IWI on a cavity wall

[Night Owl]

Looking for thoughts on how others would tackle improving the insulation on my late 1950's dormer bungalow in the East Midlands.  Wall make up is -

 

Outer face 110mm red brick

Inner wall 110mm red brick

Cavity fill insulation is a white cotton wool material but lighter.

 

Due to issues with damp I have had to clean out the lower part of the cavity walls above (and sometimes below) the slate DPC due to being full of material waste from the build.  The cavities are now clear but as a result I have large voids around the lower part of the walls where the insulation has been lost.  As I have removed all internal plaster from the walls I am looking to improve the insulation in and/or on the walls.  Don't want to go EWI as I like how the bungalow looks as it is and it would be quite expensive. 

I'd like to batten out the internal walls that form the outside walls and add insulation and an Intello plus vapour barrier, then board and skim however I am concernedabout potential condensation issues going this route.  Should I get the CWI removed, replaced or try and top it up?  There is space in the rooms to add around 50mm extra IWI without looking odd.  I have used sheep wool to insulate the loft which I like as a product although very expensive.  50mm in the walls won't make a lot of difference so would be interested to know what others would install.

 

Thanks.

[Iceverge]

Get rid of the blown in mineral wool in the cavity and replace with EPS blown beads. 

 

Create an airtight layer by parging or applying a membrane to the external walls. Seal every penetration, windows doors, joist ends, wires and pipes to this layer with tape or airtight paint. Seal the walls to the floor also and ceiling. You'll need to return your airtight layer to the internal abutting walls too to prevent air leaks passing through them. 

 

Batten out the walls for a service cavity. Once all your wires are in place install 50mm mineral wool and then plasterboard and skim. 

 

Ensure you have some mechanical ventilation somewhere. MVHR is the best but often impractical in retrofits.

Something like dDCV might be preferable. 

[Night Owl]

Thanks for the reply and advice on how you would tackle this situation.  I have decided not to add anything to the exterior brickwork which is in reasonable condition and quite sheltered from the elements.  I'm slowly going round sealing up areas where air can escape.  I am realistic in thinking I won't plug every gap but will at least be able to make a decent improvement.

 

Interesting that you would get rid of the cavity insulation and replace with beads.  I understand that the beads are probably the best option for my situation and will look to getting a quote for this.  Is it vital that the cavity is filled if adding IWI to avoid condensation issues?  I presume the idea is that it will help keep the inner wall a little warmer?

[IGP]

Can I ask a stupid question - what is your ultimate aim?

[Night Owl]

No such thing as a stupid question!  My aim is to improve the overall comfort within the bungalow without compromising the fabric of the building.  I want the property to feel warmer, use a little less energy and have no future issues with damp (as a result of my work).  

[IGP]

I recently (over the last year) renovated my house - 1920's semi with 65mm cavity, I had grand plans of insulation everywhere - doing EWI  & IWI. 

 

Any way, long story short I worked out that loads of insulation is overrated in our climate - law of diminishing returns.  300mm of loft insulation, cavity wall insulation if you can and if you've got suspended timber floors and happen to be doing work there do so.  Beyond that, I'd go so far - controversially as saying don't bother. Exceptions would be thermal bridges that would cause condensation. Sort the ventilation strategy.

 

Ultimately insulation doesn't *make* you warmer - it means the space loses heat slower, you still have to heat the space (caveats apply to that broad statement around thermal bridging / uneven insulation across elements)

 

The focus then should be heat pump system designed to the lowest flow temperature you can (<40c) and is weather compensated. This is where you would get the comfort and lower energy bills.  

 

Save your money for a well designed heating system.

[IGP]

Oh and empty your cavities and fill with EPS beads as described earlier for nice full and even insulated walls. 😃

[Night Owl]

Thanks for your comments IGP.  Interesting thoughts on insulation.  My floors are unfortunately solid concrete with a bitumen DPM that has worked to a point but not completely.  I'm planning on digging up some of the floors in the house to get some insulation in but finding someone willing to do it is proving a little difficult.  I am lucky to have a blank canvas for the heating system that goes in, every option available bar mains gas.  I have looked into heat pumps but always thought they only work well in a well insulated leak free house.  I'm also nervous of being held to ransom by the electricity companies.  Don't have the appetite for solar panels and battery storeage yet so may opt for oil heating.  Has anyone ever heard of an oil heating system working with oversized radiators to run at a cooler flow temperature in a similar way heat pumps do?

[Iceverge]

That would work fine with oil. 

 

Have you considered air to air heat pumps? 

 

 

[IGP]

I'm actually sort of jealous of your blank canvas!  

 

If you can get someone to do your floor - think about underfloor heating - this is an enabler for the highest efficiencies in a heating system as its basically the biggest radiator you can have. 

 

On heat pumps, if your knowledge comes from reading anything in any newspaper or from the comments at the bottom of articles on heat pumps - forget what you think you know.

 

They don't need highly insulated and leak-free houses, they could heat a barn if designed to do so. Insulation helps make the heat pump you buy smaller and the emitters you need to be smaller, i.e. if i put loft insulation in my home it's heat loss goes from 6.3kW to 3.8kW (obviously already done it) - and it means i can by the 4/5kW heat pump size rather than a 7/8kW. (side note that efficiency can actually be marginally higher on higher heat loss properties, but that isn't a reason not put insulation in, see the graphic below where the 10kW model at 35c is 5.03 vs the 4.48 on the 5kW, but the 5kW is much cheaper to buy)

 

Whilst you could do it with oil and have big radiators and low flow temps - the efficiency gains on oil / gas are in the region of going from 75% to 85% efficiency - so like 10% increase, not nothing. With heat pumps you can be talking multiples. In the example below on a Valliant heat pump, going from 55c flow to 35c there is a increase of 142% on the 5kW model.

 

 

On your point about being held to ransom by electricity companies - i would argue its almost exactly the same with heating oil companies or whatever. Potentially think about a woodburner as a backup if you're nervous? 

 

All I would say is that you need a good engineer - not just any joe bloggs off the street to do a heat pump job - whilst they are actually technically simpler than oil / gas systems - the training across the industry is patchy at best. To find good engineers - look at some of the ones on HeatpumpMonitor.org that are clearly doing great installs. 

 

 

Last thing i'd ask is a simple question - do you think oil heating is the future? 

[Night Owl]

Thanks again for the reply.  Whilst I am lucky to have a blank canvas for my heating choice, I don't unfortunately have a blank cheque!  I'd love to have a ground source heat pump but don't have the space or money for a bore hole system.  I have however already installed the wood burner (multifuel actually) and am slowly building up a decent supply of logs for future use.

 

Interesting info on air source heat pumps which I need to look into in greater depth.  Interesting link you provided to see how heat pump installations are faring.  Sounds like there is a lot to consider when designing a system for maximum efficiency.  Finding a good installer who knows what they are doing could prove challenging in my area - only one listed in the link who also do gas and oil so not a dedicated renewables company.  I agree that the price of oil is likely to go only up in the future as less and less gets extracted from the ground (if you believe this) but you can at least buy it when the price is cheaper during the summer months and store it for winter - something you can't do with electricity.

 

I've never lived in a house where the heating is constantly on.  I'm used to only having it on when needed which I know can be quite inefficient however my bills have always been reasonably low.  As I'm getting older now and at home full time it could well make sense to have a system running all the time at a lower temperature for efficiency and overall comfort.  Would be nice to not need an oil or gas tank freeing up some space.  Lots to consider and research!

[IGP]

If you’re going down the rabbit hole of research, this is where I found the most easy to digest information by just going through the back catalogue of the videos in these channels. Completely eye opening. 

 

https://youtube.com/@heatgeek?feature=shared

 

https://youtube.com/@urbanplumbers?feature=shared