Fabric spec - what can I improve easily?

[Buzz]

Hi @Indy reading some of the replies above and based on my own experience i would agree with the fact that your architect has given you a build spec without any real consideration as to how difficult the wall and roof detail will be to build in the real world. 

The floor is a odd thickness, i was told a block is 225mm tall and the 1st row will sit on the B+B floor with a layer of DPM on top so you will need to lay DPM across the floor and up the walls 225mm, so you have 225mm to play with i opted for 175mm insulation and 50mm flow screed, your only adds up to 185 so not sure how that is going to work out on site for you .

The walls with PIR are going to be a nightmare to install correctly personally I wouldn't even entertain the idea, I would even consider if you don't want to increase the size of the cavity going for full fill batts and accepting 0.017 u value done properly rather than 0.015 with PIR done half arsed. 

Totally agree with @Iceverge regarding the roof .

IMHO putting PIR in between rafters should be the choice of last resort, fluffy stuff between the rafters and PIR above or below or combination of both .

But most importantly whatever you choose to do make sure that it will be achievable on site in the real world, you would be far better of have a house built to basic building regs but done properly than having a higher spec but a difficult design and ends up being build poorly. 

 

[Iceverge]

Flow screed isn't a bad idea actually. I forgot that it was above block and beam so 100mm of concrete wouldn't be really required. 

 

I can't see how the Architect could possibly get to a U value of 0.15 from 100mm PIR. Closer to 0.22. 

[Indy]

Thank you @Iceverge - that's a really terrific input and lots of food for thought. I will admit I don't know what the difference is between yours and the one I posted originally apart from the areas where there's greater insulation and wall depth, and therefore assuming U-Values must be better. I take your point about buildability and cost as well. 

 

The tough part will be how do I get this all incorporated in my BR drawings as it could mean redoing a lot of the work that has already been done and I'm really at a point where I want to finalise the pack and start tendering the work out!

[Iceverge]

 

 

Thats a start!

 

Notes: 

 

The seperate DPC works fine for both cavities, its what we did. 

 

THe DPM is between 2 layers of EPS, protecting it. 

 

EPS 100 will take UFH staples better than EPS 70. 

 

The cavity extending well below the floor insulation is required to  prevent mortar dropping making a thermal bridge here. 

 

I haven't included any periscope vents for the void in the drawings. 

 

The French drain will really protect the floor from dampness and allow everything to stay dry and perform much better. 

 

U value is about 0.14 worst case.

[SBMS]

My makeup:

 

walls - brick and block, 200mm cavity eps blown beads a- 0.15 u value

 

floor - 200mm pir (two layers of 100mm) - 0.09

 

pitched roof - 304mm posi rafter, cellulose blown insulation with 22mm wood fibre sarking and medite smartply as inner airtight and vapour control - 0.11 u value

 

flat roof - warm roof, 200mm PIR over 225 posi joists with medite smartply again  - 0.10 u value

 

SAS aluminium windows and doors average 0.75 Uw value

 

SAS sliders and bifolds average 0.9 Uw value

 

MVHR throughout 

 

wall hung floor posi joists with passiv purple around hangers (just in case) 

 

aerobarrier throughout target 0.6 ACH

 

ASHP with downstairs UFH. Currently deciding between fancoils upstairs or traditional rads with AC units for cooling. 

 

Spent a fair amount of time detailing the above and I think it represents a high performing traditional masonry buildup. Would be wary of things like PIR in your roof. Whilst theoretical u value is 0.11, I doubt this would be achieved unless you’re all over fitting. And even then wood shifts and gaps open up. Opting for things like pumped beads and cellulose designs out any potential issues with workmanship - even the most diligent builder might have a crew on a Friday afternoon that rushed something. You’ll live with that Friday afternoon job forever. 
 

Have a look at your thermal junctions as well. Make sure where your roof and walls meet is continuous for example (beads and blown cellulose work well for example here). Check or design your thermal junctions at doors (thermally break across your cavity

for eg don’t use concrete fill cavity to sit your doors on. It’ll bridge. ) Learned a lot on last build and have tried to design out my mistakes from last time!

Edited December 3 by SBMS

[SBMS]

Also - don’t do B and B floor. Hated it on last build. Flush thresholds are difficult (can’t cover air bricks). Some of the longer spans make the crockery ‘jiggle’ when my kids stamp on the floor. Probably could have been improved with smaller spans but we are

going slab this time. 

[Iceverge]

[G and J]

12 hours ago, SBMS said:

Also - don’t do B and B floor. Hated it on last build. Flush thresholds are difficult (can’t cover air bricks). Some of the longer spans make the crockery ‘jiggle’ when my kids stamp on the floor. Probably could have been improved with smaller spans but we are

going slab this time. 

Oooh you got a blank look from Iceverge lol

 

What was you largest span?

[Iceverge]

I've never seen block and beam close up. 

 

It's ground bearing slabs all the way near where I'm from. 

 

Our first floors are precast hollow core planks. An elephant wouldn't disturb them but I think it's an overkill for a house. 

 

I understood that on some clay soils b&b was required due to ground heave etc. 

Edited December 4 by Iceverge

[JohnMo]

13 hours ago, SBMS said:

don’t do B and B floor

I never really understood why lots of here, seem to do them.  A house down the road did, on a nice flat area, basically on sand, so zero need. Front door threshold is about 3 foot of the ground. Looks strange for one, and has cold air blowing beneath it also, so not for heat loss either.

[Iceverge]

 

 

Flat roof is much the same as the pitched roof so I haven´t labled it. 

 

You could use 22mm OSB instead of the plywood in 2400x600 sheets. It would be easier to manhandle onto the roof and offer much the same pullout resistance as the ply if you missed the rafters with the screw.  

 

 

[Iceverge]

18 minutes ago, JohnMo said:

I never really understood why lots of here, seem to do them.  A house down the road did, on a nice flat area, basically on sand, so zero need. Front door threshold is about 3 foot of the ground. Looks strange for one, and has cold air blowing beneath it also, so not for heat loss either.

 

Personally I think lots of these things are a legacy of not having modern machinery. 

 

Strip foundations require minimal concrete which used to be mixed by hand or in a small drum mixer. Like wise block and beam or a timber suspended floor can be handballed into place off the back of a builders trailer with no machines needed. 

[joth]

On 03/12/2024 at 06:36, nod said:

You should be sealing the boards to stop the air getting behind them 

 

 

Doesn't this mean all penetrations for sockets switches etc need to be airtight back boxes and sealed to the face of the board?
And make the airtightness vulnerable to any future changes even of a fairly cosmetic level to the wall finishes.

 

Personally I'm happy we went with a clear service void behind the boards, and airtightness envelope behind that. (OSB lining)

 

[JohnMo]

26 minutes ago, joth said:

happy we went with a clear service void behind the boards

Air tightness layer, 50mm service battens, plasterboard. Did same treatment on walls and ceilings. Low profile down lights used, where fitted. 50mm also allows the led inverter thing space.

[nod]

2 hours ago, joth said:

 

Doesn't this mean all penetrations for sockets switches etc need to be airtight back boxes and sealed to the face of the board?
And make the airtightness vulnerable to any future changes even of a fairly cosmetic level to the wall finishes.

 

Personally I'm happy we went with a clear service void behind the boards, and airtightness envelope behind that. (OSB lining)

 

Yep they should be sealed around everything Your Probably over thinking it If your worried about later cosmetic changes Fixing a shelve really won’t make any difference 

[G and J]

5 hours ago, JohnMo said:

I never really understood why lots of here, seem to do them.  A house down the road did, on a nice flat area, basically on sand, so zero need. Front door threshold is about 3 foot of the ground. Looks strange for one, and has cold air blowing beneath it also, so not for heat loss either.

My understanding is that there isn’t a huge airflow typically below BnB floors, the vents are more to prevent gas build up than humidity control.

 

On the build of our current house we switched last minute to BnB as a land drain was discovered where the house was going, although we diverted it to where it was supposed to be on medium elasticity clay it still meant BnB was cheaper than continuing with a solid floor.

 

On the new pad we’re going for them as the slope is such that at the back of the house we’d have to dig down circa 700mm and then put lorry loads of stone is to build up, on a sight where we can’t bring in a tipper.  With BnB we cart a lot less away.

 

There’s also no need for a BnB floor to be any higher than a solid one as long as you are happy to dig down a bit to get the void under (opinions appear to vary between 150mm min and 225mm min.  In our case we might have to push some soil towards the back of the house, but that’s it.

 

However I’d go for solid if I didn’t need BnB, but I think it’s potentially a ‘save having to think’ option for an architect.  

[G and J]

On 03/12/2024 at 11:14, Iceverge said:

Floor construction: U Value = 0.13 W/m².K

• 100mm powerfloated concrete with embedded fibers and UFH.
• 500g. Polythene separation layer
• 100mm PIR from Seconds&co or 100mm EPS70. ( I prefer the EPS)
• Polythene DPM
• 100mm Pir From seconds & co or 200mm EPS. (I prefer the EPS)
• Concrete beam & block floor to manufacturer's design

@Iceverge is your preference for EPS over PIR primarily due to environmental or other considerations?

[SBMS]

7 hours ago, G and J said:

Oooh you got a blank look from Iceverge lol

 

What was you largest span?

Think it was near 5.7m. Wouldn’t do that again. 

[Iceverge]

40 minutes ago, G and J said:

@Iceverge is your preference for EPS over PIR primarily due to environmental or other considerations?

 

Gas migration, shrinkage, cost, off gassing,thermal bridging through the foil, recyclability, durability, permeability, hydrogen Cyanide gas when burnt. 

 

Grenfell. 

 

 

[Iceverge]

I'd much prefer to substitute the PIR for mineral wool in the roof above scenarios for @Indy but I think given the cost differential and the on paper reduction of U value I think I'd be shouting into the wind. 

 

This would be better is almost every way. Espically long term performance and fire prevention. 

 

 

 

Edited December 4 by Iceverge

[Indy]

Can I ask for an opinion on the option to go for a Lewis deck on the 1st floor ? This would be covered with a thin layer of screed which gives the floor a little more solidity, acoustic insulation (which I want to prevent noise transferring across floors) and a little better fire prevention capabilities. 

 

This was something the architect and I had discussed a long time ago and agreed but due to the whole process dragging on far longer than anticipated - was mostly missed in the pack I have so far. The SE design will need to be redone, the 1st floor height has been done at 2422 to account for normal floors and 22mm UFH system, whereas Lewis deck would mean I need to give up an extra 27mm of headheight as the system with UFH will come in at 52mm vs the 25mm of the existing option.

 

Has anyone does this and does it really work? This is the one we're considering LEWIS-Metal-decking-brochure-2022-EN-web.pdf

[G and J]

3 hours ago, SBMS said:

Think it was near 5.7m. Wouldn’t do that again. 

Ta.  I had that option and was nervous about bounce so I’m designing in a sleeper wall so our max span is 3.7m.

 

[G and J]

2 hours ago, Indy said:

Can I ask for an opinion on the option to go for a Lewis deck on the 1st floor ? This would be covered with a thin layer of screed which gives the floor a little more solidity, acoustic insulation (which I want to prevent noise transferring across floors) and a little better fire prevention capabilities. 

 

This was something the architect and I had discussed a long time ago and agreed but due to the whole process dragging on far longer than anticipated - was mostly missed in the pack I have so far. The SE design will need to be redone, the 1st floor height has been done at 2422 to account for normal floors and 22mm UFH system, whereas Lewis deck would mean I need to give up an extra 27mm of headheight as the system with UFH will come in at 52mm vs the 25mm of the existing option.

 

Has anyone does this and does it really work? This is the one we're considering LEWIS-Metal-decking-brochure-2022-EN-web.pdf

Re 

Lewis decking and screed upstairs, I really, really hope that’s not needed as I’ve not designed anything like that in.  Most peeps manage with just posijoists and caberboard methinks. But then, there’s just the three of us (one little dog, one grown up, and me) so maybe the requirements are less onerous.

[Indy]

On 03/12/2024 at 10:24, Iceverge said:

There is one possibility through. A product called Aerobarrier will use an airbourne caulk to improve your airtightness. I've seen videos and a few patrons of the forum have used it. It may be worth a look. 

I looked into this and seems an impressive product and would get rid of a lot of the hassle with regards to getting builders to do the right thing.

 

I've had a quote from them - looks like they do it solely based on floorspace and it's coming in at just a shade over £3k ex VAT! Seems like a pretty high number and I'm sure there are cheaper ways to achieve the airtightness? 

[Iceverge]

11 minutes ago, Indy said:

I looked into this and seems an impressive product and would get rid of a lot of the hassle with regards to getting builders to do the right thing.

 

I've had a quote from them - looks like they do it solely based on floorspace and it's coming in at just a shade over £3k ex VAT! Seems like a pretty high number and I'm sure there are cheaper ways to achieve the airtightness? 

 

It really isn't the end of the world that price. I assume it'd cover your blowerdoor test too. 

 

Beware that it might still not function if you don't do a reasonably good job with airtightness in the first place. It's designed only for blocking small holes. It isn't magic.  I doubt you'd get a satisfactory result for example with your architect's roof details. There'd be far too many gaps the size of a 50p left to ever expect a method like that to work. 

 

Whatever you do if you want a house thats not leaky as a sieve you need to have a good design from the outset.