Wall construction

[Gary Martin]

Could I build a new house with 50 mm uninsulated cavity block walls, with, say, 100 mm of celotex/ kingspan fixed to the inside walls, then service battens,vapour barrier, plasterboard and skim.

would it pass building regs ?

[G and J]

No. 
 

[Redbeard]

Internal insulation (IWI) carries the biggest risk of interstitial condensation - vapour condensing on the masonry surface of the wall behind the insulation, since the insulation make the wall cold by isolating it from the heat source. It may be a good compromise solution for existing buildings but not new-build, in my view. How about building solid block with EWI (thickness depending on material)? Why are you thinking of a cavity. What is it doing, in your view?

[Gary Martin]

Cavity for weather protection. Surely if insulation is held tighter against internal skin, there is no gap to slow condensation, also the celotex acts as a secondary vapour barrier to supplement a fully sealed one.

any moisture in the blocks could dry out via the ventilated cavity. ?

ps MVHR system will also be fitted.

 

thanks for your comments

 

gary

[Gary Martin]

Thanks G&J. Could you expand please?

 

I’m new to self building and keen to learn.

 

thanks

[Conor]

From the collective experience and wisdom here, the consensus is stick to standard, proven methods of construction. You have to take account of the what most builders and trades are used to working with. Start doing something unusual, and somebody down the line will cock something up.

[G and J]

7 hours ago, Gary Martin said:

Thanks G&J. Could you expand please?

 

I’m new to self building and keen to learn.

 

thanks

I would suggest posing the question from a different angle.   What exactly are you planning to build and what factors are you focussed on?

 

Overall cost?

Wall thickness (or thin-ness)?

DIY-ability?

etc….

[Gary Martin]

150 sq mt dormer bungalow.

was looking at timber frame as faster warm up and air tight. Heard of some with rot in base after a few years. ( I know it shouldn’t but it did )

I then considered cavity block for longevity but a slower warm up and difficult to get air tight.

as insulation is essentially “inside” with TF, but “outside, in the cavity” with block, I thought it made sense to get the best of both methods.

 

my plan, from outside in = timber cladding, batten& counter batten ( gives 50 mm air gap ) breather membrane, block, 50 mm cavity,block, 100 mm celotex, air Barr membrane, battens fixed through to blocks, plasterboard and skim.

 

I looked at SIPS and ICFs but the “ none standard construction “ label makes mortgage or resale more difficult.

I assumed the cavity block bit would be classed as standard construction and the siting of the insulation arbitrary. ??????

 

 

[joe90]

19 minutes ago, Gary Martin said:

I then considered cavity block for longevity but a slower warm up and difficult to get air tight.

I don’t buy that block as difficult to get airtight, it’s down to attention of detail (I don’t like membranes and tapes) I looked at loads of different build types (from straw to sips) and ended up with brick and block with 200mm cavity and rockwall batts. 

[Russell griffiths]

Icf is not considered non standard construction any more, mortgage is not a problem. 
timberframe rotting at the sole plate is down to poor detail and design, it was a thing in 1980, but not now. 
lots come on here and ask for some details at the sole plate, then choose to ignore it because of cost. 
You can’t help some people. 
ask here for correct design and then price up the methods. 
don’t try to reinvent the wheel, it’s all been thought of before. 
 

having worked on half a dozen new builds in the last few years I would not hesitate to use a high quality timberframe on an insulated raft foundation, but i would put a block skin around it, even if adding cladding. 

[Iceverge]

Welcome @Gary Martin. 

 

Always nice to hear of new ideas. I tried plenty on our build. Almost every time I had pushback. It falls into two camps.

 

The first being a genuine logical, regulatory or cost reason. 

 

Second being people don't want to expend the energy to think about it.

 

I'll try to avoid the first camp!

 

[G and J]

8 hours ago, Gary Martin said:

150 sq mt dormer bungalow.

was looking at timber frame as faster warm up and air tight. Heard of some with rot in base after a few years. ( I know it shouldn’t but it did )

I then considered cavity block for longevity but a slower warm up and difficult to get air tight.

as insulation is essentially “inside” with TF, but “outside, in the cavity” with block, I thought it made sense to get the best of both methods.

 

my plan, from outside in = timber cladding, batten& counter batten ( gives 50 mm air gap ) breather membrane, block, 50 mm cavity,block, 100 mm celotex, air Barr membrane, battens fixed through to blocks, plasterboard and skim.

 

I looked at SIPS and ICFs but the “ none standard construction “ label makes mortgage or resale more difficult.

I assumed the cavity block bit would be classed as standard construction and the siting of the insulation arbitrary. ??????

 

 

So a total wall build up of maybe just shy of 500mm and a u value I would guess just better than building regs. Not something I’d be pleased with.  
 

Block outer skin helps mortgagabilty, but quite a few lenders are taking against external wood cladding even on block.  
 

If going block on block then I would think standard block-insulation-block-plaster type would yield a cheaper build with a nicer feel.  

[Iceverge]

On 31/10/2024 at 16:07, Gary Martin said:

Could I build a new house with 50 mm uninsulated cavity block walls, with, say, 100 mm of celotex/ kingspan fixed to the inside walls, then service battens,vapour barrier, plasterboard and skim.

 

Yes but there would be issues. 

 

The structure it would normally need some buttressing from inside with other walls. These would make a very large thermal bridge to be dealt with. 

 

Fixing through the battens into the wall and PIR would need very long screws, maybe 250mm. These would quickly get very expensive. Cavity blocks can be hit and miss to fix to as they're quite thin by concrete standards. 

 

You wouldn't save any money on blocks as standard solid blocks are about half the price of cavity blocks.

 

Like @joe90 we did a block+ EPS beads insulation+block+wet plaster wall after considering loads of methods.  It was by far the cheapest in 2019. 

 

Time over again I would stick build a timber frame on site and put a layer of blocks on the outside I think. 

 

Sole plate damage is typical of poor detailing which TF is intolerant of. Done properly it can last indefinitely though.

 

 

 

 

 

 

 

 

 

 

On 31/10/2024 at 16:07, Gary Martin said:

 

 

 

[scottishjohn]

On 06/11/2024 at 22:58, G and J said:

So a total wall build up of maybe just shy of 500mm and a u value I would guess just better than building regs. Not something I’d be pleased with.

500mm  that would be very thick 

300-350 is more like it  in most builds 

 yuo do not actually need a brick outer skin 

if you want you could do it all in wood 

 as in sips construction 

 I f you areself build and can lay blocks  ,block construction in and out will be cheapest and can get you want ever you want 

 I would  say.my own opinion  that wood cladding outside is more likley to be a problem over time if attention to detail and quality of the cladding is not as good as promised  

yes itis very much in fashion at this time  but there are not many 200year pld wood houses around in our climate 

scandinacia has cold but dry winters nad hot summers ,we have wet ones annd that includes the summers sometimes 

 

having lived in s/w scotland for 40 years nowI have seen   even the rendering and stone walls can go green on the sides of the house not facing south ,so hence my dislike for wood cladding

 forestry commsion used to build all their  office and workshop structures from wood  ,but not anymore as they have found the problems 

 

 

 

 

[Gary Martin]

Thanks for your comments guys.

timber or cement board or Plastic timber effect cladding was stipulated by planning to give an “ agricultural feel”  to the build. It is replacing a Part Q permitted development. “ cladding boards “ was the term used.

I know planning dept shouldn’t dictate like that but it’s where we are.

please no further comment on cladding.

 

Im now 100% fixed on cavity block construction, but not really convinced why internal insulation is not better. Any internal walls will be stud, so no cold bridging. Wall thickness won’t be any different to conventional as the cavity will be reduced by the same amount 100mm? As the internal walls will be increased.

I realise that “ I pays my money, and I makes my choice, and have to live with it” but am genuinely hoping to learn from your collective wisdom.

 

is it “ That’s just the way it is. Some things will never change” ( Credit Bruce Hornsby ) or are there more concrete reasons ?

[Iceverge]

Ok, whats the plan something like this?

 

20mm vertical cladding boards.

35*50mm battens across the wall

38*50mm battens up the wall @ 400cc You'd need at least 8 fixings per m2 at a guess. 30p per fixing. 95mm wafer head screw and rawlplug.

215mm cavity block wall.

120mm PIR foamed to the internal walls. Internal face of boards taped. 

20*70mm vertical battening for service cavity @ 400cc fixed at 400mm vertical spacing or 6 per m2. 60p per fixing 200mm wafer headed screw and rawlplug. 

Plasterboard ands skim.

 

U value 0.18

 

You have 8 fixings externally so £2.40/m2.

6 internally but longer screws so £3.60/m2. Total £6/m2 and 14 holes. Add some extra for openings and waste £7/m2 and 16 holes. 

 

Assume your house is a quite small external wall of 200m2 thats £1400 on fixings, not too bad sounding actually. 

 

However you would have to drill 3200 holes into cavity blocks! At least 10% of them would pull out and fail because they're awful to fix into I think 3500 is still conservative. 20 fixings per hr say. 175hrs at a cheap labour rate of £25/hr. £4375 for labour of drilling  and 5 weeks of absolute torture. 

 

 

 

 

 

 

Edited 10 hours ago by Iceverge

[Iceverge]

 

My proposal is to ditch the concrete blocks actually in this scenario. 

 

20mm vertical cladding. 

38*50mm battens across the wall

38*50mm battens up the wall at 600cc. 

Breather membrane

220*42mm studs at 600cc infilled with blown in cellulose. 

Airtight membrane.

45*50mm studs for insulated service cavity at 400cc. 

Plasterboard and skim. 

 

U value 0.16. 

 

Edited 10 hours ago by Iceverge

[scottishjohn]

10 hours ago, Iceverge said:

My proposal is to ditch the concrete blocks actually in this scenario. 

question ? what are walls going to be ,ade of then