Extension questions

mad m

Getting a double extension plus attic conversion at moment. The builders are doing a 100mm outer leaf with 50mm insulation then a 100mm inner leaf. They are going to sand&cement and skim as a finish, is this ok for good insulation? Also the attic is a bit of a concern as we have no membrane between the roof tiles and the rafters, the roof tiles are cemented (house was built in or around the 1940's)

Whats the best option to insulate in between the rafters?

muffler

You are looking at a very deficient insulation method for the walls. It may be fine providing you dry line/insulate internally. The roof is a bigger concern. If you are renovating and extending would you not consider re-roofing also?

mad m

Whats the best way as far as insulation then with a build that I described. I thought the 4inch solid with 50mm insulation with another 4inh solid is a good solid build. Re-roofing would run into thousands but I suppose would prove worth while down the line.

Mellor

50mm insulation hasn't been enough (to meet min regs) for some time.
As for roofing running into thousands. Were you expect that a two story extension and attic conversation wouldn't? Reroofing won't make the whole job cost much more (as a % of total cost)

mad m

Mellor remember you said to me in a pm that 100mm outerleaf and 40mm insulation with a 100mm innerleaf, how come the 50mm insulation in between the two 4inch solids isn't enough..I'm confused....

As I said the builders were going to go with a hollow cavity with battons with a 65mm insulated 3/8's plasterboard, so I asked that they do a solid 100mm with a 50mm insulated cavity,100mm innerleaf.

Mellor

I believe I said 100 outer, 40 cavity, 100 inner.where the wall was to be insulated internal, as an alternitive to a solid block wall insulated internally.

The 40 mm is the min size of a cavity. The insulation to go with this will need to be at least 80mm*. Many people are aiming for 100mm

*80mm may not meet the regs for DEAP assessment, but on an extension there is only so much you can do.

Builderfromhell

It is my understanding that house extensions do not need to comply with current building except where it is reasonable to do so. When extending a house you are not required to insulate to current regs. I expect the logic being that this may not always be possible and/or the exisitng house may be very poorly insulated.
You also would not fit a radon barrier as radon can get into existing house. Neither would you fit adisabled toilet etc. etc.

sinnerboy

No BFH - there is an entire section (2) for existing buildings

points to note

min u value 2.0 for replacement glazing ( or new in case of extended parts )
min u value for UFH - 0.15
min replacemnet boiler efficiency 86%
got to detail out cold bridges
got to build air tight
got to achieve u values - as if new build

mad m

So to my understanding would be that after doing a 100mm outer leaf,50mm cavity and 100mm inner leaf, batton the walls and use insulated plasterboards? Whats the most insulated in mm you can get on a 3/8's plasterboard?

Mellor

Builderfromhell wrote: »

It is my understanding that house extensions do not need to comply with current building except where it is reasonable to do so. When extending a house you are not required to insulate to current regs. I expect the logic being that this may not always be possible and/or the exisitng house may be very poorly insulated.
You also would not fit a radon barrier as radon can get into existing house. Neither would you fit adisabled toilet etc. etc.

You to to comply with current values, there is no situation where it is not possible to do so. As for radon barrir, you have to enstall a randon barrier as there might be a radon barrier in the existing, also the RB acts as a DPM and a DPM is needed regardless.
Sinnerboy covered the main points.

The only only exception to this would be replacement elements on a protected structure where said elements would be protected. eg, glazing wouyld be replaced to match and would not have to comply with min U-values, a boiler being replaced would need to comply. An extension to a protected structure would have to meet min values.


mad m, The thickest size I have seen is 70mm insulation fixed to 12.5mm board

mad m

Mellor wrote: »

mad m, The thickest size I have seen is 70mm insulation fixed to 12.5mm board

It really is a mind field, I used to work on sites but I've lost touch in so many ways.