Improving insulation an existing cavity filled bungalow.

[SteveG40]

Hi all

 

First time poster on the forum, although have been a long time lurker. 

 

Just looking at getting peoples opinions/advice on what would be the best way forward with our bungalow renovation.

 

Building control came around and noted that we had removed an amount of plaster from a wall. Said wall incidentally has been removed entirely for the installation of a wide sliding door. But the BCO mentioned that really as we had removed the plaster that it triggers us having to thermally upgrade the rest that wall around the new slider. No mention of the other outside walls in that room! But seeings as we are doing so much other work it makes sense to try and improve the outside walls as much as we can.

 

Property is a 1950s block and block (both inner and outer skins medium dense concrete block) Outside skin is painted render in good condition, sadly our eaves overhang is only 75mm to the eaves vent. So I'm thinking EWI is not the best choice with this restriction.  Cavity has been filled with Urea Formaldehyde foam insulation in a 65mm cavity. This has been installed possibly as far back as the 1990s or older, but appears in good condition, possible due to the previous owners regular repainting of the render. Costs prohibit removing/refilling this of the time being and so I think I'm looking at the IWI route.

Now this opens up a can of worms/confusion when speaking to various insulation suppliers!  The PIR boys say cover the walls in PIR laminated plasterboard. Others say that's a bad idea as the full fill nature of the cavities mean that I should treat the wall as if its a solid brick wall and go down the lime, breathable, wood fibre route. It must be noted that all these people are the sellers of products and so will naturally push their own stuff. not necessarily what's best for my house.

 

Searching on the net doesn't bring up much about what I thought was a common construction. I find lots of EWI, or suggesting I fill the cavity with PIR boards which is impossible with the house already built and filled. All information on IWI are largely related to Solid wall construction, rather than improving an existing cavity wall construction.

 

Anybody got any advice on what would be a wise choice going forward?

 

Many thanks

 

Steve

 

 

[Redbeard]

I do not (and as I read it the Approved Document L does not) agree with your BCO. Page 26 of Approved Doc L has a table (4.3) showing a threshold U value (0.7) and an improved U value for filled cavity walls. I am pretty sure that UF foam (if still intact as you suggest) would give you 0.55W/m2K in a 65mm cavity, if rockwool is reckoned to do in a 50mm cavity. Anyway, as far as I am aware (although it is often misinterpreted) the 0.7 threshold means that if it is worse than 0.7 you have to achieve the target (of 0.55). I am even more sure it will have achieved 0.7. It sounds like your BCO is trying to make the 0.3 target for solid walls stick. I would write to them saying your wall already complies and that, accordingly, you will be taking no further action.

 

Edit: Seen a lambda figure of 0.029W/mK for UF foam. At 65mm, with a base-case R value for the original unfilled wall of around 0.66m2K/W, that should give you a U value of around 0.35W/m2K, WAY better than table 4.3 requires a cavity wall to achieve.

Edited October 21 by Redbeard
Checked UF foam lambda value

[SteveG40]

18 minutes ago, Redbeard said:

I do not (and as I read it the Approved Document L does not) agree with your BCO. Page 26 of Approved Doc L has a table (4.3) showing a threshold U value (0.7) and an improved U value for filled cavity walls. I am pretty sure that UF foam (if still intact as you suggest) would give you 0.55W/m2K in a 65mm cavity, if rockwool is reckoned to do in a 50mm cavity. Anyway, as far as I am aware (although it is often misinterpreted) the 0.7 threshold means that if it is worse than 0.7 you have to achieve the target (of 0.55). I am even more sure it will have achieved 0.7. It sounds like your BCO is trying to make the 0.3 target for solid walls stick. I would write to them saying your wall already complies and that, accordingly, you will be taking no further action.

 

Edit: Seen a lambda figure of 0.029W/mK for UF foam. At 65mm, with a base-case R value for the original unfilled wall of around 0.66m2K/W, that should give you a U value of around 0.35W/m2K, WAY better than table 4.3 requires a cavity wall to achieve.

To be fair our BCO is quite good (Went private). He just mentions that we 'should' not 'must' and that we would feel benefit from upgrading the walls while the house is in such a state of refurbishment. He actually suggested even if we just chuck 25mm PIR up it's better than nothing (His words!). But I'm looking for perhaps a more educated/thought through view on this.  We are re plastering all of the walls anyway so it makes sense to perhaps get something on the walls before we plaster. We are fitting an ASHP so minimising heat loss will all help. I'm not trying to get to passive standards, but something to help with comfort and I'm happy to loose a little space off the room size in doing so.

[Redbeard]

Absolutely fine to add insulation, but your initial post suggested that he saw the removal of the plaster as a 'trigger' point *requiring* you to comply with Part L (which in this case I am pretty sure it isn't, as the wall already complies). You could do just PIR, or you could consider an insulating plaster, or wood-fibre, or whatever you are comfortable with. I like wood fibre a lot. Whatever you decide to do it's worth getting a WUFI dynamic condensation risk assessment done.

Edited October 21 by Redbeard
Edited re CRA

[SteveG40]

Understood. Probably didn't mention the slightly laid back attitude our BCO has. With wood fibre, Do you install this as batts up agains the wall? or as some form of infill in a separate stud wall?

[Roger440]

56 minutes ago, Redbeard said:

Absolutely fine to add insulation, but your initial post suggested that he saw the removal of the plaster as a 'trigger' point *requiring* you to comply with Part L (which in this case I am pretty sure it isn't, as the wall already complies). You could do just PIR, or you could consider an insulating plaster, or wood-fibre, or whatever you are comfortable with. I like wood fibre a lot. Whatever you decide to do it's worth getting a WUFI dynamic condensation risk assessment done.

 

WUFI assesment is an awful lot of money to do just a wall or one room though.

 

As for BCO wanting things done that dont need doing, well im am surprised. Not.

 

With you on the wood fibre.

[Redbeard]

58 minutes ago, SteveG40 said:

With wood fibre, Do you install this as batts up agains the wall?

 

Yes. As the existing plaster will be an unknown quantity in a 1950's house (?breathable?) you may be best to hack that off and do a 12mm lime parge coat. Tooth it for about half its thickness and bed the boards in, then mechanically fix with plastic hammer fixings. Finish with a toothed coat of lime plaster, then mesh, then another coat, and just trowel up if you like a coarse finish, or use a fine lime finish if you like 'mirror-polished'.

 

@Roger440 said "WUFI assesment is an awful lot of money to do just a wall or one room though."

 

Yes, agreed, and practitioners sometimes hard to find, too, but not if the OP is maybe going to add IWI throughout the house, and also not if your WF merchant offers the service for free, as at least one does.

Edited October 21 by Redbeard