Insulating cavity walls advice

[sambotc]

So, the garden currently looks like the Somme. Managed to get a 3/4 tonne digger in over the weekend and if it wasn't for the massive lump of concrete found underground, everything would have gone fairly smoothly!

Anyway, I need to shutter off the crater that's left and I've seen some people mentioning 150mm cavity with full fill Rockwool or similar?

I have dug the majority of the footings 600mm so I was intending on 100mm block, 100mm cavity, 50mm partial fill celotex (or similar) and either 100mm internal block or thermalite? Could I go 75mm batts and have a 25mm air gap still?

Is there a nack to thermalite as they seem to draw in the moisture quicker and move about a bit, puts me off using them a bit but I'm guessing they will be needed to keep the U value down with partial fill?

Appreciate what views others have on the best way to go? Prefer to avoid the 150mm cavity if I can as the ground is a nightmare to dig and it would mean trimming up the trenches to widen the footings in places!

Thanks in advance

Sam

 

[garyo]

If you're doing your own brick/block laying then why not keep it simple and go for 100mm full-fill cavity and thermalites on the inner skin?

 

[^woody^]

150mm cavity? Wtf

 

[garyo]

OP - have you been looking at the new building reg U values that only apply to new build, rather than extensions?

 

[sambotc]

Hi,

Thanks for all the replies. I know, I thought 150mm cavity was stupid when someone mentioned it to me but then I heard another guy mention it as well and thought I'd missed something?

I am doing my own blockwork, yeah. Never been that comfortable with thermalite though but happy to give it another crack. Do you bed them on a slightly wetter mix to allow them to soak up or even wet the blocks, or is that silly?

Can I use standard blocks on the inside? At least it gives you something to fix things too inside!

Is a partial fill not better than full fill for keeping an air gap and also being celotex I'd have thought a better U value? It's a bit more expensive then rockwool mind?

Appreciate your time

Sam

 

[noseall]

I am doing my own blockwork, yeah.........Do you bed them on a slightly wetter mix to allow them to soak up or even wet the blocks

You seem very naïve to the whole building thing yet are comfortable doing your own block-work? The fact that you even explored partial fill cavity system or 150mm cavities screams - get someone in!

 

[garyo]

Celotex is 'about' twice as good as rockwool, but you'll need to leave a air gap meaning that you might only use half as much of it, so you end up back where you started. PIR will also require skill and attention to detail in the fitting if you actually want it to work well too.

Normal muck will be fine for the thermalites. Pay more attention to keeping them as dry as possible (out of the rain pre and post laying). If you lay them damp/wet they'll cause the wobble problems you describe due to lack of suction and you'll also be more at risk of the internal leaf cracking as it dries out and shrinks.

 

[sambotc]

You seem very naïve to the whole building thing yet are comfortable doing your own block-work? The fact that you even explored partial fill cavity system or 150mm cavities screams - get someone in!

Hi Noseall,

Certainly not naive otherwise I wouldn't be asking the questions!

I'm not a bricky, no. I'm a heating engineer so I'm not completely wet behind the ears in the building trade. I'm also up for doing most things and like a bit of a challenge.

Obviously I'm asking you guys who have the experience in this what you'd personally do, as there is often more than one way to skin a cat. I want to do a professional job and I'm a bit of a perfectionist, so advice from you guys is very helpful.

Don't ask and you won't know, but also, don't try and you'll never learn

Thanks

Sam

 

[sambotc]

Thanks Garyo,

I think it's the mental block with having a cavity and then filling it with something that absorbs so much moisture. In my head I'm thinking it's better to have an air gap but get your point on being able to fit the celotex batts well for them to perform as designed. Is it quite tricky to do so?

Full fill would be easier, cheaper and by the sounds of it, quicker too so perhaps that's the best way to go?

Thanks again

Sam

 

[sambotc]

Just to add, 150mm cavitys were talked about by builders I've spoken too, both online and in person. Having never come across 150mm cavitys in all my years of drilling through walls was one of the main reasons for posting in the beginning!

 

[garyo]

The new Part L came out in 2013 and I think came in to force 2014. It's a slightly different system, but in simple terms the external walls for new builds now need to achieve 0.18 rather than 0.28 w/m2, but a new extension of an existing building can still be 0.28w/m2

I've not seen a newer version of the old 'approved wall construction' table that was in the previous building regs... the one that listed all of the permutations of outer skin, cavity width, cavity insulation spec, inner skin, internal finish (plaster or dot/dab) and the resultant U value. Does such a thing exist?