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Strip off or add to?


Gyn_Paul
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I'm renovating a bedroom and wondering which route to take.

It's a first floor addition (probably 50's/60's built on to a stone ground floor) of rendered breeze-block exterior with those cellular terracotta blocks, about 30cm x 20cm (ish) and 10cms thick. With the plaster applied directly to the interior face. No insulation (other than the block's air gap).

Insulating is the main purpose of the exercise (apart from the race to replace the ceiling before it lands on my head one night!), so the choice is either to add polystyrene-backed plasterboard directly to the existing plaster, or to strip off the plaster/terracotta blocks back to the breeze blocks, stud it, and re-board it. Then I would have a nice 200 mm cavity which I can fill with fibreglass.

Either way, I need to take out the window and re-fit it flush to the new inside level, so that's not an issue, but the room is small already, so losing 212mm by adding the fibreglass+plasterboard on top of the existing wall, isn't really an option.

So I suppose it boils down to which route gives the higher 'U' value.

Anyone been down this road?

p

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You said earlier in your post you would use foam backed PB which has a higher thermal value than glass fibre so will you still lose 212mm?

Most builders will tell you the terracotta blocks are really good insulators.  It sounds a lot of work removing it all but the end result will be more comfortable and lower heating costs for a good long time.

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I have gone down the latter, a few rooms in the main building had been done with doublage and I was very unimpressed with it, it literally fell of the walls and had trapped damp which had created some awfull mould which must have been a health risk, in my own flat the damp was so bad that when I pulled off the doublage the polystyrene behind had decomposed and fell to the floor in millions of little billes.

I now use metal stud and laine de verre, there are no damp problems in my flat after 7 years of occupancy so that speaks for itself.

I dont have the space to lose so have traded off some insulation for thickness, I put the 48mm rails against the brick walls with a tiny seperation and use expanded foam to rigidise them and prevent a cold bridge, I use 100mm insulation either torn in half or often just compressed in, it doesnt bulge the placo enough to be noticeable.

In all the thickness is 60mm, you could go further for more insulation but we are talking walls, better for you to concentrate on your ceiling, do a suspended one with 200mm of laine de verre above, the upper rails of the wall studwork will serve you well for this.

Finally the gap behind makes cabling etc a breeze, window reveals become a proper rather than a bodged job and all you fixings for hanging things etc will be far stronger even if you dont fix to the metal stud as the plasterboard used for the doublage is only 10mm thick and not the N.F. quality.

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I would of course - if I go down the wrecking route - line the inside of the parpaing with a dpm., although the render is in good condition, and  elsewhere where the block-work is revealed, damp (even on the west side of the building where the prevailing driving rain is) isn't an issue.

As to the ceiling, I've already done the room above and replaced all the rotten floorboards, filling the the joist voids with fibreglass as I went.

With regard to the supposed insulating quality of the terracotta; all I can say is this room has an insulated floor, an insulated ceiling, one wall against 50cm stone (barn internal wall) , and two party walls to other similarly heated bedrooms, yet it can go from 20C when the CH goes off down to about 13C or so in a couple of hours, so my feeling is that terracotta is exactly as insulating as you would expect a plant pot to be, and I can't wait to get rid of it!

So  the consensus is that extruded foam (what is it? 60mm... 80mm?) has a greater U value than 200mm fibreglass?

p

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Hi Gyn,

I would recommend avoiding conventional dpm if possible. Try going for the studded alternative which allows air movement across the face of the wall then Montant and rail metal stud with Kingspan or similar insulation fitted as the studwork goes up so that it is tightly trapped then spray foam with mousse expansif any tiny gaps and board with moisture resistant plasterboard. Laying a tile batten below the bottom edge of the board behind where the skirting will go and removing the batten once board is fixed. This helps prevent any damp transmission and when skirting is fixed it isn't seen, There isn't any slump of laine de verre doing it this way just remember to allo space for gaine if putting in sockets / lights.
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