Cavity thickness

[Ljc1980]

Hi

Would a 50mm cavity fully filled with dritherm sheets be a sufficient cavity insulation for a 3m x 5m extension? Cavity walls will be built with blocks for both skins.

Would like to keep cavity to a minimum to maximise floor space.

Thanks

 

[tony1851]

If both skins lightweight block (rendered externally) then probably OK.
Watch out for the inevitable shrinkage cracks, though.

 

[freddiemercurystwin]

Will they meet Scottish Regs then tony?

 

[tony1851]

Ooops, err...I'm off - just realized I'm missing Downton.

 

[Ljc1980]

I was thinking outer skin 100mm dense blocks and inner 100mm lightweight celcon.

Outer will be fully rendered - for extension on side of below - limited space due to boundary wall so trying to keep thickness down

Untitled

• Ljc1980
• 25 Oct 2014
• 5

 

[noseall]

Hi

Would a 50mm cavity fully filled with dritherm sheets be a sufficient cavity insulation for a 3m x 5m extension? Cavity walls will be built with blocks for both skins.

Would like to keep cavity to a minimum to maximise floor space.

Thanks

Unlikely without having to add more insulation either externally or internally.

If internal space is a premium, look at EWI systems or similar. However, if you are considering building right on the boundary edge then EWI will not be suitable.
Unfortunately there is no magic space saving formula other than costly and impractical timber and celotex systems.

 

[noseall]

You are gonna have some fun digging a foundation right close to that rather tall wall.

 

[Jaxta1]

The foundations are going to be a nightmare to dig!

The boundary wall is one thing, but the ground is almost solid rock. The house is built on the side of an old hill/mountain/volcano! On the other side of boundary wall. It took me 2 months to chiseled out a small strip foundation for the small garden wall in pic!

So do you recon a 100m fully filled cavity (for ease of building) is minimum or could I go any thinner?

 

[freddiemercurystwin]

There are ways to get the wall a bit thinner but IMO it’s not really worth the hassle. I don’t think you will get much slimmer than a 100 cavity if you go traditional. Do a search on the forum, there have been a few threads where people have researched thinner walls. What U value do you need to achieve to meet Scottish Building Regs?

If you are literally on rock then foundations depth won’t be in issue.

 

[tony1851]

If the existing gable wall is a cavity wall, have you thought of taking out the outer skin for the extent of your side extension, (when it will then be internal) and putting a beam in at ceiling level to support the rest?
Could give you an extra 6" or so.

 

[Jaxta1]

I'll need to check the regs for the U value. I'm starting from scratch in terms of building knowledge!

Tony I would like to keep the wall so it's a separate dining room eventually. A few inches here or there I suppose won't be the end of world.

From an earlier comment - are cel on blocks prone to cracks? Should I have both skins in dense blocks?

I'll need to see if I can get a building inspector round to look at ground so he can tell me how deep foundations will be

 

[tony1851]

I would like to keep the wall so it's a separate dining room eventually. A few inches here or there I suppose won't be the end of world.

I wonder if you might have missed the point; the inner skin would still stay in place so you would maintain the separation.

If you were considering thinning off the cavity of the new wall, you would only save 1/2" or so. Taking out the outer skin of the existing wall (if it is a cavity wall) would give you an extra 6" in width.

 

[Ljc1980]

Ah right I see what you mean!

I'll look into it, sounds expensive though!

Thanks for your help

 

[theprinceofdarkness]

Years ago when I built an extension right up to a boundary, I had to have two brick skins because of their fire ratings. The person who "owns" the wall one day might decide to take it down and build a building there.
Frank