Insulated Concrete Slab Garden Office - Questions

[Ticky]

@Nickfromwales You say heat it 'somehow' - you feel it's not going to be easy to heat with anything 'above' the concrete?

 

@saveasteading I'm definitely open to options when it comes to fixing the threaded bar/bolts - think I've seen the epoxy approach on Oakwood Garden Rooms. My concern in drilling once dry was that i'd be close to the edge of the concrete and don't want to cause it to 'blow out'.  If I have 5x2 soleplate, 50mm is covering the upstand leaving 75mm over the concrete. Image below is to scale.

 

Would aiming to fix the threaded bar, in the middle of that 75mm (approx 37mm in from the edge) be about right? And how deep does it need to go?

 

I'll have to make sure I'm not hitting any rebar/mesh too

 

 

 

[Nickfromwales]

6 minutes ago, Ticky said:

@Nickfromwales You say heat it 'somehow' - you feel it's not going to be easy to heat with anything 'above' the concrete?

Mine will be an office ( and defo NOT a cheeky guest bedroom c/w en-suite ) so will defo need heat vs 'a heater'. A portable fan heater or oil-filled rad would of course suffice, but I want A/C in the summer for sure, so will get heating from the same unit in with the price.

 

I will add an in-screed heater wire even if I only use it one or two months of the year for space heating, but the main reason for the UFH is for when I have long conversations with clients / Zoom meetings etc and I don't want any noise whatsoever in the office.

 

One half of me thinks to do away with the insulated raft idea altogether and just go for an insulated timber structure over 5" of reinforced concrete, then 2x 22mm deck for 44mm of total floor thickness > under laminate Warmup / similar heat mat.

 

My head is hurting from thinking instead of doing.

[saveasteading]

How about this?

100 hardcore with sand blinding.

Dpm

25mm pir board.

Dpm

100 concrete.

wall plate on edge of concrete.

Cladding oversails. 

 

Live with the heat loss the slab perimeter.

 

Air to air heater/ chiller.

 

Roof  osb then battens and metal. The gap will allow a lot of the sun heat to dissipate.

 

This will save you a lot of time and construction cost, and enough insulation to avoid the worst extremes. 

Logic? If heating the air only in daytime, by the time the slab has warmed up, you are turning it off again.

[Ticky]

13 minutes ago, Nickfromwales said:

My head is hurting from thinking instead of doing.

Tell me about it!! ha ha. 

[saveasteading]

27 minutes ago, saveasteading said:

100 concrete.

Pir stops short of the edge by 150. Concrete at perimeter sits on the hardcore.

Hard-core is min 150 outside the slab.

[Ticky]

On 10/03/2023 at 15:19, Ticky said:

Thanks @IanR, that makes sense. I've literally just got the same answer back from the supplier.

 

It was the load of the building itself that I didn't take into consideration. Thanks for pointing that out

Looking at EPS - Would I want something like this? Jablite Jabfloor EPS150 100mm Polystyrene Insulation Board 2400mm x 1200mm - FWDirect (flooringwarehousedirect.co.uk)
This data sheet for it PowerPoint Presentation (flooringwarehousedirect.co.uk) says that to meet the u-value I'd need approx 100mm (my P/A ratio is 1.067) as all perimeter walls of my 5m x 3m room will be exposed.

 

@saveasteading - my original idea included PIR but I was later informed both by the guys on here and a vendor above, that PIR isn't suitable for a raft.

[saveasteading]

37 minutes ago, Ticky said:

PIR isn't suitable for a raft.

This isnt a raft. A raft is a very heavily constructed slab, linked to a ring beam, that all supports the structure. 

In your case the structure should sit on tge thickened edge which sits on the hardcore base.

The slab supports you and furniture, and pir can do that.

 

If it was a house, then you would take the footings deeper. I see this as a garden shed with insulation and draught proofing.

 

People have a point though....will a skinny footing support the structure and stop it blowing away?

 

So yes, dig a trench around and make the footing deeper and heavier.

 

People on here aren't wrong becuase we are suggesting different solutions.

We might all do it different ways depenfing on cost, performance and risk.

 

It is make your mind up time soon.

 

 

[Nickfromwales]

2 hours ago, saveasteading said:

It is make your mind up time soon.

Yup, I think I'll do it properly, in my instance, and just bite the bullet. Prob an extra 1 day of machine and muckaway, plus another one day for importing all materials to the top of the garden. Powered barrow and micro is around £700 p/w so I'll just accept that, and then use the spare day to dig a soil pipe in down to the house and drop a couple of ducts and a SWA in there at the same time.

Nowt worse than regret, and I've done a few jobs here when tired / short of time, and they piss me off to this day when I look at them.

Mechanical pencil is coming out. I'm going to print!

200mm EPS with 100mm concrete raft / slab with anti-crack mesh for reinforcement. Final answer.

[Ticky]

I was thinking.... as I'm going to have OSB + double battens and then cladding extending the depth of the walls, does that not give me wiggle room to bring the timbers more over onto the concrete? Even if it was by an extra 25mm, that would mean 100mm of the wall is over the concrete with only 25mm over the upstand.

Any issues with that?
I drew it out but missed out the OSB layer.

 

[Ticky]

Also, want to have bifold doors. Are there any considerations I need to make when fixing those to the concrete? Do they need to be on plastic packers to prevent cold bridging or can they be fixed directly to the slab?

[Iceverge]

 

Beware water ingress via this route. 

 

At the very least the OSB sheathing should be flush with the insulation so that the breather membrane can lap over the insulation. Preferably outside it in my opinion. 

 

 

Hence my suggestion earlier of a 100*50 sole plate with 150*50 studs on top. 

 

The 50mm overhang will allow you to put insulation outside the sole plate too breaking the thermal bridge of the wall plate. 

 

For the doors, fit them that  that they rest partially on the concrete and partially on the insulation upstand. This will sort your thermal bridge here.  

 

Are you committed to bi-folds? They are draughty. Would a french door with a fixed panel suffice? Cheaper and better IMO. 

 

 

 

 

 

 

 

 

 

 

 

[Ticky]

36 minutes ago, Iceverge said:

Hence my suggestion earlier of a 100*50 sole plate with 150*50 studs on top. 

Apologies, but i'm struggling to visualise again. Is that an extra strip of insulation? and I thought the sole plate was what the studs were attached too. Part of the wall frame? (I'm probably wrong again ha ha)

 

1 hour ago, Iceverge said:

Are you committed to bi-folds?

Not really, just thought they looked better. Didn't realise you could get Aluminium ones that looked like bi-folds.

[saveasteading]

I dont like the exposed polystyrene on the outer face. It will get damaged, and isnt easy to fix or to fix a face onto.

How about a compromise and use a thin but durable insulating board like Wedi  Board? I would just use 12.5mm to take the edge off the cold bridge. but thicker is available.

Sole plate to edge of concrete, then wedi over the concrete and the sole plate.

 

[Ticky]

I'll take a look at that..... My plan was to go down the rendering route as discussed in this post

 

More specifically this link in that post

 

 

Edited May 4, 2023 by Ticky

[Iceverge]

3 hours ago, Ticky said:

Apologies, but i'm struggling to visualise again. Is that an extra strip of insulation? and I thought the sole plate was what the studs were attached too. Part of the wall frame? (I'm probably wrong again ha ha)

 

Have a look at this one I did earlier. 

 

You may need to notch the studs to fit over the wall plate but you'll have a continuous line of insulation from the upstand to the wall. 

 

Use at least 150mm screws for the bottom layer of sheathing so that they go all the way through the  studs into the wall plate. 

 

You'll have a 100mm bearing on the concrete which will be fine. 

 

 

[Ticky]

That's the image that I was looking at and couldn't fully work out.

 

What purpose does the notch have? That seems like it would be a lot of effort to cut out a notch for every stud.

Wouldn't the OSB finishing to the top of the upstand do a similar job?

Like this? (corrected my previous attempt)

[saveasteading]

This actually is a raft (as previous;y discussed) as it isn't sitting on anything other than eps....which isn't strong enough., the slab isn't thick enough, and needs more reinforcement.

I fear the problem is trying to build it like  a garden shed, esp cost-wise, but looking for house performance.

 

Which is the priority?

[Ticky]

Why do you feel that EPS isn't strong enough?

 

I thought EPS was designed for heavy loads similar to XPS

 

[Iceverge]

1 hour ago, Ticky said:

That's the image that I was looking at and couldn't fully work out.

 

What purpose does the notch have?

 

This is what I mean. The notch will sit outside a 100 x 50 soleplate. Without the notch you'll not be able to "squeeze" the OSB sheathing into the sole plate.  It'll just bend the OSB. The notch will create a 50mm gap outside the sole plate to stuff some mineral wool to break that thermal bridge. Despite what @saveasteadingsays I think It's important to keep the soleplate warm. Without it there's a risk of it getting cold, especially in contact with the slab and then you get condensation and rot. Not an issue with a permanently heated house but an intermittently heated garden room it'll save you in the long run. 

 

Line up all your studs on a bench and clamp. Snap a line and set your saw to 50mm and cut them "end on" along Line A . Set your circular saw to max depth/100mm and cut along line B. You'll probably need to do some tidy up with a hand saw but it'll be quick. 

 

 

 

 

 

Have a look at the  detail above. 

 

I would use all 50mm sheets of EPS.

Use 150mm under the main part of the floor and then thin that down to 100mm under the thickened edge of the slab.

Also use 50mm for the insulation upstand.

Lay the DPC as with plenty of spare over the edges. 

Lay your at the centreline (50mm) of the  concrete on chairs.

 Tie the foundation bolts to the mesh in the correct place with some wire or cable ties. 

 

Pour your concrete. Level with a stick and float by hand. 

 

When it is set take your soleplates and use the hammer method to find the right hole location. Drill out holes and ensure it's correct/square. 

 

Notch your studs and build the wall as normal including OSB sheathing. 

 

Fold DPM over Upstand. 

Apply A/T tape for air sealing. 

 

Similarly tape all joints in OSB externally. Very effective and simple. Tape outside of doors and windows similarly. 

Apply breather membrane, batten and clad. 

 

 

[Nickfromwales]

Just put the 100mm soleplate down, then lay the 150mm soleplate for the footer of the stud framing onto that. Carry the upstand insulation up another 44mm ( or whatever the thickness of the soleplate is ) and for 50mm of overhang the OSB will be as happy as Larry.

 

[Iceverge]

That would work too. I'm just exceptionally cheap.

[Nickfromwales]

@Ticky

Why the 25mm anchors? Do you live in a hurricane zone? 🤔

[Ticky]

53 minutes ago, Nickfromwales said:

Carry the upstand insulation up another 44mm

Wouldn’t this prevent me from levelling the concrete? Unless it was ‘added’ after the shuttering was removed. 

 

2 minutes ago, Nickfromwales said:

Why the 25mm anchors

😂😂 Just a guess. What size would be suitable? 

[TonyT]

12mm

[saveasteading]

1 hour ago, Nickfromwales said:

Why the 25mm anchors

25mm would be used on a 30m span portal frame, albeit longer and into much more concrete.

12mm sounds fine. 

Eps is ok for sitting the floor on but not the structure. There is heavier duty material for that.

Either build a heavy raft, or sit the structure onto the stone.

 

I've done my bit on this now.