26 Mar 2020
Reaction score
United Kingdom
new built detached outbuilding/store/workshop modern build

concrete blocks/cavity insulation/thermalite blocks inside (won't be skimmed or boarded)
floor insulated

mono pitched low pitched roof

occasional heat but more about keeping stuff dry as opposed to getting moisture damage in old shed etc
The roof is cold roof so inside;
100mm celotex fixed /taped flush with rafters (50mm gap above/ vented soffits)

How can i finish the ceiling ???

Been told Vapour check /foil backed plasterboard to finish ceiling is an option but would prefer an alternative

would prefer to fix a board such as OSB

been told VCL is the problem

would have to staple VCL and then batten out before putting any timber (air gap) apparently ..whats my simplest options ?
Sponsored Links
been told VCL is the problem

would have to staple VCL and then batten out before putting any timber (air gap) apparently

The boards can go direct on the VCL - just like foil backed plasterboard is :rolleyes:

You do need a VCL else risk timber rot. But this will require that the roof membrane is breathable, and not normal felt

If you use timber, then thats a fire risk.
sarnafil , underlay on OSB is main roof as its a single mono / low pitched roof
(so roof not breathable like tiled /breathable felt type)

There is 50mm gap above the 100mm celotex
Sponsored Links
Even with a ventilated air gap, there is still a need to reduce the potential for moisture to get into the roof structure, hence a VCL.
Woody Im confused little , please ignore my lack of knowledge;
Please note the construction -
Im' just concerned with finishing the ceiling.
Celotex is tight to rafters...from inside looking up

So if I fix a polythene vapour barrier straight to the celotex ?

Then the OSB 3?
I posted my post above before reading yours above that! So was not an answer to your post, just a comment.

So yes you can staple the VCL and then board directly on that.

You could instead, tape the joints of the Celotex, and across the rafters with a foil tape or duct tape and this will then form the VCL. Otherwise polythene will do.

The only proviso with this, is bear in mind that moisture from the room will still hit the VCL and potentially remain there and soak in to the OSB. Normally this won't be a significant amount and will be fine. But keep your eye on it over the first few seasons to make sure there is not a lot of moisture being held there causing damp stains on the OSB.
Thanks woody
Do you mean tape over all the celotex or just the joints and joists
I normally tape over joints and joists as a rule - will this enable me to avoid putting polythene up , as long as monitor things as above
The foil is effectively a vcl so taping joints completes its seal. Why osb? If just aesthetics maybe use 4mm ply, cheaper, thinner so less weight and easier to install

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links