Was about to finish insulating the ceiling today and noticed condensation n the insulated parts. The boards are celotex equivalent, foil faced both sides. I’ve left a 40-50mm gap between the ply roof and the boards thinking that would be best to have an air gap but think that was wrong!

Should I put the boards against the ply or anything else that would stop the condensation?

I’ve bought a Meaco dehumidifier to use in the ‘cave’ but would like to prevent the problem as much as possible.

Thanks

You left an air gap but does the air gap have any ventalation ?

Infact does the shed have any ventalation at all.

You’ve got a cold roof there so any moisture in the air will condense on it.

You need to have good ventilation above the insulation.

I’d put a plastic sheet across the insulation once it’s up and try and seal it as much as possible.

Celotex do a ply bonded insulation, so you could try and bond yours directly to the ply (not sure you can do this with foil backed celotex though). You will definitely need to make sure you get a good vapour barrier, to make sure no warm vapour can get to the cold surfaces.

Celotex bonded to Ply

In a traditional roof you leave 55mm air gap between insulation and the tiles / membrane, but that gap is ventilated with soffit vents:

Soffit fascia vent by brf, on Flickr

If there’s no way for air to escape you’ll just get moisture condensing on the cold surface in there.

Sounds like I’ve cocked it up then.
There’s no ventilation, not sure how I could put any in above the insulation either?

If you have no ventilation then bond the insulation to the Ply. Or, cut a hole in the ply under the eaves and still a grille in to let air flow in / out.

Or install your insulation as a warm deck roof, which IMO is better.

A few things to and work out then.
Any idea what adhesive for sticking the foil to the ply?
Trying to work out how to do the ventilation, hope the pics can help?

Science, do you mean insulate on top of the ply under the felt?

Would be curious to see a picture at the other end. My 2p … its not condensation.

In the picture the water runs down the face of the plywood at the highest point of the roof!

Condensation forms and drips if sufficent. With the mono pitch this would collect at the low end not the high end by the door.

Without going into condensation / classification or useage there is insufficient source in the building to create that amount of “condensation”

The verge overlap / detail (drip) is “mean” over the door and thus the junction is suspect. Nicely highlighted by the internal water staining on the original picture.

Put a nice big fascia around it to give it some weather protection.

I’d echo the sentiments above, It looks more like water running in.

Pretty sure it’s not water coming in, for a change we’ve had no rain for 3 days! There were few areas of water marks along both sides as well I only took pic of the front wall as it was worse and this has only been evident since I put the insulation up.

Point taken about the fascia, as the mo it’s 3×1 but could go for 6×1 I guess.

Any idea what adhesive for sticking the foil to the ply?

Use spray on foam adhesive – you’ll find a few in the Screwfix catalogue or local Builder’s merchants.

Just to clarify having read through the above.

The celotex bonded to ply is for placing on top of joists to for a warm roof with a roof finish on top which does not need ventilation. You cannot simply stick the ‘celotex’ to the underside of the ply you have installed already.

All insulations that have a a rigid sarking board over need to have 50mm void ventilated at eaves and ridge, otherwise you are asking for trouble with condensation which in this case warm moist air has travelled up the void, got cold, condensed and is now running down the roof.

Usually celotex joints in boards are butted tight together and sealed with a foil tape to create a vapour control layer. You have nothing at present stopping moist air reaching the cold side of the insulation.

Unfortunately looking at the photos you appear to have purlins up against the underside of the ply that would stop being able to effectively vent void.

Otherwise in simple terms you could have had a continuous nominal 10mm gap top and bottom – with a bit of insect mesh to stop critters getting in and making your cave their own.

Without going into condensation / classification or useage there is insufficient source in the building to create that amount of “condensation”

Unless of course the OP was working in there with the door closed – humans, especially when working physically, create a fair bit of moisture.

All my comments are based on what would need to be done on a home, but it’s little consolation if stuff in the cave gets knackered.

Edit: removed as on reflection was probably too much.

Cheers.
The only way I can think of how to vent the void above the insulation is to drill a series of 25mm holes along the top feather edge board just under the eaves and fix some sort of mesh/grill over them.
Not sure that I want to go down that road but if that’s the only option then I’ll have to.
mrben, I was going to try and bond the insulation to the ply but if that wouldn’t solve the condensation any other ideas that would or is it time to get the drill out?

Thanks.

IMO if you want to insulate between/beneath the joists then you should have the ventilation. But as i mentioned, looking at the photos you have cross members that would potentially stop the ventilation running from high to low which is the principle used with a house roof.

As mentioned by science and picked up on by yourself a warm roof, insulation on top of the ply is technically better however you still need to provide a vapour control layer somewhere between your ceiling finish and the insulation – standard manufacturers details general have a vcl on top of the plywood before the insulation and finish. This option is more reliant on workmanship however as there is no ventilation to get rid of any moisture that could get through a poorly installed vcl.

If you put holes make sure you have adequate protection from wind driven rain and drips to stop water tracking back along the soffit into the roof void.

Just want to qualify i only ever deal with the theory of these kind of things, the practical side……..there’s usually a man for that 😳

Just taken down the insulation until I decide what to do next, couldn’t believe how much condensation there was in just a few days so have put the Meaco to work.
I dont want to put back the boards as they were and drill vent holes above as I mentioned was a possible option so looking as seeing if I can bond the boards to the ply and still get an effective vapour barrier somehow, still trying to understand that bit!!
Or is there another option I can use internally?
Or look into insulating on top of the roof but that would need another layer of felt so more cost……

Bleeding sheds……

mrben can you put a link re vcl please, couldn’t find it when I googled thanks.