Cavity Wall Insulation problems & External Wall Insulation

Hi. I am at the start of my retrofit journey and need an answer to this basic question: If cavity wall insulation is not possible due to debris in the cavity & issues with the damp course (as reported by a surveyor), is External Wall Insulation still an option? Will EWI provide a good thermal envelope for the house, and be effective, in the absence of CWI? Or is CWI a necessary prerequisite before fitting EWI? If so, is there any way of addressing the debris problem? The house is a 1960’s detached & has never had CWI.

The debris in the wall could act as thermal and damp bridges so is best removed, however the cost and hassle of removing the debris probably makes it something to be aware of and act upon if issues arise. Therefore any work undertaken should not make such undertakings more difficult.

The damp course needs fixing anyway and insulation should not be applied until the DPC works and the damp has dried out.

Now, to actually answer your question:

EWI with an uninsulated cavity is slightly better than pointless, as the warmer air in the cavity will be drawn up into the roof space and dissipated, drawing in new colder air after it.

IWI will work but will need to be thicker than otherwise. IWI needs extra planning to eliminate thermal bridges and reduces the thermal mass of the property.

Thanks for this Tim. I don’t know how much of an issue there is with the damp proof course. We haven’t noticed any damp inside the house, but I guess we need to get that checked out as a first measure. Removing debris from the cavity sounds like it could be tricky. We don’t know how extensive it is, so again I think we need a thorough survey to determine this, and then get advice as to how to remove it.

You say that EWI with an uninsulated cavity is slightly better than pointless. If I am going to be forking out a small fortune on this though I will need to know that it will be more than slightly better, so again I will need to investigate this further.

Does your last sentence refer to EWI rather than CWI?

Thanks for pointing that out. It should be IWI. I have amended the original.

So am I correct in thinking that if the debris can be removed and cavity wall insulation fitted, that EWI would be much more effective?

I’d be interested to know what you do here too.

In theory, you can remove the external brick cladding - assuming it’s non load bearing. Then apply air control, insulation and your preferred render or cladding system to the block.

Reclaimed bricks would cover some of the demolition costs.

You’d have to provide a detailed schematic for the wall to foundations to ground floor assembly. A ventilated floor system would probably not be viable as air tightness would be tricky to maintain across the cavity

In a word, yes.

Whether that the best way to go is for you to decide when you have all the estimates/quotes in.

EWI with CWI is a very effective solution.

The sequence of the layers of wall/insulation does not affect the overall U value, however it does affect possible locations for your airtightness layer. Cavity wall insulation will not give airtightness, although injected foam reduces rate of movement. The airtightness needs to be on the warm side of the insulation to avoid condensation but can be as much as 1/3 of the way into the the structure, measured by U value not distance if absolutely necessary. This explains @Frank_Reif’s suggestion for the outside of the inner skin.

For example. If we’re dealing with: a slab on grade ground floor system, with retrofitted closed cell foam CWI, a vapour open insulation EWI and back ventilated cladding. Then, would you be able to get away with not installing a DPC across the original external brick? It probably wouldn’t cost a lot extra to remove several brick courses to access and detail a continuous membrane across from the block dpc. * Edit, compared to an additional dpc only to the original exterior brick cladding (tedious to write exactly what I mean!)

Furthermore, there may be set of precautionary jobs, such as structural cracks, or brick tie integrity checks, that should be done alongside the rest. Your GC better be experienced if this is all necessary.

By way of explanation, closed cell foam is pretty well airtight but is not the normal foam used by the main CWI companies. You would need to specially request it and pay a premium.

The type of CWI recommended normally depends on how waterproof your outer leaf is and the site’s exposure but if you are planning EWI very soon after then those are not considerations, as the new weatherproof layer will be whatever covering your EWI has. This enables you to decide on cost, U value and airtightness benefits. However if you are not planning on undertaking both in quick succession you will still need to follow the normal route, so as not to damage your building between projects.

Any advice beyond that given above needs a better understanding of your house age and structure.

An update on the above problem: I don’t think I can go ahead with any king of wall insulation until the debris problem is resolved. I know that there are lumps of cement etc in the cavity (I’m told that the technical term for this is “snots”, which seems an apt description).

I’m not confident that my DIY skills are adequate to rectify this, so I’m looking for professional assistance. The problem is that while there are many companies out there offering CWI fitting or CWI removal, there don’t seem to be any that advertise that they will clear the cavity of these snots. Does anyone here know of any company that might do this?

You are wise not to try this yourself. It is definitely a professional job but I have heard it is very expensive. Unfortunately I can’t help with any recommendations.

You could try looking for an architect that specialises in retrofit and ask them who they use.