Bungalow renovation - insulation, vcl and airtightness

[Rob000]

Hi,

 

We are renovating our 1950s detached bungalow. I'm currently trying to figure out the best approach for insulating the whole ceiling and the new walls and would welcome advice from knowledgeable members. Apologies in advance as this post is a bit long but I'm not sure how to condense it. 

 

Walls

 

The original walls are brick cavity (approx 60mm) and have loose fill insulation (possibly mineral wool but I haven't taken a close look yet). There is a small amount of old rear extension wall that is brick and block with a cavity (55mm), which also has the loose fill in it. The new walls have a 100mm cavity with 90mm PIR or similar in it (I'll add the exact type later). I don't know how good the existing cavity wall insulation is but wouldn't be surprised if it is patchy. 

 

The ideal upgrade given all the work we are doing would be external insulation but as it would change the look of the building we'd need planning permission which takes on average about 6 months here and me and my family of 4 are currently living in a caravan in our garden so I'm not sure I'd get away with it. My first idea was 75mm PIR covered by constivap membrane secured with 63mmx38mm cls timbers to create a service void (the orientation of the timbers varying depending on what's going in the void and on the wall) and covered with plasterboard. A friend then suggested I look at multifoils (insert groans here) and I discovered that for a 10mm thicker build up (50mm PIR, 25mm batten, ybs super quilt, cls, plasterboard) I could further improve the total r value by about 1.2 (although the foil would be compressed to about 30mm rather than the full 40mm wherever there is a socket or switch). 

 

Any thoughts on the pros and cons of these two options (u values, airtightness, vapour control) or any potentially superior alternatives are welcome. Please note I don't want to shrink the rooms any more than I would with the options above.

 

Roof

 

A warm roof would require planning (see above). The pitch is only 22.5 degrees and the majority of it has 90mm rafters. The roof was replaced a few years ago so it's got a breather membrane. There are a number of tricky corner sections in the rafters which makes PIR tricky to install in the rafters and the shallow pitch and small rafters would make a ceiling PIR install also very tricky. I am keen to have some kind of service void to avoid having to massively increase the size of cables because they are running in insulation. I've investigated using multifoil as the airtightness layer/vcl on to of the ceiling joists and putting rockwool on top and was advised by the multifoil manufacturer that (despite their drawings for flat roofs showing otherwise) it was best to run the multifoil (if using it as a vcl) beneath the joists to avoid interstitial condensation - I couldn't get to the bottom of why though. Is that accurate? I thought the vcl was just supposed to be on the inside of all insulation so can only guess that it might matter because a joist could be a cold bridge where it sits on the wall plate? 

 

If the vcl needs to go from underneath I reckon I can create a service void by stapling it up the sides of the joists and taping over the staples with an airtightness tape. Can anyone forsee any issues with using multifoil? Would I be better using a breathable hydroscopic membrane instead as I'm putting rockwool above or will that not matter? For info, I'm also installing a MVHR.

[MikeGrahamT21]

Don’t discount EWI quickly as it solves a lot of problems, plus you can achieve pretty much any finish you want, so shouldn’t be a planning issue. I’ve used it on my bungalow (60s) to good effect. Take a look at my blog on here, I’ve posted a fair bit as I’ve been doing it.

 

never used multifoil, though I do remember seeing the buildup spec many years ago suggesting that to achieve anywhere near a good u value it required supplementary PIR, whether that’s changed since I’m not sure.

[Iceverge]

1 hour ago, Rob000 said:

are renovating our 1950s detached bungalow. I'm currently trying to figure out the best approach

 

Knock and rebuild off the cards? 

 

1 hour ago, Rob000 said:

The original walls are brick cavity (approx 60mm) and have loose fill insulation (possibly mineral wool but I haven't taken a close look yet)

 

Suck it out and blow EPS beads in instead. 

 

1 hour ago, Rob000 said:

The new walls have a 100mm cavity with 90mm PIR

 

I really don't like PIR boards in a cavity. Bump the cavity to 150mm and use EPS beads or mineral wool instead. 

 

1 hour ago, Rob000 said:

My first idea was 75mm PIR covered by constivap membrane secured with 63mmx38mm cls timbers to create a service void

 

This would make the external wall very cold and unless you were really confident in doing a top job with airtighness you would risk moisture accumulation in the wall. 

 

How about 

External block

60mm EPS beads

Internal block

Sand/cement/lime parge coat

50mm Mineral wool batts between battens for service cavity. 

Plastered

Skim

 

[Rob000]

On 29/01/2024 at 21:19, MikeGrahamT21 said:

Don’t discount EWI quickly as it solves a lot of problems, plus you can achieve pretty much any finish you want, so shouldn’t be a planning issue. I’ve used it on my bungalow (60s) to good effect. Take a look at my blog on here, I’ve posted a fair bit as I’ve been doing it.

 

Thanks Mike, I didn't realise brick slips were an option. I think I'll reduce the inside insulation to 50mm PIR, get a survey of the cavity wall insulation done (and remove and replace is necessary), and do an additional 60mm or 70mm of K5 externally in the summer/early autumn. 

 

Having done a bit more research on AVCLs I think I'm going to use a non-combustable one and batten off it too create a service void rather than lapping it up (the a2 rated ones I've found are foil based so the less you can scrunch them up the better). I'd feel far more comfortable fitting downlights under one that wouldn't potentially take a fire all over the house.

[MikeGrahamT21]

Be careful with that build up, you are putting two very non permeable (in terms of vapour) materials on either side of the main structure, moisture will have no where to go and there will be plenty of that in the bottom of your cavity. Your best off sticking to only EWI, or if you must use PIR internally then use something like wood fibre or rockwool boards for the EWI as the moisture will always travel toward the cooler side of the cavity, in winter this will always be the external leaf.

[Rob000]

21 hours ago, MikeGrahamT21 said:

Be careful with that build up, you are putting two very non permeable (in terms of vapour) materials on either side of the main structure, moisture will have no where to go and there will be plenty of that in the bottom of your cavity. Your best off sticking to only EWI, or if you must use PIR internally then use something like wood fibre or rockwool boards for the EWI as the moisture will always travel toward the cooler side of the cavity, in winter this will always be the external leaf.

Ok thanks Mike, but is K5 on the outside and no PIR on the inside (but still a VCL) going to make any difference to the moisture at the bottom of the cavity? It still has nowhere to go? Or will moving the dew point further out remedy that?

 

I've had a look at wood fibre, which may have the advantage of warming up and drying out the existing cavity insulation (if it needs it), but haven't seen anything yet that confirms your can use brick slips on it. 

[Roger440]

2 hours ago, Rob000 said:

Ok thanks Mike, but is K5 on the outside and no PIR on the inside (but still a VCL) going to make any difference to the moisture at the bottom of the cavity? It still has nowhere to go? Or will moving the dew point further out remedy that?

 

I've had a look at wood fibre, which may have the advantage of warming up and drying out the existing cavity insulation (if it needs it), but haven't seen anything yet that confirms your can use brick slips on it. 

 

I face a similar issue with my 70's extension.

 

On a slightly diffrent point, if you go with EWI, then surely the cavity will need to be aitight. And thats not going to help if there is any moisture in there.

 

Unless the cavity is 100% fill?

[MikeGrahamT21]

On 01/02/2024 at 19:17, Rob000 said:

Ok thanks Mike, but is K5 on the outside and no PIR on the inside (but still a VCL) going to make any difference to the moisture at the bottom of the cavity? It still has nowhere to go? Or will moving the dew point further out remedy that?

 

I've had a look at wood fibre, which may have the advantage of warming up and drying out the existing cavity insulation (if it needs it), but haven't seen anything yet that confirms your can use brick slips on it. 

dew point ends up at the render/insulation board interface. No requirement for an internal VCL with EWI alone, your free to wet plaster for airtightness.

 

moisture and moisture vapour are two very different things, the foundation below DPC is designed to remain damp, but the rest needs to be managed correctly.

 

brick slips can be used on any insulant, they get stuck to the basecoat render. There are multiple types of brick slips too, some made from real brick which can be applied with a metal mesh behind them, or lighten resin slips like I used which are stuck straight on with special adhesive.

 

23 hours ago, Roger440 said:

 

I face a similar issue with my 70's extension.

 

On a slightly diffrent point, if you go with EWI, then surely the cavity will need to be aitight. And thats not going to help if there is any moisture in there.

 

Unless the cavity is 100% fill?

some properties cavities are capped off with slate, mine isn’t and is still open topped, but filled and over covered with loads of dense rockwool. I did speak to someone once who told me the more airtight the cavity, the better the EWI performance, makes perfect sense but isn’t feasible, and is impossible when you have a semi detached property like I do.

[Roger440]

On 02/02/2024 at 21:59, MikeGrahamT21 said:

dew point ends up at the render/insulation board interface. No requirement for an internal VCL with EWI alone, your free to wet plaster for airtightness.

 

moisture and moisture vapour are two very different things, the foundation below DPC is designed to remain damp, but the rest needs to be managed correctly.

 

brick slips can be used on any insulant, they get stuck to the basecoat render. There are multiple types of brick slips too, some made from real brick which can be applied with a metal mesh behind them, or lighten resin slips like I used which are stuck straight on with special adhesive.

 

some properties cavities are capped off with slate, mine isn’t and is still open topped, but filled and over covered with loads of dense rockwool. I did speak to someone once who told me the more airtight the cavity, the better the EWI performance, makes perfect sense but isn’t feasible, and is impossible when you have a semi detached property like I do.

 

If its not airtight, what possible use will EWI be if there is ambient air circulating in the cavity?

 

[MikeGrahamT21]

10 hours ago, Roger440 said:

 

If its not airtight, what possible use will EWI be if there is ambient air circulating in the cavity?

 

 

If the cavity is correctly packed with insulation, air movement will be limited at best. The chimney effect is what you are thinking of, which would bypass EWI, and this would not occur with full filled cavity, there could still be a small amount of infiltration but not enough to cause serious degradation of performance.

 

 

This is a comparison in energy use from the main place I did EWI last year, along with a replacement window for 3G from 2G.