Fabric spec - what can I improve easily?

[Indy]

Following the recommendation on another thread, putting up the details of my build here to ask for recommendations on things can be improved. 

 

We're not aiming for a Passivhaus level of spec, as the law of diminishing returns means that the payback probably is not worth it. Aiming for a Building Regs compliant comfortable family home - and this is going to be a massive step up from a 1930s semi with no cavity wall (where we currently live). 

 

Ground floor is Beam and Block, and First floor is 225mm posi joist, Lewis deck with screed (for more stability). 2 sets of sliding doors (4.1m wide each) at the back which is West facing. 

 

Putting in ASHP, MVHR, in roof Solar panels (battery to be added later), 300l water tank + 100l buffer OR 400l water tank (3 bathrooms and cloak room), UFH heating on both floors including bathrooms, some form of air con in the 4 bedrooms and living/dining area (5 in total), Towel radiators with electric element in bathrooms. No carpets in the house - tiles in the hallways and living/dining areas and engineered wood in the bedrooms. 

 

My main questions are: 

 

1. Given that this is a block and block build - how do we target airtightness? The 2 options I'm looking at are either

• Parge coat and dot and dab plasterboard (cheaper and faster to put up)
• Wet plaster though I've been told this will be more expensive, take longer and also reduce thermal efficiency which means speccing thicker insulation in the external walls. 

2. What are the other things that are easily addressable given the details below to make it more efficient, without drastically altering the spec?

 

 

Floor construction: U Value = 0.14 W/m².K

• 65mm. sand: cement screed
• 500g. Polythene separation layer
• 120mm. Celotex XR4000 insulation
• Polythene DPM
• Concrete beam & block floor to manufacturer's design

Wall construction: U Value = 0.15 W/m².K

• Render
• 100mm. Thermalite Hi Strength 7 blockwork
• 50mm. clear cavity
• 100mm. Celotex CW4000 insulation
• 100mm. Thermalite Hi Strength 7 blockwork
• 12.5mm. plasterboard on dabs dry lining, plus skim

Pitched roof construction (insulation at rafter level): U Value = 0.11 W/m².K

• Roof tiles
• 25x46mm. treated tiling battens
• 25x46mm. treated counterbattens
• LR Breathable roofing membrane
• 150mm. rafters fully filled with 150mm. Celotex XR4000 insulation
• 25x47mm. battened services void beneath rafters
• Celotex PL4060 (60+12.5) insulated plasterboard plus skim
• Proprietary felt support tray

Pitched roof construction (insulation at ceiling level): U Value = 0.11 W/m².K

• Roof tiles
• 25x46mm. treated tiling battens
• 25x46mm. treated counterbattens
• LR Breathable roofing membrane
• 150mm. rafters
• Unventilated loft space
• 19mm. plywood decking or flooring plywood for loft storage
• 75mm. Celotex GA4000 insulation above ceiling joists
• 150mm. ceiling joists fully filled with 150mm. Celotex XR4000 insulation
• Visqueen High Performance vapour barrier
• 12.5mm. plasterboard plus skim

COLD Flat roof construction: U Value = 0.11 W/m².K

• GRP weather surface
• 18mm. plywood deck
• Firrings to give 1:40 fall
• 195mm. joists with 150mm. Celotex XR4000 between joists
• 90mm. Celotex GA4000 insulation below joists
• 12.5mm. plasterboard plus skim

 

[SteamyTea]

7 hours ago, Indy said:

law of diminishing returns means that the payback probably is not worth it

Are you just looking at the price of doubling the insulation thickness and comparing that to the cost of energy?

You need to look at the marginal price of the whole structure i.e. a wall or floor, not just the insulation.

 

Are you going for UFH?

[nod]

Insulation levels look quite low 

Wet plaster and dot and dab work out about the same Certainly if you are going to the bother of page coat Which is a waste of money 

If your looking at payback times 

PV in well insulated house will take many many years 

[Russell griffiths]

Change your block type, nothing in the world would make me use both inner and outer skins in thermalite blocks. 
if you must use them on the inside then there only. 
100mm celotex in the cavity only just meats regs, you need to improve this somewhere. 
 

how will you fit lights in the flat roof area, does this need a service cavity. 
no mention of vcl in pitched roof. 
no mention of vcl in flat roof. 
 

on the pitched section insulated at rafter level, change the insulated plasterboard for normal celotex, fit this directly under the rafters, then service batten then normal plasterboard. Again no mention of vcl. 

[Andehh]

Insulation seems very reasonable, your well into law of diminishing returns at that stage! Maybe eek the flood up a bit with UFH. Ours is 0.11 floor, 0.22 walls (shoulda done better but realised too late) and 0.14 roof.

 

Air tightness either becomes a focused investment or DIY, you will need to be all over the trades at all times realistically. I used to drive past the house after work nearly everyday with expanding foam and sealant doing what I could around the edges (DIY).

 

We then stuck a wood burner in with external vent and lots of sliding doors  🤪

[G and J]

Noting the wildly differing views on insulation levels may I suggest playing with Jeremy’s spreadsheet might help, it did for me.

 

 

[G and J]

7 minutes ago, Andehh said:

Insulation seems very reasonable, your well into law of diminishing returns at that stage! Maybe eek the flood up a bit with UFH. Ours is 0.11 floor, 0.22 walls (shoulda done better but realised too late) and 0.14 roof.

 

Air tightness either becomes a focused investment or DIY, you will need to be all over the trades at all times realistically. I used to drive past the house after work nearly everyday with expanding foam and sealant doing what I could around the edges (DIY).

 

We then stuck a wood burner in with external vent and lots of sliding doors  🤪

In another thread @Indy expressed doubts about them having a woodburner.   As you have added a woodburner @Andehh do tell, do you use it and do you leave sweat puddles or simply watch the main heating system ease off (or both of course?)

[G and J]

9 hours ago, Indy said:

First floor is 225mm posi joist

What spans are these across?

[Andehh]

2 hours ago, G and J said:

In another thread @Indy expressed doubts about them having a woodburner.   As you have added a woodburner @Andehh do tell, do you use it and do you leave sweat puddles or simply watch the main heating system ease off (or both of course?)

 

We went with a big twin sided Stovax jobbie, that has 45sqm of kitchen on side and 25sqm of lounge on the other, both with 3.5m ceilings. So it actually does a remarkably below par job unless you really fire it up hard.. Which then consumes enough wood to keep Drax's power generation spinning.

 

It was ungodly expensive to install, paid for over 2 years of building the house and making it work (400mm deep into a 140mm thick wall and lintel...) but we wouldn't be without it as a 'wow' factor.

 

We use it when the house has been too warm for the UFH to kick in, but gets chilly later in the day (Ufh runs through the night as default). Also for the ambiance on a Friday and Saturday etc.

Edited December 2 by Andehh

[ProDave]

Section B-B

 

If that flat roofed dormer is only there for headroom for the stairs, then re design the stairs and avoid the dormer.  Fewer steps on the first flight to the half landing then more steps on the second flight, until you achieve required headroom on the upper flight without the dormer.  That will save you a lot which you can spend elsewhere.

 

And why do you want concrete stairs?

[Indy]

19 hours ago, SteamyTea said:

Are you just looking at the price of doubling the insulation thickness and comparing that to the cost of energy?

You need to look at the marginal price of the whole structure i.e. a wall or floor, not just the insulation.

 

Are you going for UFH?

Yes, wet UFH currently planned on both floors.  

[Indy]

19 hours ago, nod said:

Insulation levels look quite low 

Wet plaster and dot and dab work out about the same Certainly if you are going to the bother of page coat Which is a waste of money 

If your looking at payback times 

PV in well insulated house will take many many years 

I have heard the same re cost of wet plaster vs dot and dab being the same from a contractor I spoke to - just a little more work for the plasterer. However, my architect is convinced this was more expensive and would increase the drying time by several months - which would mean we couldn't paint the property or to expect cracking for at least 6m afterwards. So we changed tack to parge coat and dot and dab as a compromise. 

[Indy]

17 hours ago, Russell griffiths said:

Change your block type, nothing in the world would make me use both inner and outer skins in thermalite blocks. 
if you must use them on the inside then there only. 

What alternatives should I look at?

 

  

17 hours ago, Russell griffiths said:

100mm celotex in the cavity only just meats regs, you need to improve this somewhere. 

Somewhat by design - given that BR is already a huge step up. I'm not too keen on losing internal space and we're limited on how far out we can go as we need to leave a 1m gap on either side of the property from the fence line. 

 

  

17 hours ago, Russell griffiths said:

how will you fit lights in the flat roof area, does this need a service cavity. 
no mention of vcl in pitched roof. 
no mention of vcl in flat roof. 
 

on the pitched section insulated at rafter level, change the insulated plasterboard for normal celotex, fit this directly under the rafters, then service batten then normal plasterboard. Again no mention of vcl. 

I have to admit I will need to spend time understanding all of this - though I assume VCL is a Vapour Control Layer?

  

[Indy]

17 hours ago, Andehh said:

Insulation seems very reasonable, your well into law of diminishing returns at that stage! Maybe eek the flood up a bit with UFH. Ours is 0.11 floor, 0.22 walls (shoulda done better but realised too late) and 0.14 roof.

 

Air tightness either becomes a focused investment or DIY, you will need to be all over the trades at all times realistically. I used to drive past the house after work nearly everyday with expanding foam and sealant doing what I could around the edges (DIY).

 

We then stuck a wood burner in with external vent and lots of sliding doors  🤪

I am keen on having an airtight house though we're working towards a target of 3.5 at the design stage. It's something I will discuss with the main contractor and stress on as the build goes on.

 

However, we do like to open the windows from time to time and the sliding doors are pretty non negotiable I have to say. Wood burner is something I like (as do both the boys) so I guess it will stay but I've been told you can get chimney stacks that aid airtightness when not being used?

[Indy]

14 hours ago, Andehh said:

 

We went with a big twin sided Stovax jobbie, that has 45sqm of kitchen on side and 25sqm of lounge on the other, both with 3.5m ceilings. So it actually does a remarkably below par job unless you really fire it up hard.. Which then consumes enough wood to keep Drax's power generation spinning.

 

It was ungodly expensive to install, paid for over 2 years of building the house and making it work (400mm deep into a 140mm thick wall and lintel...) but we wouldn't be without it as a 'wow' factor.

 

We use it when the house has been too warm for the UFH to kick in, but gets chilly later in the day (Ufh runs through the night as default). Also for the ambiance on a Friday and Saturday etc.

 

Same for us - it's really there for wow factor more than actual utility. It will come on for about a week between Christmas and New year (based on the usage of our current gas fire) when we have friends and family over for entertaining and showing off. Not really meant to be the primary or even secondary heating system. 

[Indy]

14 hours ago, ProDave said:

Section B-B

 

If that flat roofed dormer is only there for headroom for the stairs, then re design the stairs and avoid the dormer.  Fewer steps on the first flight to the half landing then more steps on the second flight, until you achieve required headroom on the upper flight without the dormer.  That will save you a lot which you can spend elsewhere.

No, there is a room behind it which is my study and this gives extra headroom and there's a skylight in that room on that flat dormer. 

  

14 hours ago, ProDave said:

And why do you want concrete stairs?

 

Because I hate creaking stairs in our current house and the original thought was to build an ICF house with concrete stairs, walls and floors. Moved away from this due to cost reasons but decided to keep the concrete stairs requirement. This would go well with the Lewis deck and screed floors that I'm planning for the 1st floor. 

 

Have the option of either though - and my initial quote of a pre-cast staircase wasn't as horrible as I was expecting. £4k including delivery but it would need cladding and installation etc.

 

Getting quotes for timber stairs ranges from £3-4k for pine to about £10-12k for hardwood oak stairs - so the differential isn't as large as I was expecting, though this is all desktop research for now. We'll finalise which route to go when we get firmer prices back on either option and the SE designs/calcs we have cater for both. 

[nod]

4 hours ago, Indy said:

I have heard the same re cost of wet plaster vs dot and dab being the same from a contractor I spoke to - just a little more work for the plasterer. However, my architect is convinced this was more expensive and would increase the drying time by several months - which would mean we couldn't paint the property or to expect cracking for at least 6m afterwards. So we changed tack to parge coat and dot and dab as a compromise. 

I’ve run a plastering company for forty years 

Wet plastering labour is more but materials are less So evens itself out 

Settlement cracks are about the same 

I could have done either on our builds 

But chose dot and dab as it’s a warmer feel 

I used to use thousands of bags of Parge each year Mostly for sound deadening But even the housing associations have dropped it now 

it’s a complete waste of money 

Only self builders seem to use it 

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

Our air test has tested as a 2 Traditional solid build Lotss of glass and three vaulted ceilings 

Which is good enough for comfort 

If you do decide to use Parge Don’t call it airtightness coat Tell the dry liners that it’s what it says on the back 

SOUND BLOCK coat Or they won’t bother sealing anything up 

Fifteen years ago when we used Parge on nearly every job the blockwork was shocking Gaps in joints and snots everywhere Then overnight BC clamped down and all joints where pointed and bar finished 

No need for Parge 

[SteamyTea]

6 hours ago, Indy said:

Yes, wet UFH currently planned on both floors.  

Then look at your floor U-Value, while 0.14 W.m^-2.K^-1 seems impressive, the heated slab is not room temperature i.e. 20°C [293K], it is probably closer to 30°C [303K], so losses are higher, and highest when you need the heating most.

The ground floor/slab is also one part you cannot improve later.

 

The Celotex XR4000 has a k-value of 0.022 W.m^-2.K^-1, which is a normal value for a PIR material.  Increasing the thickness from 0.12m to .2m changes the R-value from 5.45 to 9.05 m².K.W^-1.

That is the kind of improvement that would negate the last 2 years energy cost rises for that element.

 

Many people on here have found that second floor (or is it first floor, upstairs) UFH is not necessary in a well insulated and airtight house. Natural convection and conduction though the internal fabric usually suffices.

Having said that, not many people believe that is the case, so it does offer some reassurance, and is relatively cheap to install.

6 hours ago, Indy said:

I am keen on having an airtight house though we're working towards a target of 3.5 at the design stage

By Buildhub standards, that is pretty poor, and if you are fitting MVHR, which you should be, then sub 1 ACH is better.  Be careful with the air test calculations as the m³.m^-2, air volume divided by surface area, at 50 Pa, calculation is for all exposed elements, not just the volume of air divided by the floor area.

A Vapour Control Layer (VLC) is NOT there to improve airtightness, it is there just to stop the high humidity internal air passing into the structure and condensing into liquid water.  It is on the wrong side of the insulation i.e. the warm side (in the UK) to improve airtightness.

Ideally an air test should be done before the VLC is installed, but no one every wants to pay for several air tests, so very few people really know how effective the insulation is, until cold patches and mould appear.  Some people have made their own fan units and test as they build.

 

There is nothing to stop you opening windows, in fact, if you have to open a window, it shows the hard work of getting the airtightness right was worth it.

And you have control of where the 'leak' is, not the builders.

Edited December 3 by SteamyTea

[G and J]

1 hour ago, SteamyTea said:

A Vapour Control Layer (VLC) is NOT there to improve airtightness, it is there just to stop the high humidity internal air passing into the structure and condensing into liquid water.  It is on the wrong side of the insulation i.e. the warm side (in the UK) to improve airtightness.

Accepted that the primary role of the VCL is to keep moisture out of the structure, in our case a standard 140mm stud based OSB skinned timber frame.
 

But I am working on the notion that from floor to ceiling VCL, with wall VCLs joined by Tony trays, is effectively a balloon with as few penetrations as possible.  At all points those VCLs are sandwiched between either OSB, plasterboard or insulation, so there should be no possibility of ballooning.  
 

 

[Iceverge]

On 01/12/2024 at 22:54, Indy said:

We're not aiming for a Passivhaus level of spec, as the law of diminishing returns means that the payback probably is not worth it

 

Passivehaus really isn´t about minimum costs (althought it can make finantial sence) , but rather a comfort, building longevity and efficency. 

 

 

On 01/12/2024 at 22:54, Indy said:

1. Given that this is a block and block build - how do we target airtightness? The 2 options I'm looking at are either

 

Ok I´m going to throw a spanner in the works here. I don't think if you build a blockwork house you'll get better than 3ACH on a blowerdoor test.  I read many times that before we built our house that most airtight house builders had abandoned masonary as it was just too hard to get a result. None the less I pressed ahead. We ended up with a blowerdoor test of 0.31 which I was pleased about. HOWEVER, i spent 4 years researching the topic to an obsessive level of detail. I was off work during covid so I was personally able to do all the detailing myself. I spent days on end with a DIY blowerdoor fan and a tube of airtight sealant assessing every crevace and corner. The amount of time and effort that went into it was huge. 

 

It would be completely unreasonable to expect any normal houseowner or builder to go down this route. 

 

If you want a truely airtight house I would consider other methods. Timberframe liek MBC's twinwall or ICF would be much easier. 

 

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. 

 

 

 

 

 

[SteamyTea]

1 minute ago, Iceverge said:

There is one possibility through. A product called Aerobarrier will use an airbourne caulk to improve your airtightness

Seals from the inside though, so may initially mask badly fitted insulation and cold air paths to the inner walls.

 

I was at @joe90's when the blower test was done (think it was 1.45).  It was quite obvious where the holes were when the fan was running (though I failed to mention the WBS air inlet leak at the time, but then I hate WBS) , so well worth knocking something up.  Why make life hard for 50 quid at the scrap yard.

[joe90]

17 minutes ago, SteamyTea said:

I failed to mention the WBS air inlet leak at the time,

I did notice it and sealed it afterwards, but as you say, droughts are easily noticeable with smoke of some sort (I used joss sticks).

[Iceverge]

Ok with that in mind I will revise your blockwork build spec accordingly. The U values your architect has specced are pure fiction. 

 

 

 

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

 

 

 

On 01/12/2024 at 22:54, Indy said:

Wall construction: U Value = 0.15 W/m².K (lol yeah right) 

• Render
• 100mm. Thermalite Hi Strength 7 blockwork
• 50mm. clear cavity
• 100mm. Celotex CW4000 insulation
• 100mm. Thermalite Hi Strength 7 blockwork
• 12.5mm. plasterboard on dabs dry lining, plus skim

 

DON'T USE BOARDS IN THE WALLS. TERRIBLE IDEA!!

 

My suggestion. 

• Render
• 100mm 14N dense blocks. 
• 200mm full fill cavity with EPS blown beads (or mineral wool batts) and stainless steel wall ties. 
• 100mm. Dense blockwork. 
• 15mm Sand cement render. 
• 5mm skim. 

 

 

 

The roof requires some thinking about. 

 

I have come to the conclusion that it is far too varied and complex to make a good job of it unless you take the airtightness layer to the outside of the rafters. It'll be easier to build too.  Get rif of the cold loft. 

 

On 01/12/2024 at 22:54, Indy said:

Pitched roof construction (insulation at rafter level): U Value = 0.11 W/m².K

• Roof tiles
• 25x46mm. treated tiling battens
• 25x46mm. treated counterbattens
• LR Breathable roofing membrane
• 150mm. rafters fully filled with 150mm. Celotex XR4000 insulation
• 25x47mm. battened services void beneath rafters
• Celotex PL4060 (60+12.5) insulated plasterboard plus skim
• Proprietary felt support tray

 

 

• Roof tiles
• 38x50mm. treated tiling battens
• 25x70mm. treated counterbattens. Screwed into the plywood deck with 200mm screws. 
• Roofing membrane
• 50mm PIR joints taped layered over the rafter tails. 
• 100mm PIR with joints staggered from above layer. 100mm rafter tails screwed into the true rafters with 175mm screws. 
• 18mm ply with all joints taped and returned to the wall airtightess layer at the wall plate and eaves. 
• 140mm rafters with 100mm of mineral wool insulation. 
• 40mm service space 
• 2 x 12.5mm plaster board layers
• Gypsum skim. 

This approach would require stub rafters to be screwed to the top of the existing rafters for rafter tails and overhangs. AKA this video.  I would change some of his details though. 

 

 

 

 

 

 

 

 

On 01/12/2024 at 22:54, Indy said:

COLD Flat roof construction: U Value = 0.11 W/m².K

• GRP weather surface
• 18mm. plywood deck
• Firrings to give 1:40 fall
• 195mm. joists with 150mm. Celotex XR4000 between joists
• 90mm. Celotex GA4000 insulation below joists
• 12.5mm. plasterboard plus skim

 

This is a really risky approach unless you are hyper on top of your airtightness. 

 

Also it's tricky to joint to the pitched roof. 

 

I would more or less copy the above detail. However I would include ventilation below the deck as I don't like the deck straight on top of the insulation. It is very failure intolerant. 

 

 

• EDPM
• 38x50mm. treated across the rafters. 
• 25x70mm. treated counterbattens. along the rafters.  Screwed into the plywood deck with 200mm screws.  53mm ventialted space with mushroom vents throught the deck and continious with the pitched roof ventilation. 
• Roofing membrane
• 50mm PIR joints taped layered over the rafter tails. 
• 100mm PIR with joints staggered from above layer. 100mm rafter tails screwed into the true rafters with 175mm screws. 
• 18mm ply with all joints taped and returned to the wall airtightess layer at the wall plate and eaves. 
• Tapered firing pieces to create 1:40 fall.
• 140mm rafters with 100mm of mineral wool insulation. 
• 40mm service space 
• 2 x 12.5mm plaster board layers
• Gypsum skim. 

 

 

 

[Iceverge]

45 minutes ago, SteamyTea said:

so may initially mask badly fitted insulation

 

In my view the primary objective of airitightness is building health by stopping drafts carrying damp internal air into the structure. 

 

Badly fitted insulation is solved relativly easily. 

 

Just use a blown product like cellulose of EPS beads OR fit board insulation over a flat surface like with warm roof or a floor. 

[Iceverge]

 @Indy

 

I've put a good bit of consideration into the above. 

 

Materials etc weren´t selected at random so if you have any specific questions fire away. 

 

I think it'd actually be much cheaper than what your architect proposed too by the way.