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Worth doing EWI with a 50mm cavity?


House man
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Hi guys,

 

Ongoing extension 

 

Have 50mm cavity insulation with 50mm air gap, suspect 50mm insulation boards not fitted properly

 

Just thinking wouldn't it be kind of useless to have 100m external insulation wall insulation  since I have a 50mm air gap between walls? 

Thoughts?

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3 minutes ago, nod said:

I do a lot of these and they external insulation works really well 

The house below is a 80s build with a 50 mil cavity with hardly any insulation in the top part of the building 

Extremely quick and easy to fit

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Edited by nod
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Hi Nod thanks for your input, what I really am trying to comprehend is the the effectiveness of the EWI while there's a ventillation gap. If the air gap would mean the outside 100mm EWI has its heat saving properties reduced due to the 50mm gap in the cavity.

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41 minutes ago, House man said:

Hi Nod thanks for your input, what I really am trying to comprehend is the the effectiveness of the EWI while there's a ventillation gap. If the air gap would mean the outside 100mm EWI has its heat saving properties reduced due to the 50mm gap in the cavity.

You can go the other way and fix insulated plasterboard on the interior walls 

62 mil will give you 50 mil of insulation 

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1 hour ago, nod said:

Extremely quick and easy to fit

 

Says the master! ?

 

What system is that out of interest?

 

I presume it all starts with a bottom rail, straight and true? 

 

Do you block any existing air bricks?

 

What's the m2 cost on materials excluding labour roughly?

 

Cheers

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24 minutes ago, Onoff said:

 

Says the master! ?

 

What system is that out of interest?

 

I presume it all starts with a bottom rail, straight and true? 

 

Do you block any existing air bricks?

 

What's the m2 cost on materials excluding labour roughly?

 

Cheers

It is actually very quick 

We normally screw a batten along the dpc

and sit the tray on top Bedded on with a bit of backing 

Then simply sit a row of insulation in Drill five 9 mil holes Then tap the plastic knock-ins in 

Two men a couple of hours for an average gable 

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3 minutes ago, nod said:

It is actually very quick 

We normally screw a batten along the dpc

and sit the tray on top Bedded on with a bit of backing 

Then simply sit a row of insulation in Drill five 9 mil holes Then tap the plastic knock-ins in 

Two men a couple of hours for an average gable 

 

Is it an issue if there's a gap between the back of the EWI panel and original wall face?

 

I'm thinking that may be unavoidable anyway if your original wall isn't plumb or the finish stippled, rough etc. Presume the new EWI goes in nice and plumb?

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A friend of mine had EWI added to a standard timber frame bungalow. It was done free of charge by some grant scheme.  As far as I could tell it was EPS fitted to the outside and standard (looked like cement based) pebbledash done as a dry dash (stones added dry to wet render)

 

It appeared the EWI went all the way to the ground and blocking all the air bricks that were present.

 

Sadly they got divorced and sold the house not long after so I don't know what issues it may have caused re the probably now bridged DPC and blocked air bricks.

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2 minutes ago, ProDave said:

A friend of mine had EWI added to a standard timber frame bungalow. It was done free of charge by some grant scheme.  As far as I could tell it was EPS fitted to the outside and standard (looked like cement based) pebbledash done as a dry dash (stones added dry to wet render)

 

It appeared the EWI went all the way to the ground and blocking all the air bricks that were present.

 

Sadly they got divorced and sold the house not long after so I don't know what issues it may have caused re the probably now bridged DPC and blocked air bricks.

 

Indeed, I was assuming to do it properly you would need to add MVHR. Saying that there must be cold bridging issues at the floor adding EWI to existing housing stock and no continuity at the ceiling. Is it "worth it" without addressing those other two areas?

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The original question was effectively 'is this pointless when I have an air gap and air bricks?'

 

The air bricks are there to ventilate under a suspended floor, so should stay uncovered IF you still have a suspended floor.

The 50mm air gap is to stop dampness moving from outer skin to inner. It also assists the thermal insulation considerably. There is no appreciable air movement within the air gap, although you wouldn't design a new house this way. It can remain and does not preclude extra insulation outside.

 

As to how to do it, I defer to Nod.

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22 minutes ago, saveasteading said:

The original question was effectively 'is this pointless when I have an air gap and air bricks?'

 

The air bricks are there to ventilate under a suspended floor, so should stay uncovered IF you still have a suspended floor.

The 50mm air gap is to stop dampness moving from outer skin to inner. It also assists the thermal insulation considerably. There is no appreciable air movement within the air gap, although you wouldn't design a new house this way. It can remain and does not preclude extra insulation outside.

 

As to how to do it, I defer to Nod.

Quick answer is NO

It isn’t pointless 

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1 hour ago, saveasteading said:

The original question was effectively 'is this pointless when I have an air gap and air bricks?'

 

The air bricks are there to ventilate under a suspended floor, so should stay uncovered IF you still have a suspended floor.

The 50mm air gap is to stop dampness moving from outer skin to inner. It also assists the thermal insulation considerably. There is no appreciable air movement within the air gap, although you wouldn't design a new house this way. It can remain and does not preclude extra insulation outside.

 

As to how to do it, I defer to Nod

 

Thank you, was more the answer I was looking for in the beginning but got a lot of insight from nod, now so I have made my mind up to go ahead with the 100mm XPS under the render, but would like to ask if I should fill that cavity at all? The roof isn't on yet so wondered if you would think of giving it a fill of some sort of leave it as it?

Edited by House man
Typi
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2 hours ago, House man said:

if I should fill that cavity

I wouldn't.   The air is helping and any infill would have to be pumped polystyrene beads or cellulose (which concerns me a bit).

 

Cavity wall with 50 insulation and 50 air, and the 100 of outer insulation is pretty good.

 

Retain the air bricks if you have a suspended floor. Insulation no use below the floor level, so stops off as on Nod's pics.

 

100 is  a lot to stick on the outer face, so the windows/gutters geometry etc need looking at to check this all works.

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1 hour ago, saveasteading said:

100 is  a lot to stick on the outer face, so the windows/gutters geometry etc need looking at to check this all works.

 

Just because you have 100, 90 or whatever mm on the walls doesn't mean you have to have it so thick at the reveals.

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2 minutes ago, Onoff said:

Just because you have 100, 90 or whatever mm on the walls doesn't mean you have to have it so thick at the reveals.

No, but there might be details such as canopy/porch. The sills will disappear and need replacing. The over-clad may clash with the gutter, which they can't simply move outwards as the tiles won't reach.  And some other things I haven't thought of.

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12 hours ago, House man said:

Just thinking wouldn't it be kind of useless to have 100m external insulation wall insulation  since I have a 50mm air gap between walls? 

 

You are correct here. Its pretty pointless unless you fill the cavity or seal it 100% from drafts which is almost impossible. Even with EWI you will have to consider an unfilled cavity as vented.

 

Pump EPS beads or a suitable injected or poured foam into the residual cavity before EWI if you want to realise the full benefit. 

 

I had a play with this calculator online. If you could guarantee absolutely zero air movement to inside or outside in the residual cavity, then EWI will help. otherwise its a waste of money.BCB29C3A-5838-4B49-9CC5-5C81FD6FFD4E.thumb.png.4676eb5067ca192d2205d4c5e745488d.pngEF9C877E-35B3-4019-B6A4-ADE3F4A0437A.thumb.png.64f7801d756bb268bc8d15fcc9233233.png91A24C47-10C0-4928-B3E5-4F69A52E46A2.thumb.png.ec50fac075d5c665e34919b7461bd7b5.png947CB467-57B6-4FC7-8B77-2F127E77A3A2.thumb.png.cfc4e9366f1e6c360ee192449ef76ebd.png

 

 

 

 

 

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41 minutes ago, Iceverge said:

 

You are correct here. Its pretty pointless unless you fill the cavity or seal it 100% from drafts which is almost impossible. Even with EWI you will have to consider an unfilled cavity as vented.

 

Pump EPS beads or a suitable injected or poured foam into the residual cavity before EWI if you want to realise the full benefit. 

 

I had a play with this calculator online. If you could guarantee absolutely zero air movement to inside or outside in the residual cavity, then EWI will help. otherwise its a waste of money.BCB29C3A-5838-4B49-9CC5-5C81FD6FFD4E.thumb.png.4676eb5067ca192d2205d4c5e745488d.pngEF9C877E-35B3-4019-B6A4-ADE3F4A0437A.thumb.png.64f7801d756bb268bc8d15fcc9233233.png91A24C47-10C0-4928-B3E5-4F69A52E46A2.thumb.png.ec50fac075d5c665e34919b7461bd7b5.png947CB467-57B6-4FC7-8B77-2F127E77A3A2.thumb.png.cfc4e9366f1e6c360ee192449ef76ebd.png

 

 

 

 

 

 

Very interesting. I find it surprising that adding 100mm on the outside equals an absolute zero advantage in terms of U-value. And that the 50mm gap is the equivalent to an infinite gap if you know what I mean. Any disadvantages with a complete cavity fill with 50mm inside and 100mm outside? Anything im missing? Except not paying proper attention and getting it right the first time?

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12 hours ago, Iceverge said:

you will have to consider an unfilled cavity as vented.

 

Please explain this. The cavity wall has a dpc at the base and should be closed at the top too. Any vents are gaps in the mortar, which covering would close off.

 

I appreciate that a wide cavity allows air to circulate due to thermal differences but it stays in the cavity.

 

A cavity wall has much better insulation than a solid wall.

 

On top of this, an external masonry wall loses heat through wetting and evaporation, which will not be the case when clad.

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You’ll never get a cavity 100% sealed unless it’s built that way.

 

having the air gap will render EWI almost pointless, you’ll save a little on thermal bridging from the wall ties, but hardly any. Thermal bypass will occur in the gap.

 

filling with slow expanding foam is an option, but you MUST use fully breathable insulation on the outside if you do.

 

a better option is likely insulated plasterboard unless you have a good reason not to

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5 hours ago, saveasteading said:

Please explain this. The cavity wall has a dpc at the base and should be closed at the top too. Any vents are gaps in the mortar, which covering would close off.

 

They are crudely sealed at best. Many gaps in mortar and around windows. Also blocks are quite porous. With any wind substantial amounts of air will blow right through them. Good rendering can improve the situation but I wouldn't rely on it. 

 

5 hours ago, saveasteading said:

I appreciate that a wide cavity allows air to circulate due to thermal differences but it stays in the cavity.

 

Even on a windy day ?

 

5 hours ago, saveasteading said:

A cavity wall has much better insulation than a solid wall.

About 10% on average from https://www.bre.co.uk/filelibrary/SAP/2016/CONSP-16---Wall-U-values-for-existing-dwellings---V1_0.pdf

image.png.8cb936b9196a6519765ccb694157993b.png

5 hours ago, saveasteading said:

On top of this, an external masonry wall loses heat through wetting and evaporation, which will not be the case when clad.

True. EWI is expensive, as a rainscreen where most of it's value will be lost through cavity wall thermal bypass. 

 

 

For @House man I think EWI is too disruptive for the benefits you'll likely achieve. I would put EPS blown beads into the cavity. If you want to go further polyurethane foam would add a good level of airtightness and lower U value but it is expensive. 

 

Then I'd consider internal wall insulation or EWI if that'd suit better. 

 

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Just sat and worked out I've 8 different external wall make ups here.

 

1) Render/9" block/1"batten/1/4" hardboard.

 

2). Render/9" solid brick/render/plaster.

 

3) Render/4" brick/2" cavity/4" brick/2 - 4" pir/vcl/12.5mm plasterboard or Aqua Panel/tiles

 

4) Render/4" block/60mm cavity with 50mm Rockwool batts/4" block/50mm battens with 50mm Rockwool batts/12.5mm plasterboard.  

 

5) Render/4" block/render/plaster

 

6) Render/9" block/render/plaster

 

7) Render/4" brick/2" cavity/4" brick/render/plaster

 

8)Tiles / felt / 12mm ply / 4" studs with 100mm roll insulation / 3/8" plasterboard.

 

4) Is the warmest room in the house. 

 

Be easier to knock my place down ?

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Having read the latest comments i assume the best action would be to have 50mm of insulation inside behind the plasterboard rather than 100mm on the outside, especially since im suspecting the builders of not taking the care to thoroughly execute the installation of the 50mm boards without gaps around the dozens and dozens of wall ties. The silicone K rend alone should probably more than suffice for providing the additional protection to reduce bridging and condensation.

 

Now would 50mm rockwool or 50mm celotex be the best option between the 50mm battens behind the plasterboard? Any better alternatives?

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Could you afford to lose nom. 82.5mm inside?

 

50mm pir, 25mm battens then 12.5mm plasterboard? 

 

It'd give you an uninterrupted, nom. airtight, 50mm insulation layer, a 25mm cavity for cabling. You could use 35mm dry lining boxes for sockets and switches. 

 

I'm thinking of doing exactly that.  

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10 minutes ago, Onoff said:

Could you afford to lose nom. 82.5mm inside?

 

50mm pir, 25mm battens then 12.5mm plasterboard? 

 

It'd give you an uninterrupted, nom. airtight, 50mm insulation layer, a 25mm cavity for cabling. You could use 35mm dry lining boxes for sockets and switches. 

 

I'm thinking of doing exactly that.  


It is possible but why not put the 50mm PIR between the battens and plasterboard screw straight infront of it saving the 25mm? 

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