Insulation upstand when not having skirting board?

[flanagaj]

With a conventional build, the thickness of the 'dot & dab' / float and set and skirting board will hide the 25mm thickness of the PIR upstand of a screed floor.  If you are not planning on having skirting board, then a conventional build up risks the PIR showing.  In such a scenario, is the only option to use insulation backed plasterboard or bring the plasterboard off the wall using battens?

 

This obviously only applies to a polished slab finish or an insulated garage floor.

Edited June 19 by flanagaj

[Nickfromwales]

35 minutes ago, flanagaj said:

With a conventional build, the thickness of the 'dot & dab' / float and set and skirting board will hide the 25mm thickness of the PIR upstand of a screed floor.  If you are not planning on having skirting board, then a conventional build up risks the PIR showing.  In such a scenario, is the only option to use insulation backed plasterboard or bring the plasterboard off the wall using battens?

 

This obviously only applies to a polished slab finish or an insulated garage floor.

Mildly confusing, so which one is it?

 

Are you having a floor covering?

 

More info and less ambiguity please!

[Spinny]

Dot & Dab brings the plasterboard off the blockwork anyway. 12.5mm plasterboard with 2.5mm skim = 15mm. Dabbed off the wall by 10mm and the front of your finished wall is circa 25mm off the block work.

 

Why no skirting board ?

People do use flush skirting with shadow gaps and stuff - is that what you mean ?

 

We have a 25mm upstand but levelling compound and flooring will cover it. 

 

Laying the top part of the screed to cover the top of the upstand could be one answer. Yes creates a small thermal bridge between floor and wall but does this matter much ?

[flanagaj]

7 hours ago, Nickfromwales said:

Mildly confusing, so which one is it?

 

Are you having a floor covering?

 

More info and less ambiguity please!

No floor covering

[flanagaj]

2 hours ago, Spinny said:

Dot & Dab brings the plasterboard off the blockwork anyway. 12.5mm plasterboard with 2.5mm skim = 15mm. Dabbed off the wall by 10mm and the front of your finished wall is circa 25mm off the block work.

 

Why no skirting board ?

People do use flush skirting with shadow gaps and stuff - is that what you mean ?

 

We have a 25mm upstand but levelling compound and flooring will cover it. 

 

Laying the top part of the screed to cover the top of the upstand could be one answer. Yes creates a small thermal bridge between floor and wall but does this matter much ?

My post was confusing.  I have an insulated garage floor slab, but I wasn’t planning on plaster-boarding or fixing skirting board.  Rather than having a 25mm slice of PIR showing, I could either omit the upstand, but that sort of defeats the point of insulating the slab.  
 

[Spinny]

Why insulate the slab, are you heating the garage ?

[Nickfromwales]

If bare walls, who gives a hoot about seeing a bit of PIR around the perimiter?

[flanagaj]

On 19/06/2026 at 20:27, Nickfromwales said:

If bare walls, who gives a hoot about seeing a bit of PIR around the perimiter?

I've subsequently discovered that you can cut a 45 degree angle on the PIR to hide the edge.

 

One final question.  How do you get a smooth enough finish on the slab for PIR.  Do you simply use a piece of timber with a side to side motion and then use a bull float at 90 degrees.  I really need to be able to do the slabs myself with my brother as I struggle to find anyone to do it without changing lots of £££

[Mr Punter]

1 hour ago, flanagaj said:

do the slabs myself with my brother

 

Well that is great news for your project.  Although you may find siblings tricky, at least they know your foibles and are not such a PITA as contractors can be.

[Nickfromwales]

11 hours ago, flanagaj said:

I've subsequently discovered that you can cut a 45 degree angle on the PIR to hide the edge.

 

One final question.  How do you get a smooth enough finish on the slab for PIR.  Do you simply use a piece of timber with a side to side motion and then use a bull float at 90 degrees.  I really need to be able to do the slabs myself with my brother as I struggle to find anyone to do it without changing lots of £££

YouTube vids should help, as seeing pictures and methods will be much more benefit to you.

 

Self Compacting Concrete may help massively as this will want to find it’s own level without much coercion, but I would still recommend getting one chap in to help you finish the slab; you only need them for 1 day max and that can’t be too much £££  vs the risks. 
 

You also need to be able to adapt to the mixes being inconsistent, unless one wagonload will do you?

 

You can also blind the finished concrete with sharp sand, before laying, but if you’re laying a screed on top then this doesn’t need to be that ‘spot-on’ anyways. 

[flanagaj]

12 hours ago, Nickfromwales said:

YouTube vids should help, as seeing pictures and methods will be much more benefit to you.

 

Self Compacting Concrete may help massively as this will want to find it’s own level without much coercion, but I would still recommend getting one chap in to help you finish the slab; you only need them for 1 day max and that can’t be too much £££  vs the risks. 
 

You also need to be able to adapt to the mixes being inconsistent, unless one wagonload will do you?

 

You can also blind the finished concrete with sharp sand, before laying, but if you’re laying a screed on top then this doesn’t need to be that ‘spot-on’ anyways. 

I will have a look at self compacting concrete.  When you say someone to help finish the slab, will a bull float suffice?

[Nickfromwales]

13 hours ago, flanagaj said:

I will have a look at self compacting concrete.  When you say someone to help finish the slab, will a bull float suffice?

It’s knowing how / when to use it. You would also need to seal it as you lay to stop the moisture evaporating too quickly.

 

Not a job you can just ‘do’, if you’re not used to doing it.
 

I know some very good lads who would work a weekend, if you need (want) some help, (and if you can get concrete there on a Saturday). Let me know. 

[Tony L]

Thanks for bringing my attention to SCC, @Nickfromwales.  @flanagaj, according to a conversation I had with Gemini AI, you'd use a dapple bar, rather than a bull float with this.  Based on watching one YouTube video, it looks like an easy enough DIY job for my small garage, although I think I'd be inclined to to pay some experts to do this for me if this was my house.  The sealant, which goes on soon after the pouring & dappling, is super important.

[Nickfromwales]

19 hours ago, Tony L said:

Thanks for bringing my attention to SCC, @Nickfromwales.  @flanagaj, according to a conversation I had with Gemini AI, you'd use a dapple bar, rather than a bull float with this.  Based on watching one YouTube video, it looks like an easy enough DIY job for my small garage, although I think I'd be inclined to to pay some experts to do this for me if this was my house.  The sealant, which goes on soon after the pouring & dappling, is super important.

You’re welcome

 

caveat; you’ll have one guy on the dapple bar, another at his side raking the concrete about, in his other hand the bar for the dumpy laser for getting TOC within a few mm, and unless it’s a small slab you need a backpack spray bottle loaded up with the sealant so you can spray sections after the dappling has been done.

 

So many facets to this, plus watching the consistency of the mix(es) to adjust pace accordingly.

 

In this weather, those with a brain or experience have cancelled all concrete pours. Just too hot atm. 

[Onoff]

How wide is your garage? Set up screeding rails either side and tamp to them. Can't go wrong and more level than a level thing. If mixing concrete yourself add deaerator. 

 

 

Edited 11 hours ago by Onoff

[saveasteading]

37 minutes ago, Onoff said:

add deaerator. 

I've never heard of this. Poker for deep pours yes. Tamper for thin. Is this an additive. Can you point me at a reference?

[Onoff]

You bolt / screw a rail on each opposing wall (I used unistrut), set dead level. You then make a tamping bar that "hangs" on the rails, with it's underside at finished slab level. Then just barrow in your screed or concrete and drag/fill/tamp to level. I did 4:1 concrete. Used Everbuild 202 integral waterproofer as an admix that also incorporates a deaerator i.e less bubbles & less tamping to get them out. I'd do it this way again in a heartbeat:

 

 

 

 

 

I am however a bit odd. You can set your screed rails in the floor so to speak, fill/tamp to them then pull them out and fill where they were. 

[Tony L]

7 hours ago, Onoff said:

How wide is your garage? Set up screeding rails either side and tamp to them. Can't go wrong and more level than a level thing. If mixing concrete yourself add deaerator. 

 

 

 

About 5.3m wide & 6.4m deep (external measurement for the slab).  I'm planning a ring beam, hung off screw piles then I'll build up the middle with hardcore & 70mm crushed concrete, non-woven membrane then sand blind the middle, get my DPM in, tape it up to prevent leaks around the screw piles & joins (self compacting concrete leaks almost like water, I have learned), rebar mesh on some castles & tied to the screw piles so it doesn't float, then I'll have some ready mix SCC delivered.  I'm thinking a chute will do - I don't want to pay for a pump.  I'm expecting the SCC will just flow to where it needs to go & won't require too much work to push it around.  I could tamp using the top of the shuttering, which I will set with a laser, but I don't think tamping will be required.  For SCC, I know I definitely shouldn't work it with a poker, as this would ruin the homogenous nature of the mix, & weaken it considerably.

 

I've decided to forgo a layer of insulation in the slab, so I only have to pay for one concrete delivery.  A while ago, I worked out that mixing this volume of concrete myself would not be any cheaper than ready mix (for ordinary concrete), & I'm expecting the SCC to be around £300 more than ordinary concrete, whilst saving a lot of bother.

 

I have very little practical building experience, although I have succeeded in making two concrete garage bases, most recently 20 years ago, & not with SCC, so any critique of my plan will be most welcome, please.  

[Gone West]

7 hours ago, Onoff said:

I did 4:1 concrete.

How wet was the mix and how long before you could tile it?

[Onoff]

56 minutes ago, Gone West said:

How wet was the mix and how long before you could tile it?

 

Wet as in slopping around the barrow. Left it about two years I think before tiling just to make sure!