MortarThePoint

They're a law unto themselves. My over 70 father approached the planners asking about replacing the bungalow he grew up in now that my grandmother has died. She had expressed her hope that he would. They said they would expect 3 or 4 houses to go in a plot of that size (3/4 acre in middle of village). Consequently it has been more of a fight than he's been subconsciously prepared to take on and now 5 years later he said to me, after reflection during Covid, that he realises he'll never get to live there. Sad, he could have been living there by now, but he doesn't handle stress. He has dutifully mowed the lawn regularly for all this time.

It's a lot of a short span, but on a long one its pretty small. Bear in mind its not going to be deflecting up and down on a daily basis but is going to get loaded out and deflect. I think you'd fall over if a floor deflected by 12mm whilst you walked on it. @SuperJohnG could probably hold River Dance lessons on that floor.

I'll hold my hand up as a bit of a hypocrite here as I got my garage RIR trusses designed based on 2.5kN/m2, but they have a clear centre of 4.5m and overall span of 6.5m I am surprised you have ended up with so may joists as those attic trusses are 222mm on 600mm centre. Shows the benefit of those vertical members in an attic truss

Yes, I think 8mm is probably too strict for longer spans. I know the normal guidance is 12mm (and 0.003x span) and for longer spans it's probably less important to tighten that figure.

I know that MVHR can divide opinions, but the numbers don't work for me as my house has been conceived (having ASHP and not hyper insulated). I respect that it works well for some people's use cases. I had planned to use window trickle vents, but take exception to how they look and how they would likely be lived with (I think most people just leave them closed). Trickle vents act as a point of ingress and egress for fresh, but cold, air from outside. They are often perceived as 'free' but will of course have a cost when included in a window quote. Though some have filters to keep out insects etc, I suspect they are not very effective. I think if I was starting from scratch I would consider airbricks with internal gratings, but that ship has all but sailed now. PIV looks to offer a happy middle ground for my needs: It is relatively low capital cost and actually likely to be cheaper than or similar to window trickle vents. It has reasonable filters, so should provide good air quality It is centrally controllable so I won't find all the trickle vents closed It only has a single eyesore Benefits from solar gain in unheated loft space (dubious) I don't perceive it as any more expensive to run than trickle vents except for the power consumption of the pump itself ~5W --> ~45kWhr/yr --> ~£9/yr. Both waste warm air in the winter. Key concerns: Egress: My main concern is where does the warm air egress and does it cause condensation there. I'm not timber frame so am not bothered about OSB walls. If it is in the cavity that would likely be OK as it would drain. The graph below suggests that if the humidity in the house is at 50%RH and the temperature is 18C, then condensation would occur when that air is cooled down to about 8C. For the vast majority of the year it shouldn't be an issue then. [Please challenge this if you can think of why, I don't know much about dew point]. Draught: It creates a centralised point of draught rather than a distributed one like trickle vents. The plan would be to put it in the 2 storey hallway that would make it >1.5m from people and have a direct 'drop' to ground floor level. I wonder about detecting peoples movements (plan to do it for lighting) and turning off the PIV when people are on the landing or stairs. You can have heated PIV, but the efficiency would be rubbish. Summer Heat: The PIV would be drawing hot air from the loft and so heat the house. We can counteract this by opening windows and I suspect the heat gain in the loft would be less due to the air being pulled through it. We'll have conventional extractor fans in bathrooms that will likely turn off based on %RH. Why not MEV: Where is the air coming in from and what is it bringing in with it No filter (similar to above) Multiple points of extraction so ducting required (and multiple eyesores) Draws in moist air so mould growth and mechanical consequences possible I'd love to rant about how I'd design and build my own system but contrary to the playground quip, we don't live in a free country.

But I expect the 8mm specification is excessive for large spans as I have calculated. It's probably vital for a feeling of quality at lower spans, e.g. <=4m

If you'd doubled up again you wouldn't need any OSB3 ? It'll be nice a firm though

In terms of people walking on a floor and a perception of bounce less deflection is always going to help, but a short span that has 8mm of deflection will bounce much more than a long span with a deflection of 8mm. I presume the deflection figure is based on 1.5kN UDL live load. You'll effectively have a mass/spring/damper arrangement and with a larger span you'll have more mass, spring and damper. That may or may not help from a resonant frequency perspective. Hand waving and most importantly ignoring effects of resonant frequency: A 6m beam with 1.5kN/m2 * 0.4m UDL applied has a peak moment of 2700NM [1kN/m2: 1800] - Adding a 1kN point load midspan increases that to 4200NM, so 56% more [3300, 83%] A 5m beam with 1.5kN/m2 * 0.4m UDL applied has a peak moment of 1875NM [1250] - Adding a 1kN point load midspan increases that to 3125NM, so 67% more [2500, 100%] On that basis, if both floors are designed to have the same max deflection, a 5m span that has the same deflection specification will feel 20% more bouncy. I suspect it's more important to understand floor dynamics (which I surely don't) than set a specific maximum deflection value. However, setting a maximum deflection figure is probably more important with shorter spans.

You've bags of room to get insulation in there. If you've used EPS there up until now it could be replaced with PIR and that's 2/3 the thickness for same performance. You could research spray foam as may work well to fill the gap. Likely 20mm of PIR plus spray foam a nicely workable solution.

@Bobo just double check they have used the correct sheet material for the membrane as well since they may not have understood what was required and I can't see any red membrane showing in your photo (actually some Radon membranes look to be green as well). I expect joints would need to be overlapped and taped as well which may not be the case for DPM. Potentially a bigger problem if they have used the wrong membrane, but it may make for an easier solution as it potentially narrows down your options as you'd probably have to have a fresh membrane over the top of the concrete slab. It would be a pain to live with during the build though, so you may prefer to go a 'perimeter and infill' approach where you put gas proof DPC in all the walls sticking out enough (e.g. 100mm) and then later, when less likely to get damaged, you stick in areas of gas proof membrane attached to the DPC using that gas proof tape. Hopefully obvious, but that gas proof DPC would have to be under ALL walls, so include internal walls. You'd be making a continuous barrier out of the DPC and later applied membrane. You don't seem fully convinced you need a Radon barrier. Double check that as I don't think there are any grey areas in it from a regulations perspective. Maybe you wanted it as you are outside (but near) a required area, but is it in the existing house and if so how are you joining to it?

No worries, happy to try to help. I've found these bumps in the road can be stressful, but are part of the journey. Solve them promptly and completely so they don't come back to haunt you and cost much more to resolve

The suggested fix in green is likely to be the most pragmatic solution. They will have to scrap out the perimeter insulation a good depth (e.g. 75mm), tuck in some double sided butyl tape and the attach the green portion. Pig of a job unfortunately I think this is the sort of tape you'd want. https://www.permagard.co.uk/radbar-double-sided-tape-dpm-tape

I think your idea would work against the damp as the perimeter insulation may not wick water, but it wouldn't satisfy the Radon requirement as far as I know, but my knowledge there is very limited.

I think it's more about the Radon gas side of things. Something in your favour is it looks like there is around 50mm of edge insulation.

Ah, that's very different then. To protect against gas you need to have it continuous across the cavity. You either have to have a radon barrier or not and if you have to have it it has to be installed correctly. Could be very expensive to correct later if building regulations get worked up about it, or your Warranty provider. Is the material you have referred to as DPM actually a certified radon barrier. To key options come to mind: Take down the first course of bocks to expose the orange DPM/Radon barrier and make a good solid joint to new sections. Apply a Radon barrier on top of the concrete floor slab, across the cavity. This would require a cavity tray on top of the radon barrier which is a nuisance.

That answers my question, the first course of blocks is in place (pink in the sketch below). Drastic, but you could remove that course of blocks to join to the orange DPM, but I'm sure someone has a better idea

I suspect the orange DPM would only really need to extend up another ~100mm join with the DPC (blue) to perform its function * . The DPM doesn't need to cross the cavity as far as I understand. You have a break in the damp protection between the top of the orange and the blue DPC. Is the first course of blockwork against the concrete floor in place yet? * Please confirm it is only against damp not gas (eg Radon etc)

I'm trying to make up my mind between roof tile vents or wall based extractor fan outlets. It would be the roof option all the way if not for the possibility of condensation being an issue. Does anyone have any experience they can share. I've copied below some pictures of what the roof tile vent looks like (in this case for an SVP vent). I've also copied a very good video about installing such vents. He advocates having a horizontal section to allow any condensation to pool and then evaporate naturally. In many of my instances, the insulation will be at rafter, not ceiling, level but I expect a horizontal length is still a good idea. I probably have an irrational fear of ducting with mould growing on the inside.

Is the current DPM between the insulation and poured concrete? A sketch or photo would help understand

This is looking promising. I weighed down the closest pipe with a brick overnight. I may also try a heat gun at around 100C (testing on membrane scraps first)

Would be OK upstairs but not with the gas barrier

I was pleased to get the keep outs, edge strip and membrane down But unfortunately the UFH pipes are pulling up Any ideas as to how to solve this? I can't use anything that penetrates the green sheeting. It's not insulation underneath, it's concrete and, on ground floor, a gas membrane too.

I think you'd be lighter after releasing it as it is under pressure inside you and so more dense than the air your extra volume would displace. If you were underwater though, you would become less buoyant after releasing it.

Rats, I think I should have done the perimeter strip before the membrane

Interesting talking to one screed supplier who said liquid screeds can react with concrete, so definitely need the membrane. She also said that liquid DMs are not suitable. I had read before that screeds can react with aluminium (e.g. on insulation) and know that wet cement does too. I recall reading about an unfortunate prototype aluminium cement mixer. Looks like I have to carry on with the vapour barrier. I'd bought some spray adhesive in case I wanted to bond the membrane down against gusts of wind etc. I haven't used any of that yet though.