EWI + yellow rockwool CWI

[Innes-]

Hi,

I am looking to install EWI on my 1950s semi detatched bungalow. The walls are 300mm with a 50mm cavity. The previous owner had rockwool cavity wall insulation installed around 2012 and I have been advised to get this removed before putting up EWI due to possible damp issues. I wondered if anyone else has fitted EWI over existing cavity wall insulation? 

 

I am also planning to install an mvhr and was thinking this would take care of condensation issues that may cause damp.

 

 In another forum I read that some folk had sealed up their cavities at the top with foam but this seemed to be controversial. Someone else recommended filling cavities up with a a dry concrete mix. I am feeling a bit confused by it all! Any advice would be gratefully received. Thanks. 

[A_L]

hello and welcome,

 

The principal problem with putting EWI on a filled cavity wall is that the cavity filling can reduce the temperature of the outer leaf to below the point at which water vapour from inside the house condenses out. The more EWI insulation the warmer the outside leaf is and the less likely for this to occur.

Can you provide specific details of the original wall construction, it is probably brick/cavity/brick, does your 300mm include external and internal decorative finishes? Also what is the EWI material and thickness?

 

The cavity must be unventilated or the EWI would be ineffective, leaving the mineral wool in place would add significantly to the overall insulating effect.

 

This sort of condensation occurs inside the structure and MVHR will not have any effect.

[tonyshouse]

The existing CWI will tend to dry out even if it is damp now, not get damper providing the new EWI is vapour open, even it isn’t i cant see how the CWI could get damp apart from a plumbing leak 

[Innes-]

Thanks for the reply. Cavity construction is pebbledash, 2 rows brick, cavity, brick, plaster. The 300mm is what i measured by removing the kitchen air vent. From what I can tell the insualtion is dry in the cavity but have been advised to drill in to the wall and remove some lower down to be sure. 

 

I am in discussion with an installer at the moment about how much ewi to put on there. I have a concrete soffit/overhang of 200mm which i would like to pack out with graphite eps. The installer has advised against this saying it will block light out of my windows. I am not going to move the windows out in to the ewi layer at this stage but may do so when i come to replace them in the future. 

 

The installer recommended that i consider up to 140mm eps or go for 80mm phenolic. Their standard install is 90mm eps for a uvalue down to 0.3. He thinks i should aim to meet new build regs of u value around 0.24. Im aiming to get the u value as low as I can.

 

 

 

 

 

 

Edited November 29, 2020 by Innes-

[Innes-]

11 minutes ago, tonyshouse said:

The existing CWI will tend to dry out even if it is damp now, not get damper providing the new EWI is vapour open, even it isn’t i cant see how the CWI could get damp apart from a plumbing leak 

I was hoping this might be the case but the installer seems a bit twitchy about taking the chance on the glasswool.

 

I am looking at £1500+ to get the insulation removed though so I want to be sure its absolutely neccessary to do that. 

[Russell griffiths]

Can anybody explain how they get rockwool out from a cavity. 

[tonyshouse]

If you remove it then the cavity WILL be draughty and wick heat away from the house negating the effect of the new EWI. 
 

I would save the 1500 without a doubt

[A_L]

 

1 hour ago, Innes- said:

Cavity construction is pebbledash, 2 rows brick, cavity, brick, plaster. The 300mm is what i measured by removing the kitchen air vent.

 

That does not quite fit, two rows of brick are 225mm, but there is little effect on the ultimate thermal resistance.

 

Assuming no foil on the phenolic and breathable render there do not seem to be any problems with interstitial condensation

 

U-values               With CWI   Without CWI
90mm EPS                0.21            0.28
140mm EPS              0.16            0.19
80mm Phenolic        0.17            0.21

[MikeGrahamT21]

11 hours ago, Innes- said:

I was hoping this might be the case but the installer seems a bit twitchy about taking the chance on the glasswool.

 

I am looking at £1500+ to get the insulation removed though so I want to be sure its absolutely neccessary to do that. 

 

Theres no need to remove anything which is permeable from the cavity, this is fine and actually better for the overall effect as others have commented. There are loads of properties having this done with cavity wall, not read of any issues, just needs designing properly and simulating beforehand to ensure there is no risk. 

 

http://www.yougen.co.uk/blog-entry/2560/Can+you+have+both+cavity+wall+insulation+and+external+or+internal+wall+insulation'3F/

 

[SteamyTea]

12 hours ago, A_L said:

 

 

That does not quite fit, two rows of brick are 225mm, but there is little effect on the ultimate thermal resistance.

 

Assuming no foil on the phenolic and breathable render there do not seem to be any problems with interstitial condensation

 

U-values               With CWI   Without CWI
90mm EPS                0.21            0.28
140mm EPS              0.16            0.19
80mm Phenolic        0.17            0.21

So the U-Value will change between 0.07 and 0.04

 

Now let us just look at the units W.m^-2.K^-1

A watt is a very small unit, equivalent to 1 kg.m^-2.s^-3

 

So in more relatable values, you are talking about a kilo being either 1.07 or 0.04 kg.  You are not going to notice the difference.

 

Airtightness is much more important.

[A_L]

2 hours ago, SteamyTea said:

Airtightness is much more important.

 

Agreed, but the original question was 'Is it necessary to remove the existing CWI', and pay £1500 for the privilege?. Answer - No.

The table just illustrates that the original CWI has a significant contribution to the overall thermal resistance

Incidentally a change of 0.05W/m²/K is about 2.5kWh/m²/yr difference in energy saved/lost during heating season.

[Gone West]

18 hours ago, Innes- said:

I am feeling a bit confused by it all! Any advice would be gratefully received.

I certainly wouldn't remove the CWI and would fit as much Graphite EPS as the soffitt will allow with the rainscreen.

[rh2205]

We also have CWI on our 1960s bungalow, we are adding EWI albeit only 90mm as was genuinely concerned about shadowing as the low soffits had an affect on the space and I didn’t want to recreate the problem given those soffits have now disappeared above a new floor. Probably depends on your sunlight orientation to the main living spaces could be a benefit south facing!

 

I am all ears on how to improve airtightness internally on an existing masonry structure though? As much as we try to come up with a solution it all seems pointless when the walls get chased for radiators to keep them away from the uninsulated concrete floor (we are EWI’ing below DPC).

[tonyshouse]

Wet plaster, loving care and attention to detail, see DraughtBusters.ReadingUK.org    

[Innes-]

Thanks for all these helpful replies. Glad to hear that there seems to be consensus that I dont need to remove the cwi. The u value info is excellent -thanks. Seems like I just need to try and bring the installer along with me now. I have already had one installer walk away after I said I wanted the dpc done too - he said it was illegal and would never meet building regs!

 

R.e. air tightness I spoke with beattie passive about their retrofits and they were sayign they use passive purple airtight paint on external walls before installing ewi in their 'teacosy' system. Apparently they also put in new exterior walls and a new roof though. 

 

Although I am installing an mvhr I have accepted that I may not be able to get passive house levels of airtightness. I plan to use it mainly for improved air quality. 

 

23 minutes ago, rh2205 said:

We also have CWI on our 1960s bungalow, we are adding EWI albeit only 90mm as was genuinely concerned about shadowing as the low soffits had an affect on the space and I didn’t want to recreate the problem given those soffits have now disappeared above a new floor. Probably depends on your sunlight orientation to the main living spaces could be a benefit south facing!

 

I am all ears on how to improve airtightness internally on an existing masonry structure though? As much as we try to come up with a solution it all seems pointless when the walls get chased for radiators to keep them away from the uninsulated concrete floor (we are EWI’ing below DPC).

 

Our house is south facing but the installer is saying we need to leave 30mm minimum of sofit to meet the ewi manufacturers requirements. He was a bit unclear about whether this meant 30mm to the gutter or 30mm of overhang. I do wonder if they are being extra cautious because of the 25 year warranty. 

 

What insulation will you be installing over the dpc? Our installer is recommending xps with a drip line. Again ive heard conflicting info about whether eps would be better.

 

 

[Innes-]

22 hours ago, Russell griffiths said:

Can anybody explain how they get rockwool out from a cavity. 

Apparently there are several companies out there doing this due to damp issues in some properties. They drill in to the walls and suck it out.

[Innes-]

22 hours ago, tonyshouse said:

If you remove it then the cavity WILL be draughty and wick heat away from the house negating the effect of the new EWI. 
 

I would save the 1500 without a doubt

Would there be any benefit in sealing the top of the filled cavity? 

 

 

[tonyshouse]

Yes, but don’t bridge it thermally, insulation stuffed in is simple and should joint to loft/roof insulation 

[MikeGrahamT21]

12 hours ago, Innes- said:

Thanks for all these helpful replies. Glad to hear that there seems to be consensus that I dont need to remove the cwi. The u value info is excellent -thanks. Seems like I just need to try and bring the installer along with me now. I have already had one installer walk away after I said I wanted the dpc done too - he said it was illegal and would never meet building regs!

 

R.e. air tightness I spoke with beattie passive about their retrofits and they were sayign they use passive purple airtight paint on external walls before installing ewi in their 'teacosy' system. Apparently they also put in new exterior walls and a new roof though. 

 

Although I am installing an mvhr I have accepted that I may not be able to get passive house levels of airtightness. I plan to use it mainly for improved air quality. 

 

 

Our house is south facing but the installer is saying we need to leave 30mm minimum of sofit to meet the ewi manufacturers requirements. He was a bit unclear about whether this meant 30mm to the gutter or 30mm of overhang. I do wonder if they are being extra cautious because of the 25 year warranty. 

 

What insulation will you be installing over the dpc? Our installer is recommending xps with a drip line. Again ive heard conflicting info about whether eps would be better.

 

 

Needs to be XPS or some form of insulation which doesn't readily absorb water. EPS is open cell so will absorb a fair amount of water, no good for below DPC.

 

With regards the airtight paint (passive purple or blowerproof), for a cavity wall house, or probably any other house which is having EWI this is a total waste of money! This type of paint is meant to go on the inside face of the walls inside the property to seal any cracks etc, and give an airtight layer beneath the finish plaster.

 

All of these systems etc will have BBA approval documents which show how they should be used, and these are valid with LABC.

[rh2205]

You need XPS below DPC, no one wants to install this stuff below DPC. It is a do it yourself job if keen around here, I visited some open ecohouses last summer which have just done it themselves given they also couldn't find any installers either. Its the last of the jobs and we plan to do it in stages. 30mm is usually recommended, there is a best practice guide from INCA and there you can have a 30-40mm drip detail added to achieve that recommendation, this is not the preferred option though and as such it is better to just keep your EWI 30mm away from the fascias if you've got enough depth anyway or extend the roof line.

[A_L]

1 hour ago, MikeGrahamT21 said:

Needs to be XPS

 

18 minutes ago, rh2205 said:

You need XPS below DPC

 

Experience of all the 'Passiv slab' houses on here and plenty more elsewhere suggests otherwise.

 

 

 

 

[MikeGrahamT21]

33 minutes ago, A_L said:

 

 

Experience of all the 'Passiv slab' houses on here and plenty more elsewhere suggests otherwise.

 

 

 

 

They'll likely use high compressive strength closed cell EPS. This isn't viable to EWI plinth insulation, as it can't be thick enough, and EPS300 doesn't perform as well as XPS thermally.

[rh2205]

Didn’t even know you could retrofit a passiv slab? Doesn’t sound cost effective. I know you can make a existing house passiv using other measures but if you’ve dug that much of it up you might as well start again!

[Innes-]

10 hours ago, MikeGrahamT21 said:

With regards the airtight paint (passive purple or blowerproof), for a cavity wall house, or probably any other house which is having EWI this is a total waste of money! This type of paint is meant to go on the inside face of the walls inside the property to seal any cracks etc, and give an airtight layer beneath the finish plaster

Yes im not even going to attempt using the stuff. From what I was told the Beattie passive system turns the existing exterior walls in to interior walls i.e. wrapping the whole house with a new roof and skin. Mvhr ducting then goes in the new cavity between the old exterior wall and the new exterior wall. 

[Innes-]

Thanks for all the helpful replies r.e. insulating the dpc.

 

At the rear of the house we have a concrete plinth/step which extends out from the back door. Theres also a reinforced concrete flat roof above linking the house on to a small outbuilding. Will the concrete act as a cold bridge and if so is it possible to calculate how much the overall effect of EWI will be compromised if I leave them in place?

Edited December 1, 2020 by Innes-