A couple of building questions ?

[hammerswinger]

Hi,
Could any of you knowledgeable sorts answer these questions for me please

Cavity Closers, Do I need to use them?
My inner leaf is built from Thermalite blocks with 85mm full fill dry therm insulation, can I close the cavity with a slice of Thermalite and a DPC up against the outside leaf brick work?

Wall between Garage & Utility room.
It was suggested by the BCO that this could be a stud wall with insulation inside, just wondering without having to ring the BCO again if I could build a single leaf 100mm wall with standard 7N concrete blocks instead, I would insulate the habitable side of the wall with kingspan type insulation, how thick would the insulation have to be.


Thanks.

 

[freddiemercurystwin]

You should not close the cavity with blockwork as you'll end up with a cold bridge, you can get a Cavity Closer fairly cheaply, whatever you're local merchants can supply will do. You're BCO may allow Damcors although I think they're pretty crap and am pleased they're forbidden down this neck of the woods.

You can build your wall this way no problem although it will be thicker. You can do 1 layer of plasterboard on around 75mm of Kingspan on 25 x 50 battens @ 600 ctrs on your wall depending on the exact type of block you use. This is based on using Celcon Standard 3.6N blocks, no need to use 7N unless it needs to be for some reason not mentioned. If you use a 7N block it will be more dense and will insulate less and you may need to use slightly thicker insulation to comply.

 

[^woody^]

Aerated blocks can be returned to close the cavity without forming a thermal bridge. Concrete blocks can't

You can't use a slice of block though as it may just fall out or crack. The blockwork should be properly returned and bonded

Apart from your block wall idea needing to be thicker due to the insulation, it may likely need its own foundation too - not just built off the slab

 

[hammerswinger]

Hi,
Thanks for the replies.

I asked about the cavity closer as the stuff that has been delivered doesn't look that good, it's a wide strip of plastic with a strip of polystyrene on it.
The Thermalite aerated blocks are up & haven't been returned to close the cavity so it will be too much work to alter them to close it and keep the bond that's why I mentioned the strips but I understand why I can't do that.
I have some kingspan type insulation on site, could I not cut a strip of that to close the cavity?

The wall between the Garage & Utility will be built on top of a concrete lintel that has been put in at floor level, it spans the width of the Garage to avoid the drains below.
The wall will support the rafters for the back elevation of the lean to roof on the wall, the roof isn't very big , 2.4m wide by 1.9m span.
Sorry I'm mixed up with the blocks, I wanted to use just a standard concrete blocks for the wall, I thought these were called 7N blocks, as long as its not built from aerated blocks.as personally I don't like them.

I wish I had avoided aerated blocks for the inner leaf as I think standard blocks feel much more solid and stick together a lot better.... just out of interest if you do use standard blocks for the inner leaf what cavity do you need to have to satisfy the regs?

Thanks.

 

[simonjay]

Hi,
Thanks for the replies.

I asked about the cavity closer as the stuff that has been delivered doesn't look that good, it's a wide strip of plastic with a strip of polystyrene on it.
The Thermalite aerated blocks are up & haven't been returned to close the cavity so it will be too much work to alter them to close it and keep the bond that's why I mentioned the strips but I understand why I can't do that.
I have some kingspan type insulation on site, could I not cut a strip of that to close the cavity?

The wall between the Garage & Utility will be built on top of a concrete lintel that has been put in at floor level, it spans the width of the Garage to avoid the drains below.
The wall will support the rafters for the back elevation of the lean to roof on the wall, the roof isn't very big , 2.4m wide by 1.9m span.
Sorry I'm mixed up with the blocks, I wanted to use just a standard concrete blocks for the wall, I thought these were called 7N blocks, as long as its not built from aerated blocks.as personally I don't like them.

I wish I had avoided aerated blocks for the inner leaf as I think standard blocks feel much more solid and stick together a lot better.... just out of interest if you do use standard blocks for the inner leaf what cavity do you need to have to satisfy the regs?

Thanks.

Does the strip of polystyrene (1) fill the cavity or (2) is it a thin strip and actually an insulated DPC ?
If (2), cavity closers like that seem pretty normal. Mine had some clips you can use to screw into the inner leaf but a push fit is OK. Put plastic strip against the outer leaf - it will fold round to form DPM against side of windows, although for PVCu window not strictly needed.

I cannot see why you cannot use DIY cavity closer, since mine were just polystyrene stuck onto a strip of DPC. Worth checking with BCO first, but if you already have them why not use them ?

I used aircrete for my extension, and they do have pros and cons. Yes concrete blocks would be easier to fix to, be more solid etc.
However, most masonry houses now are build with aircrete, and it has been tried and trusted for many years, so don't worry.

Having said that, I used slightly denser blocks, thermalite shield I think. Those 2.8N solar blocks seemed a bit too lightweight !

Simon.

 

[jeds]

Technically speaking closing the cavity with aerated block - or any other type of block, doesn't comply. So I'd check your BI is happy before you get too far with that.

 

[hammerswinger]

Hi,
@simonjay,
Yes, the cavity closer I have is a white plastic strip with polystyrene on it, it looks as though it has some sort of plastic clip arrangement on it.
I haven't really looked at it properly yet, I've only seen the end of it sticking out of the bag it came in but it didn't look up to much as far as I can remember hence my question asking if I could use the Thermalite blocks to close the cavity instead.
I'll get a propper look at it later today, maybe it will be fine.

Thermalite Shield blocks have been used on this also, too late to change my mind now, but if I had though about it before hand I would have preferred standard concrete blocks, I just think they make a more solid job.

@jeds,
Having now spoke to the BI he says that the Thermalite blocks can be used to close the cavity if I want. He told me that if the wall had been standard concteete blocks a cavity closer would have to be used as you can't use standard concrete block to close the cavity with.


Thanks.

 

[noseall]

Our Bco recently confirmed that you can close with aerated lightweight blocks and dpc, although this applied to a 100mm cav.

Cav clodgers are expensive so i buy 100mm polystyrene, cut it down and friction fit it then go around securing it with a foam gun.

 

[hammerswinger]

Our Bco recently confirmed that you can close with aerated lightweight blocks and dpc, although this applied to a 100mm cav.

Cav clodgers are expensive so i buy 100mm polystyrene, cut it down and friction fit it then go around securing it with a foam gun.

Could I do the same with kingspan type stuff as I have some 120mm stuff sitting here.


Thanks.

 

[noseall]

Could I do the same with kingspan type stuff as I have some 120mm stuff sitting here.


Thanks.

Yup.

 

[hammerswinger]

Yup.

If I do this do I need to put a strip DPC down the outside edge where the kingspan will touch the outside leaf?

Thanks.

 

[noseall]

If I do this do I need to put a strip DPC down the outside edge where the kingspan will touch the outside leaf?

No.

 

[hammerswinger]

Ok thanks for that.

Now a different question

I'm having a suspended timber floor in the extension, are the sleeper walls that the joists sit on built on top of the over site concrete or do the walls need to be constructed before the over sight goes in.

Does the sleeper wall just go up against the block work that is below DPC or does a gap have to be left between them?


Thanks.

 

[stuart45]

Ok thanks for that.

Now a different question

I'm having a suspended timber floor in the extension, are the sleeper walls that the joists sit on built on top of the over site concrete or do the walls need to be constructed before the over sight goes in.
Normally on top of the oversite.

Does the sleeper wall just go up against the block work that is below DPC or does a gap have to be left between them?
A few inches off the wall.


Thanks.

 

[hammerswinger]

Hi,
Now for a drain question

With my suspended timber floor I have to have a 150mm air void between the bottom of the joists and the 100mm thick oversite concrete.

How do you go on with the drains and the oversite concrete, do the drains have to be under the over site?
I can't understand how they can be, if they are I don't think I can get the correct fall on them, so they would have to start above the oversite then sort of disappear into it as they go, but if that's the case how do you bed around the drains properly with gravel?
There's also the question of how the drains would pass through a DPM under the oversite?

Cheers.