Insulating external walls

[Steve]

My house is 50s built with cavity walls. It has cwi already, it's the soft brittle foam stuff that turns to powder when you touch it (there were two massive bergs of the stuff in the loft when we bought the house).

Anyway, I recently did a job for somebody local in a similar house and they'd also had the walls lined internally with 100mm of rockwool and boarded.

We're planning getting new triple glazed windows this year and want to maximise our heat retention. I also plan to install either a piv unit or mvhr

So in short, is it worth getting the walls lined internally? It's a job I can do myself. If I use plasterboard lined with celotex can I just stick that onto the existing wall? Do I need any kind of vapour barrier?

We have other work planned in the future, building an extension on the back and a large porch on the front. Would it be better to install external wall insulation instead as part of this work? My only concern there is changing the appearance of the house. I know they can do a brick finish but I think I'd rather have a white render finish.

So, best way to add insulation to already filled cavity walls?

Cheers.

 

[oldbutnotdead]

If you can afford it and it'll work with the roof (cos it's not a job you can do room by room) EWI will be a far better solution. If you insulate internally you'll lose 150mm from each outside wall (in terms of room size). Any power sockets and light switches on those internal walls will need to be relocated- there might be slack in the cables running under the floors, low odds there'll be enough slack on light switches. Radiators- if pipes come up through the floor good odds they'll be behind your new plasterboard. Yes you can remake but it's messy and time-consuming.
The big bonus of EWI is you get far more thermal mass inside the insulation so your room temperatures will be more stable

 

[omph]

My house is 50s built with cavity walls. It has cwi already, it's the soft brittle foam stuff that turns to powder when you touch it (there were two massive bergs of the stuff in the loft when we bought the house).

Anyway, I recently did a job for somebody local in a similar house and they'd also had the walls lined internally with 100mm of rockwool and boarded.

We're planning getting new triple glazed windows this year and want to maximise our heat retention. I also plan to install either a piv unit or mvhr

So in short, is it worth getting the walls lined internally? It's a job I can do myself. If I use plasterboard lined with celotex can I just stick that onto the existing wall? Do I need any kind of vapour barrier?

We have other work planned in the future, building an extension on the back and a large porch on the front. Would it be better to install external wall insulation instead as part of this work? My only concern there is changing the appearance of the house. I know they can do a brick finish but I think I'd rather have a white render finish.

So, best way to add insulation to already filled cavity walls?

Cheers.

What is the depth of the cavities ? You could have the existing foam removed and have some loose fill blown in. The problem with cavities is there are two sources of heat loss, one through air flow in the cavity and the other is through conduction. If the wall heat loss is through open cavities, then EWI won't fix anything. EWIs are usually geared towards single block walls for that reason.
Ensure the cavities are properly sealed if considering EWI, but you are better off having more modern loose fill insulation blown in instead.

 

[Steve]

Won't the existing foam insulation have sealed off any air flow in the cavity. I'm not sure of the cavity depth but fairly sure it isn't the full 100mm currently required. I did poke a hole through recently for a cable and I reckon it was 60mm tops.

 

[omph]

It depends on the condition of the foam (hopefully it doesnt contain asbestos or anything). As you described it, it seems like its in a pretty poor condition.

 

[Loofah]

External if at all possible

 

[jeds]

I posted this on a similar thread - about 20 posts down!

EWI can look awful and presents issues with openings, window sills, and matching into eaves overhangs and adjoining properties. Otherwise it is the least disruptive and will achieve a fairly predictable result.

IWI is disruptive (which may not be an issue, considering the scope of works you have planned) and has the advantage of not altering the external look of the house. It also provides a faster thermal response, which means you turn your heating on and it gets warm very quickly but it also cools quicker. This is ideal if you have more of an irregular lifestyle and aren't home all the time.

From your description I would go with IWI using PIR thermal laminate plasterboard. You can go room by room as and when you complete your works. 52.5 should be your minimum, but also consider 62.5 or 72.5. Fixing depends what you are fixing to? What sort of plaster do you have at present and how easily could it be removed?

PS, when comparing costs, make sure you compare like for like. Don't compare PIR board with XP board, which will be considerably cheaper.

If your house is brick, the current u-value is approx 1.72. Add 52.5 it will be 0.38. Add 62.5 it will be 0.32. Add 72.5 it will be 0.28. Neither of these complies with regs but all will make a massive difference.

You could commission a non-registered EPC from any local provider and ask him/her to prepare two or three options. One for the existing, one for 52.5, one for 62.5 - etc.