Hello,

Some year back I tried to take advantage of the green deals by getting CWI. The guy came round and quickly said it wasn't feasible, due to excessive mortar in the joint, on the inside of the cavity (did he call it mortar snot?). It's a mid 1950's build semi-detached.

He checked all the walls to make sure and while checking the party wall in the loft space, I saw on his camera up draughts causing spider webs to be blown around. !

Does that explain why the party wall feels colder than external walls? When I measure the walls internally using infrared thermometer in winter, it's around 0.5-1.5 deg colder on the party wall. Built in cupboards against the party wall get damp and mould during winter as well.

My questions are:
1/ Surely air draughts in cavity walls are a problem. It's supposed to be a semi-sealed void to limit heat loss, surely?
2/ If that's correct, how could I seal up the cavity to prevent air flow? Even just preventing airflow without sealing the cavity should be an improvement?
3/ Is there any internal cavity insulation solution for walls with uneven mortar? As far as I can tell the cavity is standard 50mm.

The front wall has some local stone around the bay window (another problem he highlighted), the kitchen has been extended out the back wall, and there is a garage on the other side wall (which we will eventually be including in the living space).

We're in SW London if anyone is able to make recommendations.

Many thanks,
Hadyn

Posted By: revorSounds like you had an honest installer. A cavity wall is meant to have air flowing through it, it is there to dry out the wall if it suffers from driving rain or other source of damp.

Im fairly sure that a CW doesnt need to have air flow to keep the inner leaf dry. Any penetrating moisture runs down the inside of the outer leaf and drains away. This keeps the inner leaf dry whether theres air flow or not. Problems arise with sloppy bricklayers dropping mortar into the cavity that accumulates on ties to bridge the cavity

Posted By: Hadyn98

He checked all the walls to make sure and while checking the party wall in the loft space, I saw on his camera up draughts causing spider webs to be blown around. !

Does that explain why the party wall feels colder than external walls?

With ambient air blowing through the cavity that wall will be no better than a single leaf wall, so will be cold in the winter which has likely caused your internal damp problems. As its an internal wall to both adjoining houses theres no risk of penetrating damp, unless the neighbours are doing something odd!! Theres no reason why you cant fill this cavity but youll need to stop whatever gets poured/blown in from travelling into the external wall cavities. A column of squirty foam either end of the internal cavity injected from holes drilled in the external wall junction??

For the external walls, Im not sure how much success youd have with a 2 storey building but Ive cleaned up a single storey cavity by stripping the roof covering and knocking motar snots off the ties with a roofing batten. Had to take a few bricks out at DPC level to clean all the rubbish out

As a party wall, you probably need to get the neighbours to agree to any work you do!

Hi

Thanks all for your comments, and thanks for confirming my suspicions regarding air draughts in cavity walls. I would be quite happy to fill all the cavities, if I could get it done correctly.

I appreciate that filling the cavities might not be sufficient insulation to significantly lower energy consumption, but given the fact we have a "wind tunnel" in our cavities currently, blocking that up and some insulation will be a massive improvement!

I've thought about this quite a bit and written and rewritten replies : the reality is I'm not sure how to proceed. I think accessing the top of the cavities to seal them up would be quite difficult for me - I did have the wacky idea of a line of expandable foam at the top of the wall to at least prevent updraught - but no idea how I would get access to that, nor whether that is a good idea or not.

Can anyone offer any guidance in terms of roughly what's required to seal the cavities?

I have to be realistic though and say I'm not going to be able to get the knowledge and experience required to implement this myself.
But having looked at comments on this forum and others, plus reports in the press about CWI problems after a few years, it seems the knowledge or experience to carry out good quality installations and remedial work, is not that common.

Is that a fair comment?

Is there a list of recommended builders somewhere who know how to implement insulation properly (air seal with tape, etc) rather than just doing the building work that looks good?

Lastly, I was wondering why solid insulation isn't retrofitted in existing cavities? I see aerogel sheeting in a variety of thicknesses. Is it not possible to push those down from above? Is blown fibre or bead insulation _really_ the best solution for all the existing properties out there? external or internal insulation is possible, but surely there is a technical solution to use the cavity to maximum benefit?

Many thanks,
Hadyn

Posted By: Hadyn98

Can anyone offer any guidance in terms of roughly what's required to seal the cavities?

Lastly, I was wondering why solid insulation isn't retrofitted in existing cavities?
Hadyn

Your cavities are likely to be connected to the adjoining house so sealing just along the top is no guarantee to stop draughts. If you seal along the top of the party wall theres nothing to stop airflow from the front of your neighbours house, along the party wall, then out of the back of your neighbours house.

Solid insulation cant be dropped down the cavity as theres wall ties bridging the cavity to hold the inner and outer leaves together. Even if you could get solid insulation into the cavity its likely to be next to useless as it needs to sit tight against the inner leaf without gaps. If theres gaps there will be draughts that will just blow out most if not all of the warmth that the insulation is trying to trap.

IMO wind driven rain won't be a problem if EWI is put on after the CWI. The EPS, adhesive and thin film render just won't allow rain anywhere near the brickwork

Wacky idea may be !

If you can fit EWI, how about filling the cavities with grout you will then be achieving some mass and would stop air movement within the walls .

Some people have taken CWI installers to court if they later get damp so I think these days any reputable company will camera the cavity and come to the same conclusion that it shouldn't be done.

However if you were to also/first render the outside of the house that might be sufficient to prevent water penetration crossing the cavity via the CWI.

Studies of void party wall cavities by Leeds Beckett for BEIS confirmed that void party walls (can) act like chimneys to promote stack effect air movement and loss of thermal performance. Their findings influenced the change in bldg regs regarding insulation of part wall cavities. see
https://www.leedsbeckett.ac.uk/-/media/files/research/leeds-sustainability-institute/elm-tree-mews/lsi_elmtree_finalreport.pdf
http://eprints.leedsbeckett.ac.uk/id/eprint/5064/
https://www.leedsbeckett.ac.uk/research/leeds-sustainability-institute/retrofit-performance-evaluation/
http://eprints.leedsbeckett.ac.uk/id/eprint/5810/

as always, evidence lead... (-: