Insulating workshop ceiling

[Omnibuswoman]

Hi peeps, further advice needed on workshop build, this time on the roof insulation.  
 

We’ve built a cold roof design - EDPM on top, 18mm marine ply, joists/noggins. No ceiling finish up yet.

 

We’re planning to put on a plasterboard ceiling with recessed LED lights, but need to work out the best way to insulate between the plasterboard and the ply.
 

Do I need to leave a ventilation gap between the ply roof and the insulation? Then another gap between that and the ceiling? How will the electric cables through the joists affect any insulation decision? Might insulated plasterboard be a better option? 

 

All suggestions most welcome!
Thanks! M

[JohnMo]

Not sure how pretty you want it.

 

But I used glass wool in the rafters, then sheets of 25mm PIR screwed underneath.  Ran electric cable in conduit to strip lights below the PIR all visible.

[saveasteading]

Thoughts, not gospel:

 

I think the trickiest thing is the practicality of fitting cables and insulation, as one will get in the way of the other.

Cables tight to joists may be the answer.

Then fixing the lights through, they  will have to push aside the insulation, and it will be a hot little area that might cause the lights to fail. 

 

In theory, fit all the insulation you can, with something that will stay in place before you fit the ceiling.

No need for any gaps that I can think of.

For all that LED lights are low power, it still seems best and simplest to not have a polystyrene layer in contact with them.

There will be a cold bridge through every joist though.

 

Or fit a membrane under the joists, then a secondary grid of timbers to create a void that the cables and lights can sit in.

 

 

There you are, more questions than answers.

 

[Radian]

11 hours ago, Omnibuswoman said:

Do I need to leave a ventilation gap between the ply roof and the insulation?

 

If you don't want problems with condensation, then I would say yes. But your nogs seem to be the full depth of the joists so when you panel over for the ceiling  there's a whole lot of closed-off spaces. Think of what happens to the water vapor in the warm inside air when it gets up to the cold underside of the ply. A vapor barrier may help but they're hard to make perfect when you bore through for LEDs and cabling. Cold roofs generally require a howling draft throughout to be foolproof.

[saveasteading]

If the void is 100% filled with insulation, and the atmosphere is dry when the ceiling is fitted, then there shouldn't be significant moisture to evaporate.

Also, if filled to the top, the summer heat does not have a volume of air (with moisture in it) at the top to heat.

 

[Radian]

21 minutes ago, saveasteading said:

If the void is 100% filled with insulation, and the atmosphere is dry when the ceiling is fitted, then there shouldn't be significant moisture to evaporate.

Also, if filled to the top, the summer heat does not have a volume of air (with moisture in it) at the top to heat.

 

Few people, certainly not me, have a solid understanding of the hygrodynamics involved. The fallback approach is to make as few assumptions as possible and all the advice I've seen is to err on the side of providing plenty of ventilation. The environment will do a pretty good job pushing vapor though from wetter to dryer areas so keeping places dry is not so easy. Good airflow will mitigate such issues.

 

[TonyT]

You could insulate.then put vapour barrier up

then put 50x50 battens at 90deg to existing timber

then plasterboard 

new leds cut out but make sure they are shallow so not damaging vapour barrier/insulation.

 

[Simplysimon]

3 hours ago, saveasteading said:

If the void is 100% filled with insulation, and the atmosphere is dry when the ceiling is fitted, then there shouldn't be significant moisture to evaporate.

Also, if filled to the top, the summer heat does not have a volume of air (with moisture in it) at the top to heat.

 

or warm deck it

[SteamyTea]

As it is a cold roof, ventilation should be there.

Or convert it to a warm roof.

 

A lot depends on how it is used i.e. permanently heated, or spasmodically heated.  That will change where the VCL goes, but down here, on the inside is probably best.

[SteamyTea]

14 hours ago, Radian said:

Few people, certainly not me, have a solid understanding of the hygrodynamics involved.

I think it is confused by practicalities as you mention here.

 

23 hours ago, Radian said:

But your nogs seem to be the full depth of the joists so when you panel over for the ceiling  there's a whole lot of closed-off spaces.

 

Really just forces and motion once you reduce the units to their most basic.

[Radian]

54 minutes ago, SteamyTea said:

Really just forces and motion once you reduce the units to their most basic.

Yes, vapor pressure is less familiar to most people than air or water pressure. A notionally 'sealed' air space that it cyclically heated and cooled is a complete monster in practice because there is really no such thing as 'sealing' in the building world. That's why sometimes the safest bet is to acknowledge that it's more fool proof to opt for a system that only relies on air being circulated to the great outdoors. This is how its been done for centuries and there's no sign of a 'were you mis-sold a cold roof?' campaign. I hope we never get into that territory with warm roofs.

 

For complete disclosure I opted for a warm roof on our garden room extension and a cold roof for our garage workshop extension. The decisions were driven by a whole lot of different practicalities rather than any ideologies.

[SteamyTea]

44 minutes ago, Radian said:

Yes, vapor pressure is less familiar to most people than air or water pressure

Kinetic energy at the molecular level. Confused by liquid water's surface tension, and capillary action.

Add to that the dynamics of temperature, pressure, humidity differentials, latent and sensible heat, and then some uncertainty, it is a miracle that all roofs keep the water out and stay up.

 

Thankfully we have solutions to partial differential equations to help, but I prefer to call them partial solutions to equations. You get marks for showing your workings.

Edited April 17, 2022 by SteamyTea

[Iceverge]

 

You could drill holes where the circles are shown. Say 50mm diameter to have a cold roof with continuously ventilation between the ply deck and the insulation. 

100mm mineral wool, airtight membrane. 50mm battened and insulated service cavity. Plasterboard. 

 

 

Alternatively warm deck. Get some seconds PIR and sandwich it between the edpm and the ply. Probably the best option and leave the inside just as it is with conduits for electrical fixings. 

 

Also, then you can make use of the joists for hooking things up to. Very useful in a workspace. 

 

 

 

 

Edited April 17, 2022 by Iceverge

[TonyT]

Probably better not to have full depth noggins/dwangs in as you are only getting airflow from outside air  to them,  beyond then there will be no air movement.

[Omnibuswoman]

Just returning to this topic after several weeks of mulling it over… 

 

The first thing that occurred to me is that the building will not be heated, so  in theory there will be no warm air to condense on the underside of the roof ply… in which case, is it worth insulating at all (like a standard shed)… 

 

We could just fit plasterboard to the underside of the joists (having first drilled a 50mm hole through each noggin to allow side to side airflow from vented soffits). In this case, it would be neither a warm roof nor a cold roof, but just a ‘same temperature as room’ roof. 
 

Am I missing something obvious??!

m

[SteamyTea]

5 minutes ago, Omnibuswoman said:

Just returning to this topic after several weeks of mulling it over…

Cornwall got to you then.

 

While not mechanically heated, it will still have a temperature gradient that can cause condensation problems.

[Mr Punter]

I had a garage with condensation on the inside of the roof.  I removed the roof sheets and replaced with insulated metal sandwich panels.  I also drilled out several vent holes in the rear wall to encourage air flow.  No more dripping condensation.

[Radian]

The problem with unheated workshops is that throughout the year there are times when the internal temperature is considerably below the outside temperature - unless it is deliberately ventilated (think slatted cow sheds). Then the relatively higher humidity of outside air condenses on relatively colder internal surfaces, tools etc. So yes, insulation can have a detrimental effect if it delays the two reaching equilibrium.

 

I think you were initially considering the reverse of this situation - where internal warm/moist air would condense on cold surfaces on the other side of the insulation. This would be non-negotiable in a normal living space but for an outbuilding you can consider increasing ventilation all round. It's somewhat frustrating I'll agree.

 

My own workshop is insulated and marginally heated by a dehumidifier and small electric convector. The dehumidifier is a compressor type that consumes around 250W and consequently raises the room temperature by a degree or two. Thus it serves two purposes. If I had to replace it I would get an electric Desiccant Dehumidifier which works better at cold temperatures but has the same positive heating effect. Even a marginally heated workshop is proving to be a luxury though so I'm looking at other options.

[Nickfromwales]

20 minutes ago, Radian said:

If I had to replace it I would get an electric Desiccant Dehumidifier which works better at cold temperatures but has the same positive heating effect

Thanks for that.

1 hour ago, Omnibuswoman said:

The first thing that occurred to me is that the building will not be heated, so  in theory there will be no warm air to condense on the underside of the roof ply… in which case, is it worth insulating at all (like a standard shed)… 

 

We could just fit plasterboard to the underside of the joists (having first drilled a 50mm hole through each noggin to allow side to side airflow from vented soffits). In this case, it would be neither a warm roof nor a cold roof, but just a ‘same temperature as room’ roof. 

If this is just a shed, why are you spending ( wasting ) any more money with plasterboard / other finises? I painted my ceiling rafters with the same grey floor paint, and all looks perfectly fine to me. Maybe time to stop now, and stop making any more problems to solve. Certainly do NOT fit recessed lighting!! Surface mounted for sure with low profile trunking. Will be plenty neat enough.

[Nickfromwales]

Oh, and get rid of the full height noggins! Replace with ones 25-50mm lower, ventilating at the top and bottom of the runs.

I bought metal racking which came with MDF shelves ( crappy 9mm POS ) but they have been the only thing to harbour moisture and grow mould. The OSB3 shows zero signs of this, so will be what I replace these crappy shelves with.

[Omnibuswoman]

Thanks peeps. I’ll have a chat with HWMBO about leaving the ceiling as-is and fitting low profile lights instead.
 

Looking at the extra cost of plasterboard etc it seems wasteful to make it any fancier than it needs to be, especially with prices going the way they are. I can’t see anything wrong with the idea of painting the rafters and making a feature out of them. ☺️