osprey

If, in your edit, you mean thermal bypass due to poor fitting of the PIR, I would contend 1) that the gap would be intermittent and small, and 2) due to the membrane, the air in the gap would be static. Air is a good insulator, as long as there is not movement due to convection currents, and, as heat rises and the room is warm, there would be negligible heat loss due to convection.

Thanks for the replies, but they are not really (mostly) answering my question, which was using PIR with the membrane hammock. Would the membrane mitigate some of the disadvantages of PIR? Knauf and similar have a worse U value than PIR, and PIR sheets come with a vapour control layer on one side. (The latter would be handy for a few areas which I can only get to from below). (And thanks, Redbeard, I had already read those links you provided)

I have a suspended wooden floor, 1920s house, with 100mm joists, and about 250mm gap below joists. Sleeper walls are ventilated. I have read MANY approaches to insulating these, and am bouncing between the choice of materials. PIR would give the best u value, but difficult to work with, needs space for battens. 'Loft roll' will settle over time, needs 120mm for spec U value etc... So I was thinking of going the Rockool/Knauf 'loft roll' approach, with breathable membrane slung across the joists. But I was wondering if I could, instead, use the breathable membrane to support the PIR sheets. So, same 'hammock' approach as with 'loft roll', but no need to allow space for supporting battens and the membrane would reduce heat loss due to any gaps around PIR. PIR would be cut for close fit, and should slide along membrane better the rough wood joists. Anyone see a major problem with this approach?

Thanks all! I like the idea of 150 cavity, and eps bead full fill, but seems a bit 'new' around here.

Maybe I was unclear. Proposed wall would be externally rendered (so block-block), cavity 100mm, of which 50mm would be 'some form' of insulation. 50mm gap. The 'room-side' of the inner leaf would have 50mm Celotex. Yes, I was wondering why not 150mm gap, and/or full fill with beads. Seems there is still a lot of caution about not having a clear gap...

I am planning a double height, 5x5 m extension to my 1920s detached house (50mm cavity brick). The architect says open cavities are 'best practice' and is planning for 100mm, half filled cavity, with additional 50mm Celotex inside the inner leaf. Outside will be rendered. I have concerns, among them if this would make it difficult to hang cupboards, TVs etc, as well as possible construction errors, and wonder if this is the best approach (being used to brick under thin plaster walls!) My plan for the existing part of the house is eps beads in the cavity, and I could add ewi to some elevations later. The most exposed elevation will be half extension, and half existing wall. So I could override the architect, and full-fill the cavity, or leave that whole elevation cavity unfilled (I guess). Any thoughts? I am reluctant to override the professional...

Thanks both Yes I am aware of the benefits and possible pitfalls, but was trying to get an idea if I needed to shop around for a specific bead. I have now gained the impression that the beads are all pretty much the same, with the source material coming from a few big chemical names in Europe. The chemicals are 'expanded' in UK by smaller companies, which may vary in uniformity of product, before going to the installers.

I am looking into CWI for my 1920s house, and apparently bonded EPS beads would be the best solution. However, I see a lot of references to graphite/platinum/grey beads, and a wondering if all beads were pretty much the same now, or if I should be looking for anything specific. For instance, my local installer claims to use Evobead, which claims to have the appropriate 0.033 W/mK thermal rating, but just seems to say the beads are 'grey'. No mention of graphite. Otherwise, I need to go to a local branch of a national company for e.g. Thermabead 'carbon bead', or similar. My normal preference would be to go for a local company. Anybody have any thoughts/knowledge on this? Thanks.

Thanks for reply. Ceilings are 2.7m high, and I could accomodate 25mm build-up if needed, and plan to include under-floor insulation, but there is a limit as to what can be done with an old house. Rads throughout look like the plan at the moment. Current CH system is about 10 years old. I will look at increasing radiator size in the future if/when needed, and also when I get a better idea of the house's thermal performance following the improvements.

I had a chat with a chap from a company which does ufh, both regular and retrofit surface mount. He said the retrofit/surface mount stuff was designed run with hotter water, both for faster response times and to be easier to mix with normal rads. Of course this would not then be suitable (I assume) for ASHP applications. A builder I spoke with about the project suggested SM UFH, as he does it 'all the time'. But I am not convinced, hence the question here: thanks for the useful comments.

OK, understood, but many houses have UFH to the ground floor, but rads to the bedrooms, so running a 'mixed system' do they not?

Thanks. The reply certainly had more brevity than the question. Any particular reasons? (Are would be part of extended kit hen/sitting area/.

I am at the earlys stages (architect involved) of planning a double height extension to a 1920s build detached house. It will convert an 'L' shaped house into a square, the extension having a 5 x 5m footprint. So one quarter of the house will have a new ground floor, the rest being suspended wooden floor. What heating should I go for in the extension? 1) UFH in the screed. The rest of the house has radiators currently. Plan to insulate underfloor rest of house, so retrofit UFH could be possible later 2) Radiators. Matching the rest of the house. 3) Radiators now, possibly surface mount UFH added to both old and new floor later if it made sense. In 1, the ufh would have a very different thermal response to the rest of the house, (slow to heat/cool) and require additional plumbing etc. With 3, I am not sure if UFH heating alone would ever make sense for an old house, but radiators + retrofit, surface mount UFH might remove the need for oversize radiators if ASHP beome the norm? Any thoughts welcomed, thanks.

Could it ge a 'Right to Light' thing? Either 45 or 25 degree 'rule' if window is facing the extension.