Workshop insulation and lighting

[rolesyboy]

Any builders on here? I have just finished building a tongue and groove workshop with a pent (sloping) roof topped with corrugated bitumen felt panels on top (Onduline).
Its all water tight now but wondered how to insulate it without causing damp/mildew etc in the future?
Starting with the roof the initial plan was to add celotex but I suspect I need an air gap and there is no void as such. The rafters run left to right and are 5 x 2 with the ply attached directly on top. From inside the workshop looking up you see the underside of the ply ie no cavity/void.
What is the best way to do this? Is it to tack say 50mm furring pieces to the upper part of the rafters, place celotex underneath?
Then to promote airflow perhaps let in some small soffit vents in the side panels of the workshop above the level of the celotex?
Do I introduce breathable TYVEK type felt???
I was then going to tack plasterboard over the celotex as having strapped a few flourescent lights to the ply rafters as the light reflection is p**s poor. A coat of white paint and Voila....
Cheers. Mark

[mab01uk]

I have done some building reg drawings for loft conversions and similar recently, standard practice is celotex or kingspan between rafters of a thickness which leaves a 50mm air gap directly under the roof covering....make sure air flow can enter and exit the air gap at eaves and ridge using suitable insect resistant vents. If more insulation thickness required place a further layer of celotex over the underside of the rafters, followed by plasterboard or you can use one of the thermal plasterboards which come with various thicknesses of insulation bonded on the back. The foil covering on the celotex will act as the vapour barrier and if you omit the plasterboard the silver foil covering may give you the light reflection required as long as you don't mind the insulation branding and logos printed on it visible?

If your air gap is above the ply and the ply is covering underside of rafters forming a flush surface, I would fix the celotex direct to underside of ply or preferably use thermal plasterboard (with the bonded on insul) mechanically fixed using the special long plasterboard screws designed to attach themal pb to batterns instead of the 'dot and dab' type adhesives usually used on vertical walls.

Other materials and methods may also be available......

[mk1]

You learn some stuff here don't you

[RS6]

Any builders on here? I have just finished building a tongue and groove workshop with a pent (sloping) roof topped with corrugated bitumen felt panels on top (Onduline).
Its all water tight now but wondered how to insulate it without causing damp/mildew etc in the future?
Starting with the roof the initial plan was to add celotex but I suspect I need an air gap and there is no void as such. The rafters run left to right and are 5 x 2 with the ply attached directly on top. From inside the workshop looking up you see the underside of the ply ie no cavity/void.
What is the best way to do this? Is it to tack say 50mm furring pieces to the upper part of the rafters, place celotex underneath?
Then to promote airflow perhaps let in some small soffit vents in the side panels of the workshop above the level of the celotex?
Do I introduce breathable TYVEK type felt???
I was then going to tack plasterboard over the celotex as having strapped a few flourescent lights to the ply rafters as the light reflection is p**s poor. A coat of white paint and Voila....
Cheers. Mark

You must cross-ventilate the void above the insulation. Depending upon the internal headroom you have, the easiest option is most likely to fix insulation-backed plasterboard directly to the underside of the rafters...but check that the rafters are of an adequate size to take the extra weight. Typically such boards come with either a polystyrene or foam insulation backing - foam having better insulation. There must be a vapour barrier between the plasterboard and insulation - i.e. on the 'warm' side of the insulation. On foam Celotex-type boards the foil face will suffice - on polystyrene backed board it is often optional. When the board is fixed make sure that all the perimeters are fully filled with mastic, including any penetrations for light fittings etc. - if you don't do this you run the risk of condensate forming in the rafter void above the insulation which can lead to timber decay. You should cross ventilate the rafter void with soffit vents along each opposing side.

If you want to provide a higher level of thermal insulation you could friction-fit some foam insulation boards between the rafters before you board out the ceiling but ensure that you've got a 50mm minimum clear airspace above the top face of the board. The easiest way to do this is to hammer some nails into the sides of the joists to act as 'stops' for the insulation depth. Then you just cut the boards to size and push them into position so they're flush with the bottom edge of the rafters. The ceiling boarding can then be fixed.

Hope this helps.

[rich@minispares.com]

or just do it 'Yorkshire style' and wear two coats....................

[rolesyboy]

What a clever bunch we have on here. Thanks for the comprehensive answers. Will get on with it now. What type of coat do you recommend

[rich@minispares.com]

What type of coat do you recommend

one of those tarten padded lumberjack ones you can buy of the market for about £6

they burn like napalm when they catch fire from either welding or grinding

ive got loads of them that have the front lower bits burnt away..............................

[Oldskoolbaby]

It's worth noting that this is for a workshop and not a house? All this talk of Kingspan/cellartex at x thickness with x ventilation room and even insulated p'boadrd is all ott for something like a work shop. A few rolls of cheap itchycoo and some tape and fill boards will be more than sufficient in my opinion.

['S'-type]

What type of coat do you recommend

one of those tarten padded lumberjack ones you can buy of the market for about £6

they burn like napalm when they catch fire from either welding or grinding

ive got loads of them that have the front lower bits burnt away..............................

Nothing worse than having your front lower bits burnt away.

[minimans]

What a clever bunch we have on here. Thanks for the comprehensive answers. Will get on with it now. What type of coat do you recommend

What a question!! an Anorak for this forum surely..................................................................says he putting on his as he leaves..............

[rolesyboy]

Ok ok I'll get me coat...,.,