Insulating workshop roof - help / advice needed

8 Mar 2013
Reaction score
United Kingdom
Thanks Alastair

I would like to do option 2 but i am concerned that i may not be able to achieve a full seal and in that case option 1 to vent the roof may be more forgiving and give me more security that the roofing will not mould and rot.

The problem i have so far is that we boarded the outer with 18mm ply and tyvek but had to fix the tyvek with temporary battens to hold it down while we installed the plastic slates (Tapco)
During the slating we had lots of rain and some got through the tyvek via temp batten fixing hgoles and soaked the ply At this point the inner celotex was being installed between rafters.
The slating was completed and i thought all was ok
Last week the sun came out and the roof began to drip water from inside between hairline gaps between rafters and insulation
On removing some insulation the ply had started to mould and was extremely warm from the sun heated tiles also the inside of the ply was very wet and mouldy in some places.
I have since panicked and spent 4 days removing all the insulation to find i have lots of surface mould and wet areas - presumably from water trapped in the ply and not drying out - very slow to dry externally as the tiles are hard on the tyvek straight to ply - no battens

I have an industrial dehumidifier in the building now and i am getting an indirect marquee heater this week to thoroughly dry the building inside before i decide what to do with the roof hence my vent or unvent dilema

i dont mind venting the roof if this is the safest design by lowering the rafters and the insulation and adding vents - i can fit circular soffit vents and Tapco make ridge vent spacers which i can fit by removing the ridge tiles and cutting a vent slot in my 18mm ply along each side of the ridge

Im not sure what i would do with the velux areas or the barge ends - probably drill through to give venting to the nearest full soffit to ridge vent run ?

I understand that i need to seal off the roof - would sealant around the cellotex panel edges and maybe taped over rafters and joints and then as you say i need to cover the ceiling with polythene - that will be a challenge as there are collars/ties and joists coming out of the sloped roof - presume again a combination of tape and sealant around the polythene ? and then fit plywood - obviously ply fixings will penetrate the polythene into the rafters.

The walls i presume are not such an issue rthey are timber framed 6 x 2 with 140mm celotex infill - there is tyvek supro on the outside with green oak cladding and the inside is boarded with 18mm ply - there is no polythene on the walls.

where would the roof polythene need to finish at the eaves ? most of the wall inner ply is already fitted except the last 500mm to the eaves.

Also i have timber purlins at midpoint running length of the roof on each side and a ridge timber - presume roof poly would only be able to be fitted up to the sides of these timbers ?

will i also need the vapour barrier poly kits for the internals of my 4 velux roof windows ?