Insulation options - Full fill cavity, no cavity/no PU in Floor

conor2949

Hi folks,

I'm hoping to start building in a couple of weeks time. Like the OP, i'm trying to get to the A3 rating with concrete block construction.

Is drylining ever an acceptable method? If you use a slab that doesn't have foil backed insulation to dryline - would this avoid the mildew/mould issue?

If you build with blocks on the flat and say 300mm of dense rockwool externally and render on top of that - will that be a more effective build? Is there not an issue with this in that people claim that concrete blocks help with thermal mass,which i don't think holds up, and means that you will now have to heat the block as well as the room? or am i missing a vital part of this type of construction?

Also what is the advantage of building a cavity wall and then pumping it full of foam or bonded beads? Does this not defeat the purpose of having the cavity in the first place?

Also the use of polyurethane insulation in floors has been pointed out as a no no. (I presume this refers to the likes of Xtratherm/Quinn/Kingspan) If there is a damp proof course layed, then surely it won't be an issue of using PU insulation to insulate the floor because water won't be travelling up through the dpc layer anyway? or have i missed something else here too?

thanks to anyone who can advise me on the above.

Conor

Slig

There are lots of posts on insulation here, its a real hot topic.
Unfortunately that means that there will be several schools of thought on the subject. personally I live in a dormer house, the ground floor is blockwork (high thermal mass) and the first floor/attic space is timber frame (low thermal mass)
Upstairs heats up easily but also cools down quickly, it also means that on a scorcher of a day the upstairs is unbearably hot, while the ground floor is generally ok.
The irish climate is very up and down and I think that a high thermal mass is necessary to regulate the temperature internally of the house.
If however you are aiming for an A3 rated house then the most cost effective method is probably standard cavity and then dryline, thus reducing your thermal mass.
The reasoning behind not putting PU boards into a floor is that the properties of the insulation are compromised by moisture. In theory seperating the insulation from the concrete should resolve this but in practice its only as good as the person fitting the membrane. Traditional EPS insulation doesnt absorb water so there isnt the same problem of reducing effectiveness. But in calculation 100 PU = 150 EPS roughly and this makes a difference with floor construction depth.

I've tried to explain this without the use of too many figures and as simply as possibly, I'm sure others will be along shortly to provide those.

Slig

Oh and the Rockwool idea, as far as I know this revised construction system would have to be certified by the IAB or agreed with the DOE that a structural engineer be appointed to do an analysis of the system. Rockwool is another one that doesnt perform as well when wet. Something like EPS is a better alternative and is used with most of these systems.
prices work out roughly 50% to 100% extra over traditional cavity construction last time I checked

ardara1

conor2949 wrote: »

Hi folks,

I'm hoping to start building in a couple of weeks time. Like the OP, i'm trying to get to the A3 rating with concrete block construction.

[HTML]Is drylining ever an acceptable method? If you use a slab that doesn't have foil backed insulation to dryline - would this avoid the mildew/mould issue?[/HTML]

You cannot achieve an 'A' rated design without drylining or applying a layer of insulation externally - the thermal bridging factor must be minimized below current standards. Are you saying the foil causes the mould? - Did you get this piece of advice from the same person that advised you on the floor below?

[HTML]If you build with blocks on the flat and say 300mm of dense rockwool externally and render on top of that - will that be a more effective build? Is there not an issue with this in that people claim that concrete blocks help with thermal mass,which i don't think holds up, and means that you will now have to heat the block as well as the room? or am i missing a vital part of this type of construction?[/HTML]

when you get the fabric performanc eto very good standards - U-values - Thermal Bridging - air tighness the temperature within the building should not fluctuate that much of internal gains are pretty regular. this means you do not actually have to 'cook' the fabric' every time you put the heating on


Also what is the advantage of building a cavity wall and then pumping it full of foam or bonded beads? Does this not defeat the purpose of having the cavity in the first place?

[HTML]Also the use of polyurethane insulation in floors has been pointed out as a no no. (I presume this refers to the likes of Xtratherm/Quinn/Kingspan) [/HTML]

Why a no no?

If there is a damp proof course layed, then surely it won't be an issue of using PU insulation to insulate the floor because water won't be travelling up through the dpc layer anyway? or have i missed something else here too?

thanks to anyone who can advise me on the above.

Conor

Call Kingspan, Xtratherm or Quinn and get their take on it. - You'll have to find your way through a mze of so called 'expert advice' - just do your homework.

RKQ

I don't wish to be off subject, as I feel this is very relevant. Maybe I'm missing something. But would it make more sense to resolve these issues before applying for Planning Permission?

Changing from standard timber frame to standard 300mm cavity construction isn't a major difficulty because the wall thickness is similar.
But changing from either to blocks laid flat with 300mm external insulation is a major "detailing" job.

Such a change could affect ridge heights, soffit & fascia, Dormer cill to eaves distance, reveal details, cill, percentage of daylight to floor area etc. Not to mention basic dpc, dpm details.

Even changing from a timber floor joists to a conc slab affects construction details - ie using blocks laid flat on groung floor either results in smaller floor area or a higher ridge height! ( If you don't change the pitch! - sometimes you can't change it, as its low already)

Personally, IMO its best to do your homework first, then design your house, apply for permission etc. At least the planning drawings can be easily brought up to Construction drawings.:rolleyes: