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Improving U values by Substitution (Brainstorm) G#13507

GBE brainstorm improving u values by substitution

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Improving U values by Substitution (Brainstorm)

Improving U values by Substitution (Brainstorm)

Improving U values by Substitution

I have attached the window and door junctions I have been trying to detail. It would be very helpful if you could take a look and let me know of any comments.

The single skin wall build up we are using consists of:

  • 15 mm plasterboard and skim
  • 140 mm blockwork
  • 150 mm external insulation
  • 15 mm render (to be applied prior to frame installation)

The details are as follows:

  • Details A, B & C: Typical window head, sill and jamb details.
  • Details D, E, F & G: Door D04 head and jamb details.
  • Note: D04 consists of a fixed glazed panel facing the terrace, joined to bifolding doors at 90 degrees by a corner post.

We are using:

  • L10 Velfac windows,
  • L20 Sunflex bifolding doors (supplied by Velfac)
  • M21 Sto external render system.

I have attempted to adapt the manufacturer’s standard cavity wall details to suit our single skin structure.

It is the contractor’s intention to apply the render system prior to installation of the windows/doors due to the lead in times on the windows/doors.

I have highlighted some areas in red which we are concerned about in particular:

  • How to avoid cold bridging at the junctions?
    We need to ensure an overlap between the external wall insulation and the windows/door frames but unsure of how to fix this to the frame edges, particularly as the frames are to be installed after the render.
  • The fixings from the frames back to the structure?
    Is packing required between the frame and the structure and how close must these sit together to ensure the fixing will work?
  • Are DPCs required at the window and door thresholds?
    Where should these be located?

Wow some complications in there!

I suggest you look at:

M21 External Insulated Render: (Jargon Buster) (Checklist)

There is also a verge gutter system made my another if needed

  • Swifix Ltd.
  • For attaching all those everyday things to insulated render
  • Satellite dish, gate, name plate, washing line, hanging baskets, etc.

L10 Windows L20 Doors

  • Iso-Chemic
  • Iso Bloco One windows and door perimeter seal to opening
  • Thermally Insulating, weathertight window perimeter sealing self-adhesive strips

They should keep you busy for a while.

Assuming they are new to you.

Speak later.

L10 Windows and L20 Doors

  • Early timber templates to create openings for late arriving windows is common practice.
  • An alternative is permanent sub-frames:
  • softwood or hardwood to match the window or door
  • made to profile to suit construction and to receive window profile
  • full decoration to all surfaces before installation
  • built into the wall ad secured as it progresses
  • insulated render applied up to it
  • windows fitted to the sub-frames later

We are currently not meeting the required Building Regulations U-values on the floor, wall and roof build-ups.

What can be done to get there?


Floor: current proposed construction

  • 15 mm floor finish
  • 75 mm reinforced screed
  • 90 mm Kingspan TF70 Thermafloor insulation PIR
  • 150 mm concrete beam and block

Current U value: 0.20

(B Regs requirement 0.13)

We would not be able to change the floor insulation depth easily

but maybe we will need to change the product


  • Is there a pre camber on the beams and if so what is the height to be accommodated (minus deflection under load)?
  • Is there a structural topping? What maximum thickness?
  • Will you use anti-tickle layer and sand for leveling? What Maximum thickness?



  • Substrate: 150 mm concrete beam and block
  • Preparation: structural topping? Thickness?
  • Levelling: to make up difference: 165 minus 25 to 37.5 (2 or 3 layers of 12.5) – 120 mm insulation
    • Anti-tickle layer and sand for leveling? What Maximum thickness?
    • Fermacell levelling compound: Thickness: 10 to 100 mm
  • Insulation: (options)
    • Extruded hard foam XPS DEO 500 kPA maximum 160 mm.
    • Extruded hard foam XPS DEO 300 kPa maximum 120 mm.
    • Polystyrene hard foam EPD DEO 150 kPa maximum 90 mm.
  • Material: wood fibre reinforced desulferisation gypsum
  • Manufacturer: Fermacell, 7 The Priory, Old London Road, Canwell, Sutton Coldfield, B72 5SH
  • T          0121 311 3480            F          0121 311 1882
  • E  W
  • Product Reference: 2 E 22 Fermacell dry flooring element
  • Thickness: overall. 2 or 3 layers 12.5 mm 25 to 37.5
  • Accessories:
  • Floor glue.
  • Fermacell levelling Compound.
  • Fermacell Joint filler.
  • Fermacell countersunk cross slot Screws: 19 mm.
  • Fermacell floor perimeter insulation strips.
  • Handling & Storage: Refer to manufacturer

Consider insulation under slung the beam and block suspended concrete floor (easy to add without changing details)

P10 Insulation

Consider composite timber I beam floor with insulation in joist zone (late redesign?)

G20 Timber Structure

Consider ground bearing aerated insulating concrete floor (late redesign?)

E10 Concrete

Floor to wall junction

F30 Accessories in Brick/Block/Stone walling

Walls: current proposed construction

  • 5 mm Sto Render
  • 150 mm EPS external insulation system and fixing carrier?
  • 140 mm Medium density concrete blockwork
  • 15 mm plasterboard and skim and fixing void

Current U value: 0.21

(B Regs requirement 0.18)

The foundations are already poured.

We cannot change the blockwork or the insulation thickness of the Sto products

  • What prevents changing the blockwork materials to AAC? loadbearing capacity?
  • What prevents changing the blockwork materials to Extruded fired clay? Learning new details?
  • What prevents changing the Sto insulation thickness?
  • Drainage details? Eaves Details? Window/Door Details?
  • What prevents changing the insulated render materials?
  • From EPS to XPS > PUR > Phenolic > PIR with other manufacturers if needed
  • Reducing thickness or improving U value

but could potentially add some internal insulation to the wall build up.

Would this bring a condensation risk?

M20 Plaster M21 Insulated render M24 Clay based plasters

  • Yes, probably, needs a vapour check inside unless you use a hygroscopic moisture transport internal insulating plaster
  • hygroscopic moisture transport internal insulating plaster:
  • Diasen (manufacturer)
  • Ecological Building Systems Ltd. (Supplier)
  • Earth and Cork Insulating plaster/render
  • Diathonite Evolution should manage moisture for you
  • Clay or lime should manage moisture but adds no insulation

K11 Linings

Wall to roof junction

  • Are there any linear thermal bridges to resolve?

Main Roof: current proposed construction

  • U value: 0.15
  • Roofing slates
  • 19 x 38 mm sw battens
  • 50 x 38 mm sw counter battens
  • 50 mm AIM warm roof system stone wool ‘Rockwool’ overlay board
  • Glidevale Protect VP400 Type LR roofing underlay
  • 150 mm trussed rafters
  • 2 x 75 mm glass fibre ‘earthwool rafter roll’ between trussed rafters
  • 12.5 mm plasterboard and skim
  • (B.Regs requirement 0.13)

We could potentially put a ‘Kingspan’ insulation in the roof instead of the ‘earthwool’

and as this is not breathable we would then need to add a VCL layer.

  • Stone wool and glass fibre are not breathable
    • (i.e. Breathing construction) but they are moisture permeable.
  • Breathing construction also requires hygroscopicity
    • (absorbs moisture vapour from the interstices into the fibre helping it to insulate and releases moisture when conditions are right)
  • Stone wool and glass fibre are not hygroscopic but hydrophobic
    • (rejects moisture from fibre keeping it in the interstices reducing its capacity to insulate)
  • Stone wool and glass fibre both need a vapour check layer always
  • Stone wool and glass fibre can be oversized and fit closely to timber sections
    • and any moisture in interstices could affect the timbers

Replacing stone wool fibre or glass fibre with PIR or PUR insulation

  • Increases thermal resistance, improves U value with same thickness
  • Is often cut to size and not resilient enough to enable a tight fit to timbers and remains gappy
  • PIR and PUR are vapour resistant and so will concentrate moisture vapour at those gaps at timbers
  • PIR and PUR both need a vapour check layer always

Stone wool, glass fibre, PIR and PUR

  • offer no decrement delay to solar radiation heat and let heat in easily
  • offer resistivity to heat escaping
  • let heat in, but do not let it out, it may lead to overheating

Consider: Cellular glass, dense wood fibre, cork board, cellulose fibre flake, recycled newspaper

I am sorry to inundate you with information but if you do have any suggestions (which wouldn’t increase the contract costs unduly) they would be gratefully received!

  • I am glad you did without it I could not respond in detail
  • I know very little about prices so you will need to check out the alternatives
  • Do you want the Thicknesses of materials working out for you?
  • I have just created a ceiling U value calculator, I could develop it for these applications

© GBE NGS ASWS BrianSpecMan aka Brian Murphy
4th November 2016 – 9th November 2016

Improving U values by Substitution (Brainstorm)

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© GBE NGS ASWS BrianSpecMan aka Brian Murphy
8th November 2016

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