs lol, looks quite nice thought!

 

The 2x 3x2 walls are for separation for sound and it would be fully filled with acoustic batts ( 75mm between first studs > 50mm continuous layer in gap between > 75mm between second studs - so 190mm of acoustic ). Speakers will be on expensive heavy metal speaker stands with ground plane spikes on rubber pads.

 

I'd sister the 3x2's internally, and install at 400 o/c, and have a single synthetic 'strap', eg to tie the internal and external stud walls together whilst still maintaining acoustic qualities.

 

Won't be flimsy at all, 'cos I'm making it and I "have the technology"  

 

Tell me more! 

 

Any VCL going in? TBH if you are controlling the build yourself a taped external layer of OSB would suffice in my opinion. I'm coming strongly to the opinion that almost all interstitial condensation problems are due to bad airtightness. 

 

4 hours ago, Nickfromwales said:

I'm not in love with twin-wall, just seems a very good solution considering I'll be "in the mix" and will need the thickness and acoustics to work well. I'll be able to stick-build the whole thing in less than 4 days including the roof on my Jack Jones.

 

Nothing wrong with loving! I am a fan myself. 

[Nickfromwales]

1 minute ago, Iceverge said:

Tell me more!

I've never built anything wonky or flimsy in my life, as I am an OCD freak of many belts, and a comprehensive selection of braces to boot.

The 3x2's will feel solid and I'll get a great result from them, sister-ing up the internal 3x2's will pay big dividends and it's only an extra 4 dozen lengths, not exactly bank busting and prob £150-200 worth at most. With those being more solid than a single 4x2, and the single external studs being tied at the half height marker to these sistered pairs of 3x2, accompanied by lashings of glue and gas nails, and more gas nails, followed by more gas nails.... I'll be happy to proclaim; "She'll bee reet, laaaaad" 

[Nickfromwales]

I think I'll start by making the roof rafters up whilst I'm waiting for diggy-boy to get his skates on.

Prob going to be 4x2's with OSB plates at the ridge; a) for simplicity and pre-manufacture, but also b) to get a ready made flattened apex which I can then board to and recess spots into plus it'll form a cable way for the various drops to the power points / switches etc.

 

Ventilation remains a bone of contention. Single room MVHR are reported to be "crap", so I may just fit a small MVHR. TBH it is likely a WC / WHB / Wetroom shower pod will make it's way into what was going to be the PV / solar plant space, as a loo will be nice up there. I'll tank it 360^o round so it can be a guest suite with shower too, for overspill. When not occupied I will pulse the MVHR on / off ( 15mins on / 2hr off or something ) with a Shelly or get one where the trickle rate can be dropped to an absolute snail pace.

 

16 minutes ago, Iceverge said:

Any VCL going in? TBH if you are controlling the build yourself a taped external layer of OSB would suffice in my opinion. I'm coming strongly to the opinion that almost all interstitial condensation problems are due to bad airtightness. 

My dear boy, it'll have a full on AT membrane from the slab to the ridge, 4 sides around, installed directly to the internal studs, then a service batten ( 25mm ) for cables and back boxes, then 15mm OSB, then 9mm PB + skim. OSB for racking mostly, but nice for hanging anything anywhere too.  

AT is where it's all at, and detailing this meticulously stops a lot more problems than people appreciate.  

[Ticky]

20 hours ago, IanR said:

 

I believe we established it's an unheated floor

Yeah it will be unheated. 
 

It’s going to be used as an office just by myself. No great plans for the flooring. I even thought of it being a polished concrete finish (or something similar)

 

As for the roof, I was also planning on using insulated roof panels (again to increase head room) I believe these are pretty heavy (13.3kg/m2)

Edited May 1, 2023 by Ticky

[Nickfromwales]

27 minutes ago, Ticky said:

As for the roof, I was also planning on using insulated roof panels (again to increase head room) I believe these are pretty heavy (13.3kg/m2)

Yup, but they are a huge leap towards killing interstitial condensation issues ( which you'll otherwise get for sure the second you heat this somehow and it's cold / freezing outside ). Got the uninsulated ones on my workshop-manshed-office and the moisture on the inside of the breather membrane is a major piss-me-off. It'll become workshop / shed soon, so won't be heated anywhere near as much after the new outbuilding goes up.