Internal wall insulation - pros /cons

After confirmation from the grant scheme that they won’t fit a heat pump without insulation - internal wall insulation is the measure we are considering and wish to understand.
This is what I know so far

*It can be installed on the external face of an internal wall / or the walls of whole rooms can insulated to make sealed thermal envelopes.
*solid brick walls need to ventilate?

  • dew points are a consideration and so are thermal bridges
  • vapour control layers are used with batt and insulation
  • building fabric of external walls determines the best fit approach and type of materials
  • window frames need to be wider to accomodate added internal thickness of insulation.
  • the way installation details are completed will be finite to the success of the insulation performance.
  • internal wall insulation makes rooms smaller
  • sometimes pipes and electrics may need to moved if access is needed on the wall being insulated

Discussing internal wall insulation with the heat pump surveyor , he informed us that to procure internal wall insulation through the grant, a change that might be happening is in favor of insulating ALL internal walls to qualify as opposed to just the external facing awalls.
I have many questions about the pros and cons in all of this - especially external wall only versus all internal walls, as that will decide whether we have it done or not.
The disruption within peoples homes doesn’t enable ‘all internal walls’ to feel achievable.

Additionally if all internal walls have to be insulated for procurement of a heat pump on the grant scheme, that dramatically reduces the number of homes eligible for retrofit who want to decarbonize on the grant.

As ever, lots of questions!! Yet experience gained through this community assures me I am right to ask.

Thanks in advance for any feedback and sharing.
X Carla

If you insulate all internal walls you will hardly have room to move in the house. The reasoning is probably that IWI leaves thermal bridges at joins with uninsulated internal walls. A way to avoid that would be to cut back non-structural walls from the external wall and insulate straight through, making good afterwards.

A problem with IWI is that the house structure gets colder in winter and therefore has higher humidity. Making the inside airtight helps regarding moisture from inside but traps moisture originating elsewhere.

Thanks for explaining this Tim.
So my conclusion from what you have said indicates external wall insulation can be more effective and perhaps less problematic than internal Wall.

As a general rule, IWI creates thermal bridges and EWI covers them. This is not always true, particularly at EWI/opening interfaces, mainly windows and doors. These need very careful detailing. The one area that EWI is not so good at is abutting ceiling height loft insulation. If you have a warm roof it is usually possible to abut to the insulation. In either case insulate above ceiling height.

If you live in a semi or terrace you might still want to insulate the party walls with IWI. This assumes that either the houses either side are significantly colder than yours or you want aparticularly high level of insulation yourself.

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We are just doing this at my daughter’s house. The loss of space is trivial only about 50mm off the external side of the room. But its a very messy job. We removed all plaster back to bare brick which revealed a damp patch we had to fix first. We also tanked the first metre of the walls as we were not sure about the damp proofing. As we are re-doing the whole house it was a re-wire and re-pumb job anyway but yes you probably do need to move electrics and any plumbing pipes.

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Thanks so much for your feedback. It’s all really helping me. :+1:t3:

Thanks so much for your feedback - it’s all really helping me :+1:t3:
5cm isn’t a lot to lose for us.
Are you doing all the internet walls then.
Also what do you think about a one wall at a time approach.
Eg in my middle room - on my wall around the window.
That would be the wall shown in the photo. We’re having a new triple glazed window there.

Carla I’m sure the purist would say you need to do them all to avoid bridges but we are doing one room at a time.

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Horses for courses. If you can accommodate more than 50mm then do. The price markup is minimal and the house will be warmer.

If replacing a window do the insulation afterwards, apart from fitting a layer of insulation under the new window board.

After insulating around the window reveals (all 4 sides) your blind won’t fit, so budget for a new one if still wanted. Think about how you will fix the curtain rail and blind without causing thermal bridges or damaging your vapour membrane. Ideally nothing should be fixed to IWI walls.

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This is the response to my email sent to the GMCA - Andy Burnham - about the LADS grant funding and my complaint about how it’s being misused and failing householders in decarbonizing and that EPCs don’t seem to be adding up and are part of the problem.
Eg funding heat pump installation surveys which shouldn’t be allowed to happen because EPCs ordered by EON are accessible to heat pump installers to view and see which properties are insulated enough to proceed towards a heat pump installation.


GMCA logo
Dear Ms Smith,

Case 3127130

In response to your query received on 16 August 2021,Whilst we accept that EPCs are not perfect, it is a Government requirement that they are used to determine eligibility for the Green Homes Grant Local Authority Delivery Scheme.

We are unable to comment on the activity completed by any assessor and or personal circumstances as there are very much variable based on the individual circumstances and need.

A Retrofit Assessment is carried out for each eligible property, which takes a fabric first approach. This means that insulation measures will always be completed prior to an Air Source Heat Pump (ASHP) being sized and installed.

If a property requires Cavity Wall and/or Loft Insulation, then this will be installed first using Energy Company Obligation (ECO) funding, leaving the full Green Homes Grant available for an ASHP.

If a property requires External Wall Insulation, then this will be funded by the Green Homes Grant. However, this funding option will not provide sufficient funding for both external wall insulation and an ASHP to be funded. Pen-ultimately the final decision on what is installed is the home owners decision, as the assessment will set out the requirements and order they should be addressed.

Yours sincerely,

Oliver Fenton
Business Support Officer


Hi Oliver ,

thankyou for your correspondence and feedback.

Just to update you, I had the survey for the heat pump this morning. The surveyor said he knew from EPC details that we won’t qualify for a heat pump owing to not having wall insulation.

This raised a few red flags - first why did he come out to visit when he knew we couldn’t have a heat pump without the insulation. He said he’d visited 9 other properties in the same situation.
For householders in this situation, with solid brick walls, the choices I understand will be internal or external wall insulation. Not wanting external wall, internal is our preferred measure
We’re very aware of the internal disruption this will cause , although our household situation can absorb this.
However, in discussing internal wall insulation on external facing walls, the assessor believes it may become standard to insulate ALL the internal walls - these are the things that spring to mind - viability, cost, off putting, effectiveness, shrinking rooms and massive disruption - add to that the fact that we don’t want to have any PIR type of insulation on our home and would prefer rockwool, we wonder if we would even be given a choice in the type of insulation used on the grant.

These are the types of quandaries that householders will be finding themselves in.
This is currently the roll out of the LADS Grant - funding being wasted, householders and installers being messed around and potentially not paid for their time, while the roll out of this has barely decarbonized anything.

It’s very concerning and I hope that somehow my feedback may count towards an improved system and roll out.

Kind regards

Carla Smith

My understanding of the grant for heat pumps is that the money is recovered from reduced/no RHI payments. This makes it a loan, not a grant. You would be better off using the entire grant on insulating and then buying a sufficiently reduced size heat pump separately, assuming you can afford it, of course. That way you qualify for the full RHI.

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I 'm coming round to this way of thinking, but I want to have a say in which insulations are used in my home. I really don’t want PIR board or any of the blown insulations. The grants seem intent on stuffing our homes full of plastics and other carbon heavy waste products.
I have sheepwool in my loft and I’m considering it for the internal window walls too - It’s performing well in the loft after we remedied a stack effect moisture problem at Christmas.

We have a small spot on the damp course which is failing on one of the internal walls - we also noticed crumbling mortar in the foundation walls in the sub floor. So we need to attend to this before anything else I guess.

So I have been taking my time to really way up internal Wall Insulation and whether or not I have it through the LADS green grant .
Having mulled it over with all your help and after reading this article which explains each type of internal wall insulation approach. I’m ready to apply via the LADS grant
Here is the link I read - it’s very good - definitely worth a share. I’ll keep you posted!!–internal?fbclid=IwAR2dZQCLB1XRrrczajulczfTLYZaDQLQQnCQYu_9k5Xk1M2YN1l8zaJe_Bs

The article gives a general overview but misses some useful facts.

Strip the old wallpaper, if any, to a) improve vapour permeability (particularly vinyl papers) and b) to remove a nutrient layer that might encourage mould behind the insulation.

Please insulate the window reveals. The thermal bridging there can be horrendous, with associated condensation and mould.

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Thanks Tim. All these additional extra bits really help towards us understanding how the job can be achieved successfully and be effective.

:+1:t3: Big thumbs up from me :grin:

This was published yesterday (I think):

I’m going to read the guidance this weekend and try to understand why Eon are saying our walls aren’t suitable for insulation …

All the best Tim

No wall of a habitable dwelling should be totally unsuitable for insulation. It may need special attention first, require special fixings or non-standard insulation material. I suspect by “not suitable” they mean “not suitable for our standard cheap one process suits all approach”.