Block And Block With Render Wall Design - Questions

[Johnny Jekyll]

May I ask for some advice on wall design. It's a small detached newbuild with 65 square metre floor plan, one and a half story (second floor in the roof).

 

I'm a newby self builder and researching / organising this myself so help would be much appreciated.

 

I'm using block and block with render for the build. This is what I have so far, from in to out:
- 3mm Skim
- Plasterboard onto 38mm or 50mm battens throughout
- 100mm Thermalite Aircrete Shield 3.6N blocks (Forterra)
- 100mm Cavity with 50mm partial fill insulation (perhaps Kingspan Kooltherm K108 Cavity Board or Celotex GA3000)
- 100mm Dense 7.3N concrete blocks for load bearing
- K rend

 

What do you think? I hope to use an MVHR system, so air tightness may be quite important. Also thinking underfloor heating.

 

I have questions like:
- Should I use insulated plasterboard e.g. Kooltherm K118 Insulated Plasterboard? Or is that overkill?
- Should I use battens or dot n dab?
- Could screwing battens to the Thermalite blocks (using the correct fixings of course) cause problems or drops in U values?
- Is there a better option for the internal blocks than Thermalite?
- Should I be using teplo wall ties instead of normal metal wall ties?
- Is 50mm partial fill insulation a good choice for this house? If so, feel free to comment on Kingspan or Celotex?
- If using foil backed insulation for the wall cavities, could this cause problems for mobile phone signals? Faraday cage effect?

 

Please of course feel free to detail how you would design the walls? Products and why?

Am I missing things? I'm especially worried about one product not working with another or damp problems etc?

 

Many thanks in advance.

[Declan52]

Can you wet plaster the inside so you do away with the plasterboard.??  Easier to achieve good airtightness this way. 

Some don't like thermal blocks as they do crack more.  I have them in mine.  Can use a normal concrete block and  use the money saved to put way more insulation in your cavity. 

50mm insulation in the cavity was good practice in about  1998, won't cover building requirements now. Increase the cavity width to 150mm and use Eps beads blown in to fill the cavity. The money you save from not going with the thermal blocks should cover this. 

Ordinary cavity wall ties will be more than enough. 

Dpc built in will cover you in protecting you against damp. 

[PeterW]

I’m with @Declan52 on this

 

If you don’t want to go with insulated PB or a hard plaster then you can use 25mm batten infilled and tape to make it airtight. Board over the top and job done. 

 

Skim

PB

25mm batten / insulation

100mm medium block

150mm cavity blown beads

100mm medium block

15mm render

 

You’ve also got your skin purpose back to front - the inner skin is always load bearing not the outer. 

 

[Johnny Jekyll]

Awesome thanks guys. Hope other will also comment. This is so helpful.

Regarding the EPS beads, are you thinking Kingspan Ecobead or another manufacturer?

Also, our house is only 65 square meters, so it's small. What if I wanted to keep to 100mm cavity? What do you think would be the best solution?

[Declan52]

It doesn't really matter whose name is on the beads they will be the same type of eps beads.  

If you won't increase the  cavity then you will have to go for a fullfill type cavity board.  The problem here is that how good its put in will mean it either works or doesn't.  Badly cut boards at corners and door and window reveals leave gaps which leave cold spots. All the  cavity boards have much better  u values than eps beads in a nice science  lab but if they are installed badly they end up much worse. Eps beads are blown in and fill every single gap leaving no holes. 

Are you tied to 65sqm by planning??  Could not not build as you intend and keep all the internal dimensions the same and increase the cavity to 150mm so you are only increasing the  outside by 50mm all round the house. 

[Johnny Jekyll]

Planning is completed so 65sqm is set in stone. What about 100mm cavity using EPS beads with Thermalite? Every time I see a building lorry I see Thermalite on the back. Do people in the know just not like using them?

[PeterW]

Best route if it’s definitely thin walls you want is to go with EWI or even consider using one of the many ICF systems as it will give you BRegs walls with air tightness and as thin as you can probably get. 

 

Otherwise to get to regs you now need cavity plus internal insulation and you’re not going to gain any room space with that.

[Ferdinand]

2 hours ago, Johnny Jekyll said:

Planning is completed so 65sqm is set in stone. What about 100mm cavity using EPS beads with Thermalite? Every time I see a building lorry I see Thermalite on the back. Do people in the know just not like using them?

 

Is there a chance that you could get the bit extra via a Non Material Amendment, under an explanation such as making it more thermally efficienct?

 

F

[PeterW]

TBH an extra 150mm both ways would be not noticeable and would allow for a 175mm blown bead cavity that would meet regs and be very quick and easy to build. 

[Johnny Jekyll]

2 hours ago, PeterW said:

Best route if it’s definitely thin walls you want is to go with EWI or even consider using one of the many ICF systems as it will give you BRegs walls with air tightness and as thin as you can probably get. 

 

Otherwise to get to regs you now need cavity plus internal insulation and you’re not going to gain any room space with that.

My apologies what is EWI?

[Jeremy Harris]

Just now, Johnny Jekyll said:

My apologies what is EWI?

 

External Wall Insulation.  You build a single skin structural wall then clad it with insulation on the outside and either clad or render over the insulation to provide a rain screen.

[mvincentd]

I think your internal leaf will be load bearing, not the external leaf...so your original proposal of 3.6n thermals for inner leaf might bump to 7n depending on the house design.

[Johnny Jekyll]

Any opinions on either of the following? Thanks

 

Kingspan Kooltherm K106 - 90mm with 10mm cavity
https://www.kingspan.com/gb/en-gb/products/insulation/insulation-boards/kooltherm/kooltherm-k106-cavity-board

 

Cavity Therm - Full fill wall insulation
http://www.cavitytherm.com/ 

 

Would they be much more expensive compared to EPS blown beads?

Edited November 12, 2018 by Johnny Jekyll

[ADLIan]

Are you looking to just meet Building Regs or looking at a much better standard. The 100mm cavity makes things difficult but can work if you only want to scrape a pass. The 2 full fill options above will be expensive (especially phenolic foam as Kingspan have a virtual monopoly on this product!). Suggest you look at the SAP assessment numbers to see what is possible.

I'm not convinced with any injected insulation in new build as there is no way of telling if there are missed areas or gaps. At least with built in you can see the quality of workmanship.

[Declan52]

I used the cavity therm boards in mine.  But I was building it myself so took my time and cut each board correctly with a hand saw as I had the time.  I also scraped the walls smooth filled in any gaps in the motar beds and then put the insulation in.  Doubt any brickie will go to that much trouble without you  crossing his hand with lots of  £50 notes. 

[Mr Punter]

The rigid full fill boards are a bitch to install and the chances are the brickie will lash them in however they can - upside down, wider cavity, gaps or whatever.

[Johnny Jekyll]

Again thanks guys. The bricky is a friend of mine and I will be there helping him all day every day as a labourer, so attention to detail should hopefully be good.

Has anyone got experience with this Cavity Therm stuff? It seems to look good from the outside? Probably bloody expensive though.

[Declan52]

I made my cavity 10mm wider as it's very easy to push fresh blocks out off plumb when you put the boards in. Was  5 years ago so not sure how much they cost now. 

[Johnny Jekyll]

Hi everyone, may I have your opinions on my updated wall build-up (from in to out), thanks very much.

 

- 3mm skim
- 12.5mm plasterboard (fermacell (strength) / aquaboard (wet) where needed)
- Battens for service voids (25mm - 50mm depending what services are running where)
- Pro Clima Intello Plus membrane held against the wall by battens (fully and meticulously taped of course, this will envelope the entire inside of the building)
- 100mm 7.3N Forterra Thermalite Aircrete blocks
- 100mm full fill cavity using Cavity Therm by Xtratherm as it can achieve 0.2 U Value at this thickness
- 100mm medium block
- 15mm render

[Declan52]

19 minutes ago, Johnny Jekyll said:

Hi everyone, may I have your opinions on my updated wall build-up (from in to out), thanks very much.

 

- 3mm skim
- 12.5mm plasterboard (fermacell (strength) / aquaboard (wet) where needed)
- Battens for service voids (25mm - 50mm depending what services are running where)
- Pro Clima Intello Plus membrane held against the wall by battens (fully and meticulously taped of course, this will envelope the entire inside of the building)
- 100mm 7.3N Forterra Thermalite Aircrete blocks
- 100mm full fill cavity using Cavity Therm by Xtratherm as it can achieve 0.2 U Value at this thickness
- 100mm medium block
- 15mm render

Make sure you set the founds to allow you to increase the cavity to 110mm. You won't get the  cavity boards in if you only leave 100mm without pushing the walls out of plumb.  It will make it much easier to build.