benawhile

My small 2 storey house, built on a simple rectangular foundation, is being extended half width at the front and full width at the back. We have cavity walls existing and extended. The builders had to make vertical saw cuts in the existing brickwork at the corners where the extensions butt on, for a vertical dpc to prevent damp being carried across along the old external rear wall, which has now become an internal wall. Inside we are making openings in what was the external wall, but is now an internal wall, downstairs and upstairs. Downstairs we are able to leave a 440mm pier supporting the steel beam across the opening, which appears to also give some lateral strength to the external wall, the strength of the corner having been weakened by the vertical saw cuts However upstairs the boundary between existing wall and extension is required to be a flush join where wall which was external but is now internal is removed. There will be effectively a simple butt join between new and old wall as in the upper part of the picture with the bricks marked 1st floor A, at what was a corner. My concern is that as we have to continue the saw cut for the full height of the wall, there will be a weak spot at this join, as the new brickwork isn’t keyed in to the old, but furfixed, and the only way we can provide any strength is to continue the internal skin brickwork a bit further into the existing house thereby staggering the furfix profiles so that the internal one is set further back than the external. See the lower part of the picture, 1st floor B. Of particular note is that every alternate course in inner and outer skin is weakened by there being just two half bricks abutting each other There was a reason provided by the architect for specifying flush furfix joints rather than keying in as that would allow for more ground movement at the join Will this staggering provide appropriate strength? Obviously I will go back to the architect for more consultation and reassurance, but I wondered if I could get an objective view of whether this is a routine problem and solution.

I have previously asked about materials for loft insulation and have come down in favour of insulation roll, loft legs and standard loft board. I want to increase my loft insulation from 4” glass fibre to 10” and retain access and storage using wickes 7” loft legs and standard loft boards. Problem is according to all sources I can find, rock wool is a superior materila to glass fibre and while I would like to use it, I can't find it in 6” thickness, only 4 or 8, (100mm and 100mm) There is something called rockwool rollbatt at Jewsons https://www.jewson.co.uk/building-materials/thermal-insulation/rock-insulation/products/RWRB1503/rockwool-rollbatt-365m-x-400mm-x-150mm-pk-of-3/ Could I use Rockwool “rollbatt “ as if ti was standard roll to insulate across the joists in my loft, and board over it using loft legs? Is rock wool available anywhere in 150mm or am I misinformed about the superiority of rockwool? Finally a gripe: It is stated on US websites that rockwool has a better u value, repels moisture where glass fibre soaks it up, and is more fire resistant. It seems little has changed since I insulated my last house in 1979. There are the same two basic materials, glass wool and rock wool. Glass wool, confusingly also called glass mineral wool for some reason, confirmed by all sources as inferior to rock wool, is nonetheless almost universally available by all manufacturers, but rock wool is only made by one firm. Even products like Knauf Earthwool, which sound as though they would be more natural, and actually do feel more natural, are made of the same old glass “mineral” wool

Sorry to be late in replying, being new to the forum I didn't choose the right notification method. Thank you for these very helpful replies, especially A_L, which concurs with what a surveyor friend advised after I posted. I'm going for a cold loft. I will be getting CWI, but not until I have arranged surveys of the cavities, as there are wall tie issues in some houses in my road. The random sized graphite coated beads seem to have a good reputation. I have solid floors downstairs. I would have preferred suspended but heigh ho.

We have moved into a 1950's house with minimal insulation. There is a conventional loft space between a hipped roof and plasterboard ceiling with 4” rafters. There is 4” mineral wool insulation between the rafters, which I want to increase by about another 6 inches, either using Celotex, Steico wood fibreboard or similar, or wool insulation roll, sheepwool or mineral, and then part board over for access and storage. If I use wool then I expect I will have to use plastic loft legs. I have also found insulated loft boards available from B & Q with 100mm insulation glued underneath, so we might try wool for the non accessible areas and insulated board for the storage areas. I have read that wool insulation over the ceiling joists in a cold loft should be covered by a membrane to prevent the circulating cold air from wind etc from “wicking” off warm air at the top layer of the insulation. What kind of membrane should be used for this, as I have also read conversely that a non permeable membrane will cause condensation on the underside as water vapour rises from the house through the ceiling? Also, for the boarded sections, will the board itself act to prevent heat wicking away? If I use Steico or Celotex, laid across the joists, should I put conventional loft board above that, to protect it, or below it to stop it being compressed where it stands on the joists? For what it's worth, currently the eaves, with fascia and soffit boards, are fully enclosed with no ventilation, so the only loft ventilation is by any accidental gaps between soffit and brickwork, fascia board and felt, and at joints in roofing felt under the concrete tiles. We have white coloured but probably aluminium framed double glazing, cavity walls but no c.w.i. yet.