Insulating room in roof houses Q+A

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Homeowner double checking his own explorations on how to insulate a ‘Chalet Bungalow’ or ‘Rooms in the roof house’

Advance images of Room in the roof ‘Chalet bungalow’ with substantial pitched roof dormers with same ridge height as main roof creating a cruciform ridge.

Wants to insulate the roof adequately but the rafter depth is 100 mm.

Will/may also include roof integrated PV panels.

Regulations:

Require 50 mm ventilation slot above insulation to permit ventilation from eaves via roof to opposite eaves

Practicality:

Sloping roofs are challenging to insulate:
- Spray foam is impossible to apply and maintain ventilation gap
- Blown in cellulose fibre flake will not maintain a ventilation gap
- Pushing soft insulation from above down slope into good position is challenging
- Projecting timbers prevent pushing rigid insulation into place
Top triangle and eaves triangles need separate access hatches
Continuity of insulation between roof parts is potentially challenging

Thickness:

Rafter depth is 100 mm
100 mm is insufficient to provide winter U value and summer solar protection decrement delay
300 mm may be sufficient to do both
Internal headroom or fitted furniture may dictate if any insulation can be added internally
Planning restrictions may prevent raising the roof to accommodate insulation
Listed buildings and conservation area this may apply

Material properties:

Use thermal insulation with and k value to keep winter heat in,
And specific heat capacity and density to keep summer solar radiant heat out.

Materials choices:

Avoid: Glass wool, Stone wool, ‘Rockwool’, any plastics in timber roof construction
Consider: Plant based insulation materials
E.g. Wood fibre, Cork, Hemp, etc.
Choose if using vapour open (wind and air tightness membranes)
or vapour closed construction (Vapour barrier and breather membranes)

Suppliers will often do WUFI calculations to confirm suitability of their insulation to your building if you buy the insulation from them

Changing Details:

Pushing the roof higher by adding insulation thickness will alter details at the roof perimeters

There is a possibility of creating thermal bridges that need to be addressed by thermal breaks

Enviroform Solutions Ltd. Offer some solutions including insulatind behind rainwater gutters

Consider different densities and format to suit:

Squeeze into rafter zone,
fix above or below and span between rafters
depending upon ambitions, opportunities, scope of work, decanting of occupants, available weather window, etc.

Different approaches:

1
Choose a good weather window to complete all the work to the roof or areas to strip, insulate and relay
The roof is divisible either side of ridge and either side of dormers and their ridges
Have tarpaulin on the ready for any unfinished areas
Remove roofing tiles, battens, underlay, salvage all materials for reuse
Observe all complicated profiles and details and be able to insulate competently
Insulate the exposed timber rafter zone from above/outside
Insulate above the rafter zone
Depending upon properties of materials used consider of wind tightness membrane
Add insulation above the rafter, lay counter battens, membrane and relay roofing battens and roof tiles
2
Remove all internal ceiling linings
Observe all complicated profiles and details and be able to insulate competently
Insulate from below filling rafter zone
Add insulation below rafter
Add any air tightness or vapour control membranes
New ceiling linings
3 a bit of 1 & 2

Multi-foil insulation: (I will create a separate GBE page about multi-foils to go into more detail)

Multi-foils have a checkered history and some Greenies do not trust their claimed performance
Claim:
- 19 mm of multi-foils + 2 No. x 25 mm air spaces are equivalent to 250 mm Rockwool
- They do not work like conventional insulation so should not be tested like conventional insulation (Hot-box test method)
They are reliant upon having air gaps around them for their surface resistivity to have effect.
Many would not use multi-foils alone without conventional insulation back up
Bill Dunster has recently designed a house (exhibited at FutureBuild2023) with two lots of multi-foils, internally and externally either side of conventional insulation in timber stud framing
I believe (having see test demonstrations) they work for a short while then radiant heat starts flowing through them unhindered.
NGS was commissioned to write Robust Product Specification template clauses for one UK manufacturer of multi-foil insulation,
- Specifications were seen as part of their armory for specification sales, without them they were struggling to get their products specified
- When NGS asked for perimeter details, to know how they were terminated, fixed and left open or closed as ventilation routes, they were unable to provide the details.
- Without those details architects and constructors do not know how to complete the details or the installation
- Without those details the manufacturer is offering and incomplete and incompetent system, that can go wrong and will go wrong
- Checking their calculations they were using the wrong values for surface resistivity
- When challenged the response was the certification bodies had accepted them, who are NGS to be questioning them

When adding PV panels to roof:

Will capture solar heat and reradiate it inwards/downwards
Building integrated may reduce amount of roofing material, but may add to overheating risk
They work better if the underside can be cooled by ventilation air

Self Build centre at Swindon

www.nsbrc.co.uk

National Self Build & Renovation Centre, National Self Build & Renovation Centre Lydiard Fields Great Western Way Swindon, Wiltshire SN5 8UB United Kingdom.

Tel. 0345 223 4455.

EcoMerchant (suppliers) are based there

Suppliers:

https://greenbuildingencyclopaedia.uk/collaboration/solution-provides/suppliers-navigation/

Why not foam insulation?

https://greenbuildingencyclopaedia.uk/encyclopaedia/gbe-spray-foam-thermal-insulation-qa/

Designers & Contractors:

The Green Register of Construction Professionals

https://greenbuildingencyclopaedia.uk/encyclopaedia/code/links/organisations/tgr-the-green-register-organisations/

https://www.greenregister.org.uk/the-register/

Future Proof: contractors

https://www.greenregister.org.uk/news/futureproof-essentials-launched/