100mm cavity - Full fill?

[iSelfBuild]

Just embarking on a wee kitchen extension. Existing kitchen part of the house is 100mm cavity with no insulation.

 

Is it ok to do full fill 97mm celotex/kingspan interlocking insulation - if so what type of wall ties are used for that?

Or should I do 75mm and a 25mm gap and top up internally with insualted plasterboard? Internal skin will be thermalite.

 

Externally will be clad in 35mm Cotswold stone veneer and pointed. Only 7m of walling or so - big extension will be SIPS which I'll be posting for advice soon

[Iceverge]

Rigid boards in the walls are impossible to get right. 

 

Use mineral wool or EPS blown beads instead. 

 

Use normal dense blocks rather than thermalite. They add almost nothing to the insulation and crack far too easily. 

 

Widen the cavity and add more insulation rather than using an insulated plasterboard.  

 

Insulated plasterboard is terrible for airtightness and is expensive and wasteful. 

 

What's the reasoning for SIPs for the extension? 

 

 

 

 

[joe90]

1 minute ago, Iceverge said:

Rigid boards in the walls are impossible to get right. 

 

Use mineral wool or EPS blown beads instead. 

 

Use normal dense blocks rather than thermalite. They add almost nothing to the insulation and crack far too easily. 

 

Widen the cavity and add more insulation rather than using an insulated plasterboard.  

 

Insulated plasterboard is terrible for airtightness and is expensive and wasteful. 

 

What's the reasoning for SIPs for the extension? 

 

 

 

 

+1. I used mineral wool in a 200mm cavity with normal blocks (I hate thermalites,).so much less faff and worked well. 

[iSelfBuild]

4 minutes ago, Iceverge said:

Rigid boards in the walls are impossible to get right. 

 

Use mineral wool or EPS blown beads instead. 

 

Use normal dense blocks rather than thermalite. They add almost nothing to the insulation and crack far too easily. 

 

Widen the cavity and add more insulation rather than using an insulated plasterboard.  

 

Insulated plasterboard is terrible for airtightness and is expensive and wasteful. 

 

What's the reasoning for SIPs for the extension? 

 

 

 

 

Thanks, I'll be doing it myself so if I do the full fill I'd make sure I get the detailing right, I was going to use the interlocking ones as I know gaps are inevitable otherwise. Just trying to keep the overall thickness as slim as possible - It's only a tiny extension, original house is thick solid stone cottage so seems silly to go over the top.

 

SIPS purely for speed - I've built DIY stick framed before and just CBA with the time it will take etc - getting the whole 100m2 extension up in a week and watertight is a massive appeal so I can plod on with the finishing. 

[Mr Punter]

You can use normal wall ties with the full fill insulation boards.  You need to really take care to clean up the snots and keep the cavity clean.

[Iceverge]

7 hours ago, iSelfBuild said:

, I was going to use the interlocking ones as I know gaps are inevitable otherwise

 

Still impossible to get them tight enough to be wall to prevent thermal looping. 

 

Whilst you're at it mark the K value of the boards from 0.022W/m2K down to about 0.033 as the blowing agent will escape from all the cut non foil sealed edges in a few years to be replaced by air so you're basically back at mineral wool or EPS anyway. 

 

Full fill mineral wool batts will be far far cheaper and easier to work with and result in less heat loss despite the headline worse insulative value. 

 

Airtighess is mega important too. I would recommend wet plaster internally. 

 

I was going to suggest to suggest stick framing with a blown or flexible insulation for the extension. 

 

What's your plan for the SIPs? Are you going to clad them in brick? 

 

 

[MikeGrahamT21]

Definitely swap out the aircrete for dense blocks, look out for the ones labelled as thermal blocks but as an aggregate block.

 

I used xtratherm (now unilin) CT-PIR at 125mm in my latest side extension, which is an interlocking board, and having done previous extensions with aircrete inner and full fill 0.032 wool, I can say that the latest build is by far the best in terms of insulation and energy use.

 

if I remember right the Kingspan full full boards need their joints taping on the cold side.

 

normal wall ties are absolutely fine, and another +1 for wet plaster

[MikeGrahamT21]

12 hours ago, Iceverge said:

Whilst you're at it mark the K value of the boards from 0.022W/m2K down to about 0.033 as the blowing agent will escape from all the cut non foil sealed edges in a few years to be replaced by air so you're basically back at mineral wool or EPS anyway. 

im not sure I agree with you on that completely, yes the cut edges where the cells are open will leak the blowing agent, but the rest of the board should stay as manufactured unless punctured in some way.

 

I’ve made fresh cuts in PIR which is more than 10 years old and can still smell the gas escaping from the fresh cut

[iSelfBuild]

30 minutes ago, MikeGrahamT21 said:

Definitely swap out the aircrete for dense blocks, look out for the ones labelled as thermal blocks but as an aggregate block.

 

I used xtratherm (now unilin) CT-PIR at 125mm in my latest side extension, which is an interlocking board, and having done previous extensions with aircrete inner and full fill 0.032 wool, I can say that the latest build is by far the best in terms of insulation and energy use.

 

if I remember right the Kingspan full full boards need their joints taping on the cold side.

 

normal wall ties are absolutely fine, and another +1 for wet plaster

Thanks! What size cavity did you have with the 125mm PIR?

 

 

[Iceverge]

1 hour ago, MikeGrahamT21 said:

im not sure I agree with you on that completely, yes the cut edges where the cells are open will leak the blowing agent, but the rest of the board should stay as manufactured unless punctured in some way.

 

I’ve made fresh cuts in PIR which is more than 10 years old and can still smell the gas escaping from the fresh cut

 

It'll keep getting out! 

 

Cavity boards suffer worse as they're smaller and have more exposed edges.

 

Anyway this is far from the biggest issue. 

 

I've yet to see anyone installing any rigid boards to any satisfaction. It's just not practically possible. 

[lookseehear]

In practise how low should mineral wool go within the cavity - right down to the base or would you stand it off the bottom of the cavity somehow?

[lookseehear]

1 hour ago, MikeGrahamT21 said:

Definitely swap out the aircrete for dense blocks, look out for the ones labelled as thermal blocks but as an aggregate block.

 

I was looking for something like this recently, but struggled to find anything I could get locally for a good price. I'm paying £1.25 + VAT for dense blocks and any thermal aggregate seem to end up closer to £3/block!

[MikeGrahamT21]

7 hours ago, iSelfBuild said:

Thanks! What size cavity did you have with the 125mm PIR?

 

 

It’s a full fill system with unventilated cavity so the board and its HIPS facer gives the 125mm, which is what the cavity has to be built as, I remember it was a snug fit as per the installation instructions. They have a really good fitting outer profile which keeps everything tight and sealed. https://unilininsulation.co.uk/wp-content/uploads/sites/2/2023/01/2913_CTPIR_UK_Final_V2.pdf

 

6 hours ago, lookseehear said:

 

I was looking for something like this recently, but struggled to find anything I could get locally for a good price. I'm paying £1.25 + VAT for dense blocks and any thermal aggregate seem to end up closer to £3/block!

have a look on huws gray, they always seemed to be cheapest for blocks for me at least, loads of options too

[iSelfBuild]

Thanks again Mike! 

 

2nd recommendation for Huws Grey. Think I paid £1.06 this morning for two pallets.

 

Used to buy truckloads at £0.60 can't believe price of things creeping up!

[lookseehear]

2 hours ago, MikeGrahamT21 said:

have a look on huws gray, they always seemed to be cheapest for blocks for me at least, loads of options too

 

I've applied for a cash account, but the prices I'm seeing online don't go lower than £2.08 inc VAT for dense blocks and not many options either in Radstock or Bristol. Pretty frustrating - it isn't like we're in the middle of nowhere!

[MikeGrahamT21]

10 minutes ago, lookseehear said:

 

I've applied for a cash account, but the prices I'm seeing online don't go lower than £2.08 inc VAT for dense blocks and not many options either in Radstock or Bristol. Pretty frustrating - it isn't like we're in the middle of nowhere!

Yeah just looked for Bristol, barely any choice and nothing thermal outside of aircrete.

 

Any on eBay that will deliver to your location?

[lookseehear]

Not that I can see. I think I'm just going to give up on trying to find medium density blocks for a sensible price that can deliver and go with the standard dense at £1.25 each.