Minimum Reasonable wall width for good U values?

[Buzz]

4 minutes ago, PeterW said:

From memory the EPS graphite beads were £12/m at 150mm cavity. 

Cheers Bud , quicker and at worst cost neutral  , worth getting a quote or two then. 

Edited March 20, 2021 by Buzz

[Buzz]

16 minutes ago, puntloos said:

 

"price 32"?

 

But yeah I'd love to figure out the price per m2 of my buildup I listed above versus the price per m2 of "20 render 100 block, 150 rockwool, 100 block 20 plasterboard"

Hi @puntloos have you had any prices yet ? would you say your top. middle or low on labour rates where you are ?

[puntloos]

2 minutes ago, PeterW said:

you cannot fill the cavity when using PIR. 
 

Either use 120mm PIR with a cavity OR

Use 150mm blown bead or rockwool. 

Ah interesting. And you would do the blown bead one then.

[PeterW]

What’s your target uValue for the walls ..?

[Oz07]

If you have dpc tray I'd prefer insulation as you go, ie batts built in. 

[puntloos]

42 minutes ago, PeterW said:

What’s your target uValue for the walls ..?

 

0.15ish. But honestly, currently I don't quite know yet. The main problem I'm facing is that "today" I have to decide wall width, at least for detail design and tendering purposes. Later-ish-on I would presumably still have a chance to fully calculate through the actual (over?)heating situation and perhaps adjust things incl wall width.

[PeterW]

Medium block both sides and a 200mm cavity will do that with either Rockwool or EPS bead at 0.032

[Buzz]

decent guide for what is achievable with 0.34 at different cavity widths 

[puntloos]

7 minutes ago, PeterW said:

Medium block both sides and a 200mm cavity will do that with either Rockwool or EPS bead at 0.032

 

Wow, I'm suprised that for the extra 0.03 (0.18 -> 0.15) I'd have to increase my wall 50mm given the calculation I posted earlier with 150 rockwool (reproduced below, more details in my original post [link] )

 

 

Edited March 20, 2021 by puntloos

[PeterW]

You’ve used aerated blocks, I’ve used standard mediums as I wouldn’t use aerated blocks if you gave them to me with a £10 note taped to each one ... 

[puntloos]

1 hour ago, Buzz said:

Hi @puntloos have you had any prices yet ? would you say your top. middle or low on labour rates where you are ?

 

Sorry no prices yet. My area (herts.) is probably quite expensive. 

[puntloos]

8 minutes ago, PeterW said:

You’ve used aerated blocks, I’ve used standard mediums as I wouldn’t use aerated blocks if you gave them to me with a £10 note taped to each one ... 

Ah, OK, (why not?)

 

Anyway how about this one then:

15mm plasterboard and skim, 

100mm block standard mediums, 

170mm rockwool

100mm block standard mediums 

15mm render

 

That should retain the 0.18ish, no? Or is 0.18 too high (are U-values an exponential thing? Seems like an awful lot of struggle for a few percentage points extra..)

 

 

 

Edited March 20, 2021 by puntloos

[Buzz]

2 minutes ago, puntloos said:

(are U-values an exponential thing? Seems like an awful lot of struggle for a few percentage points extra..)

When i was talking to the timber frame boys i was told by more than one that once you reach .18/.17 that the extra insulation cost to go lower would make such little difference to my annual heating bill that unless i was doing this on principle it was not worth the outlay, and take that cost and invest in other areas of the build. 

[puntloos]

24 minutes ago, Buzz said:

When i was talking to the timber frame boys i was told by more than one that once you reach .18/.17 that the extra insulation cost to go lower would make such little difference to my annual heating bill that unless i was doing this on principle it was not worth the outlay, and take that cost and invest in other areas of the build. 

 

Yeah I heard similar things. You can of course be very principled about maximising U value, but a slightly overdimensioned heating/cooling ASHP will be able to cope nicely. (or so I'm told)

 

As for aircrete - https://skill-builder.uk/aircrete-blocks - apparently when done wrong (ha, back to my 'how do you find a skilled person') they might crack, but from this article it sounds I could consider the internal blocks to be aircrete and the external ones 'standard'.. 

Edited March 20, 2021 by puntloos

[PeterW]

17 minutes ago, puntloos said:

 

As for aircrete - https://skill-builder.uk/aircrete-blocks - apparently when done wrong (ha, back to my 'how do you find a skilled person') they might crack, but from this article it sounds I could consider the internal blocks to be aircrete and the external ones 'standard'.. 

 

Seriously.. they were designed for a purpose so that cavities didn’t need to get bigger .. and they are horrid things ..!! You can’t fix anything into them, and they crack if you stress them. Just use mediums, use a decent cavity and crack on. 170mm is over the maximum on a standard lintel so you’re on splits anyway so go to 200mm and get the benefit. 
 

24 minutes ago, Buzz said:

When i was talking to the timber frame boys i was told by more than one that once you reach .18/.17 that the extra insulation cost to go lower would make such little difference to my annual heating bill that unless i was doing this on principle it was not worth the outlay, and take that cost and invest in other areas of the build. 

It is because they aren’t set up to deal with that increase. Look at some of the MBC builds on here and they are 0.12 with pretty much zero heating bills - and soundproofing that is the biggest benefit above heat insulation. 

[puntloos]

2 minutes ago, PeterW said:

 

Seriously.. they were designed for a purpose so that cavities didn’t need to get bigger .. and they are horrid things ..!! You can’t fix anything into them, and they crack if you stress them. Just use mediums, use a decent cavity and crack on. 170mm is over the maximum on a standard lintel so you’re on splits anyway so go to 200mm and get the benefit. 
 

OK you've convinced me.. how about:

 

15mm plasterboard and skim, 

100mm block standard mediums, 

200 mm rockwool  (with split lintels?)  

100mm block standard mediums (with the sand cement slurry)

15mm render

 

430mm total.

 

 

[PeterW]

Sand cement slurry is the wrong side - it goes on the inside of the inner blockwork under the plasterboard. 

[Iceverge]

ditch the plasterboard. rubbish for airtightness, 

 

skim 5mm

sandcement plaster 15mm

medium block 100mm

EPS blown beads with stainless steel wall ties 200mm

medium block 100mm

sand cement render 15mm 

 

if you search my last few posts you’ll see how little a difference chasing the last few decimal places in wall u values makes. Unless you’re aiming to make a true air heated passivhaus or need certification your efforts are better aimed toward airtightness.

 

 

As for the mortar snots. extend your cavity as far below the ground as you can. this will stop droppings building up and bridging above dpc level (i’ve seen it happen).

 

otherwise as long as your brickies clean the ties before the mortar sets EPS blown beads are pretty fool proof.  just make sure all your through wall ducts are done beforehand as the bonding can be hit and miss.

 

[puntloos]

12 hours ago, Iceverge said:

ditch the plasterboard. rubbish for airtightness, 

 

Is it the 'plaster' or the 'board' part that's rubbish?

 

I did a little research but perhaps gone in the wrong direction: https://www.hipcouch.com/magazine/gypsum-plaster-vs-sand-cement-plaster

Sandcement render doesn't seem too bad an idea, but internally it sounds like there's some downsides if you want to have it smooth. 

 

12 hours ago, Iceverge said:

 

skim 5mm

sandcement plaster 15mm

medium block 100mm

EPS blown beads with stainless steel wall ties 200mm

medium block 100mm

sand cement render 15mm 

 

Thank you, sounds fairly logical

 

12 hours ago, Iceverge said:

if you search my last few posts you’ll see how little a difference chasing the last few decimal places in wall u values makes. Unless you’re aiming to make a true air heated passivhaus or need certification your efforts are better aimed toward airtightness.

 

Hence the sandcement everywhere?

 

12 hours ago, Iceverge said:

 

As for the mortar snots. extend your cavity as far below the ground as you can. this will stop droppings building up and bridging above dpc level (i’ve seen it happen).

 

Mortar snots. You learn something everyday. 

 

12 hours ago, Iceverge said:

otherwise as long as your brickies clean the ties before the mortar sets EPS blown beads are pretty fool proof.  just make sure all your through wall ducts are done beforehand as the bonding can be hit and miss.

 

Good tip, thanks!

 

12 hours ago, Iceverge said:

 

HK33 (200mm) ?

I think they're 5.56mm. 

 

435 I can live with. Greenhouse effect is very high though - strange? I seem to remember concrete/cement isn't great?

 

[joe90]

8 minutes ago, puntloos said:

Sandcement render doesn't seem too bad an idea, but internally it sounds like there's some downsides if you want to have it smooth. 

I had sand/cement internal skim with plaster top coat, as smooth as any other plastering!!! And no chance of air leaks and very solid to fix too!.

[puntloos]

48 minutes ago, joe90 said:

I had sand/cement internal skim with plaster top coat, as smooth as any other plastering!!! And no chance of air leaks and very solid to fix too!.

Ah but that means 2 layers of plaster, or is the 'gypsum coat' super-thin?

[Declan52]

First layer is just sharp sand and cement. Used to straighten any imperfections in the wall. Should be around 15-20mm. Next coat is the skim. Should be only 3-5mm thick and be like glass. If the first coat hasn't got the wall straight then the skim coat never will.

[joe90]

5 minutes ago, puntloos said:

Ah but that means 2 layers of plaster, or is the 'gypsum coat' super-thin?

Plaster always goes on as 2 coats (to brick or block), back in the day “browning” was used to straighten/undercoat then top coat. Sand/cement just replaces the “undercoat “. Dot and dab with plasterboard just replaces the undercoat.

[puntloos]

11 minutes ago, Declan52 said:

First layer is just sharp sand and cement. Used to straighten any imperfections in the wall. Should be around 15-20mm. Next coat is the skim. Should be only 3-5mm thick and be like glass. If the first coat hasn't got the wall straight then the skim coat never will.

 

5 minutes ago, joe90 said:

Plaster always goes on as 2 coats (to brick or block), back in the day “browning” was used to straighten/undercoat then top coat. Sand/cement just replaces the “undercoat “. Dot and dab with plasterboard just replaces the undercoat.

 

Wait, I think you gents are contradicting each other? 

 

@joe90: top layer gypsum plaster (or do you mean sandcement plaster?) and a 5mm sandcement skim (and then block)

or

@Declan52: top layer Sand Cement (15mm) with a 5mm skim of some type of other material? (and then block)

Edited March 21, 2021 by puntloos

[Declan52]

6 minutes ago, puntloos said:

 

 

Wait, I think you gents are contradicting each other? 

 

@joe90: gypsum plaster (or do you mean sandcement plaster?) and a 5mm sandcement skim

or

@Declan52 Sand Cement (15mm) with a 5mm skim of some type of other material?

Both saying the same thing just @joe90 has seen more summers than me so goes by the old school terms.

First coat is just sand and cement. You wet the wall and put this mix on and it's job is to get the wall perfect. Or you use a sheet of plasterboard and dot dab. Both methods are just used to give you a flat surface to put the skim on. You can't put skim on block wall. Just won't work.

2nd coat is the skim. This is just a powdered mix that gets mixed with water in a large bucket and ends up like cream. This goes on top of your sand cement or plasterboard.