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A question on insulating the ceiling of the Wonky Donkey

C

Cabinetman

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Ian
Not quite at this stage yet but thinking ahead, this is the general layout of the wall and joists, there is an attic type space above the joists which will be partly boarded out for storage. The joists are 8x2.
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There will be a sheet of insulation screwed up under the joists acting as the ceiling with two more sheets above.
The question I have is do I put the other two sheets in in pieces between the joists (B) or as continuous insulation on top,(A) I favour the second way but it leaves a large gap in between the lower sheet and the top two, as all the edges will b sealed I can’t see that it will be a problem but I don’t want to lose any of the effect by having a gap, any thoughts please? I think what I’m concerned about is do the sheets have to be together to get the best effect? There will be a sheet of ply on top of either solution. Thanks Ian

IMG_0255.jpeg
 
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Option B but fill the cavity with batt insulation instead of the sheet insulation. It will save a lot of time and $.
That is if you haven't already purchased the rigid insulation.
What is the R value per rigid insulation Ian?
 
I was thinking the same. Finish the ceiling with plasterboard or wood board of some type, fill the voids between joists with mineral wool (which I would do anyway) and board fully on top to provide storage. Run any cables for lights etc through prior to boarding.
 
I'd fill the void (of the 8x2) with mineral wool type insulation. I recently used some (purchased from FB Marketplace locally) made out of recycled plastic bottles of all things. None itchy etc., BUT it was only 37 cm wide instead of 40 cm/16". Price I got it for made it worth messing with. I don't know what sizes are where you are if you do mineral wool type?

For ceiling materials... plasterboard maybe 🤔... but I used uPVC type T&G effect soffit board. Saves having to put a finish of some kind to the plasterboard... plaster/paint... or such. The walls in my workshop are covered by white uPVC type sheets on top of OSB fixed to the walls. Saved on painting the concrete blocks that the wall are built with.

Good to see that you're planning ahead Ian 👍
 
The second figure with construction types A and B looks like the "hot roof/cold roof" options. As you mention an attic above it that may change things a bit, but the overall logic seems to be vapour barrier on the warm side of insulation and ample ventilation on the cold side so that warm moist air is prevented from percolating through the insulation to condense on a cold surface. The ventilation means any that does get past has somewhere to go.

Having said that, I have a boarded loft over mineral wool insulation above a plasterboard ceiling with no vapour barriers for many years, and it hasn't fallen down.
 
Well the thing is that I already have the Kingspan type 1 1/2” sheets, I’m not at home atm so haven’t got the paperwork. Pam is allergic to the wool stuff and I certainly hate the stuff, (I’m of the opinion that it will be treated like Asbestos in time) so that leaves Sheep’s wool which is a joy to use and very expensive. Haven’t come across the shredded plastic over here, isn’t it a fire risk?
No sorry these are the options, I have decided against a plasterboard ceiling as I find it too awkward compared to painted insulation sheets, there will be a 2x1/2” covering strip over all the joints, it’s a workshop after all.
 
A void between layers of insulation effectively renders the outer layer useless. Find a way of keeping it all together without gaps.
 
This is some information for the DIALL insulation product I used re fire risk:

"Diall Recycled Plastic/Polyester Insulation (Loft Roll): This type of insulation is often described as non-itch, eco-friendly, and safe to touch. It is designed to be fire-retardant, meaning it is formulated not to catch fire easily and to resist ignition. When exposed to direct flame, this material is designed to melt rather than sustain a flame, as noted for similar, non-flammable plastic insulation products."

As you already have the boards (I thought you did as I seem to recall you using some for the floor area? You cut and squeezed them in between the joists? ) you want to use them - understandable.

If you cut and put between the joists I'd do the same for the ceiling rather than fix direct. I'd have visions of the boards dropping as the fixings pull through the insulation material.
 
Ian, am I right that you do not intend to put any plasterboard or other fire rated product over your layer of insulation on the ceiling? If so check very carefully both the fire spread properties of the insulation and what poisons are in any smoke generated. I would not take the manufacturer's word for it either. I would not have exposed insulation even if foil faced now we know better.
 
Thanks for that I’ll look into it, if 4 1/2” of the stiff stuff isn’t enough I could put that in on top!
The ceiling won’t fall down though as I’m using big plastic fixing washers which will be hidden under the wood strips, or maybe not now I’ve seen this from @PAC1
PAC1 said:
Ian, am I right that you do not intend to put any plasterboard or other fire rated product over your layer of insulation on the ceiling? If so check very carefully both the fire spread properties of the insulation and what poisons are in any smoke generated. I would not take the manufacturer's word for it either. I would not have exposed insulation even if foil faced now we know better.
Click to expand...
oh heck, I hadn’t considered that! That would not be good. Rethink coming!!
 
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