Adding Insulation to a crosswall house

RN7

I have recently bought a 1970s crosswall house.

All my external walls are studwall. There is no brick outerskin.

I believe there is some ancient rockwool between my studs, and then plasterboard over that internally. I can't quite tell from the gap I've been peering into where any VCL is fitted at the moment.

I am thinking about pulling off the plasterboard. Removing the old rockwool. And putting celotex between the studs. The wall isn't very thick so I don't think I will actually get much celotex in there - but it has to be better than some ancient rockwool! My walls in total are ~200mm thick from inside face of the plasterboard, to the external render. I am hopeful that I could get 50mm celotex in.

So questions:
1) I believe I need a VCL on the warm side of that - how do you seal that against the ceiling / floor / party wall / other internal walls? I am going to end up doing this one room at a time so I think I am always going to have some gaps at the ceiling / floor, and the internal walls. Is that a problem? Especially if I go through next winter with some of the rooms done, and others not?
2) I know ideally you would have another layer inside to stop the studs being a thermal bridge. But that would result in losing floor space, and having to move electrics and plumbing which turns this into a much bigger job which I'd like to avoid. Is it actually going to cause me problems? Or is it just less benefit than I could have? I've read about shadowing etc, but I am unclear how much this should concern me.
3) In terms of a VCL - Is there any chance my existing VCL is on the OUTSIDE of the rockwool? And if so is there any problem putting more insulation inside that provided I have another VCL inside.
4) My plan would be to use foil backed celotex - however I would probably still apply a continous VCL sheet inside that fixed to the studs. Do I still need to tape up the joints in the celotex in that case?

FreeBear

RN7 said: 4) My plan would be to use foil backed celotex - however I would probably still apply a continous VCL sheet inside that fixed to the studs. Do I still need to tape up the joints in the celotex in that case?

I would use aluminium foil tape on all joints between the sheets of Celotex and also use expanding foam plus gun to seal the gaps - Tip. A sharp serrated kitchen knife gives you a cleaner cut (and less mess) than using a saw when cutting Celotex sheets.

The foil backing will act as a VCL, but adding a secondary layer of plastic probably wouldn't hurt.

plumb1_2

I would ask your mortgage/ insurance, building control if it was ok to replace rockwool with celotex 
Is the celotex you are going to use have a fire retardant added. 

RN7

Is celotex likely to be a problem with building regs in a 2 storey house? I thought the requirement was related to taller buildings - I hadn't really considered this would need building control sign off. 

If I do this though I'd like to think I am starting off by approaching building control with a good plan!

jonnydeppiwish!

I’ve just finished some work with BC and they wanted rock wool, not celotex in the external wall. Not sure if this is a shift away from celotex?

ThisIsWeird

I cannot comment on the VCL issue - you'd need to do your research - but if you are removing all the p'board in any case, wouldn't the most effective insulation be obtained by checking the existing rockwool and, if it's in generally good order, top it up with more if required ( if it has slumped or compressed) and then overboard with insulted p'board - a doubly insulated whammy?

stuart45

jonnydeppiwish! said:

I’ve just finished some work with BC and they wanted rock wool, not celotex in the external wall. Not sure if this is a shift away from celotex?

Was the insulation in the cavity?

FreeBear

jonnydeppiwish! said:

I’ve just finished some work with BC and they wanted rock wool, not celotex in the external wall. Not sure if this is a shift away from celotex?

Cork would be a viable alternative to Celotex or fibreglass/rockwool - Fire resistant, similar thermal performance to fibreglass, and doesn't have the problem with slumping. Would be a bit more expensive, and there are not that many suppliers in the UK.

RN7

ThisIsWeird said:

I cannot comment on the VCL issue - you'd need to do your research - but if you are removing all the p'board in any case, wouldn't the most effective insulation be obtained by checking the existing rockwool and, if it's in generally good order, top it up with more if required ( if it has slumped or compressed) and then overboard with insulted p'board - a doubly insulated whammy?

I don't think that is the most effective - Rockwool is half as good as PIR give or take. 

Insualted plaster board will eat into the room. 

Plus I've never really understood with insulated plasterboard how that works given it must go on the warm side of any VCL. 

RN7

FreeBear said:

jonnydeppiwish! said:

I’ve just finished some work with BC and they wanted rock wool, not celotex in the external wall. Not sure if this is a shift away from celotex?

Cork would be a viable alternative to Celotex or fibreglass/rockwool - Fire resistant, similar thermal performance to fibreglass, and doesn't have the problem with slumping. Would be a bit more expensive, and there are not that many suppliers in the UK.

I'd worry about pay back time in that case. 

Insulation that is only as good as rockwool, but more expensive. Add in the cost of pulling off and replacing the plasterboard I just can't see it stacking up

FreeBear

RN7 said:

FreeBear said:

jonnydeppiwish! said:

I’ve just finished some work with BC and they wanted rock wool, not celotex in the external wall. Not sure if this is a shift away from celotex?

Cork would be a viable alternative to Celotex or fibreglass/rockwool - Fire resistant, similar thermal performance to fibreglass, and doesn't have the problem with slumping. Would be a bit more expensive, and there are not that many suppliers in the UK.

I'd worry about pay back time in that case.

Going by some estimates of EWI ( https://www.checkatrade.com/blog/cost-guides/external-wall-insulation-cost/ ), even at the lower end of £8K, it is going to take a very long time to get a return on investment. I'll have spent close to £10K on doors, windows, and insulation by the time I've finished the thermal upgrades here. Whilst I'm already seeing some savings, I won't recover the total cost in my lifetime. However, quality of life has been vastly improved by way of killing cold draughts and the place being easier to heat to a comfortable termperature - What price can be put on that.