What fixings do I need for 112mm insulated plasterboard?

gnolan

I'm going to be buying this insulated plasterboard to be installed around the eaves of the house upstairs. I can't put normal loft insulation here as there's not enough room. What fixings do I need for this and where can I get them?

Shoog

You need mushrooms and you need them to be either 150mm or preferably 170mm. Most regular hardware stores do not stock them at these lengths so you will probably need to mail order them.

They usually work with foam glue or dot and dab.

The glue is the primary adhesive and the mushrooms are there for fire safety. Without the mushrooms the expanding foam glue is liable to push the boards away from the wall leaving an uneven finish.

https://essentialfixings.ie/product-category/fixings/insulation-fixings/plasterboard-metal-mushrooms/

There are other ways of doing this such as battening but glue and mushrooms are by far the simplest way.

gnolan

Thanks @Shoog . This sounds tricky for fixing to the ceiling. I'm planning on removing the existing lath and plaster at the eaves, which would leave only the rafters for the glue to adhere to. In your experience, would this be enough surface area to keep the plasterboard in place, and how would I keep it in place until the glue had set?

DC999

What are you trying to do? Insulate the attic? If so the insulation goes in the attic side. Not on the roof of the bedrooms.

If tight on attic roof space you use the PIR insulted sheets and cut to fit in the rafters and then airtightseal them with expanding foam or airtight tape. You don't need ones with plasterboard.

That will cost more I expect than normal attic insualtion.

The metal mushroom 'screws' are designed to hold into a solid wall. Not wooden rafters.

Shoog

https://www.boards.ie/discussion/comment/122649986#Comment_122649986

Sorry didn't realise it was going to be vertical fixing. Don't think they would work at all in this application. Never really heard of anyone doing what you are proposing so cannot really think how you would do it or even if buuilding code would consider these boards safe in this situation.

These boards are really heavy in these thicknesses, so heavy that it takes two people to lift them - so don't know how you would even lift them into place without a plateboard crane.

gnolan

This is the approach advised by an attic insulation contractor registered with the SEAI. I had him out a few months ago to take a look at insulating the attic and he advised the 4-inch insulated plasterboard at the eaves. The more I think about it the more mad it seems.

@DC999 If tight on attic roof space you use the PIR insulted sheets and cut to fit in the rafters and then airtightseal them with expanding foam or airtight tape. You don't need ones with plasterboard.

I have considered this. Do you know if there's any risk of damp forming at the rafters with PIR? I've read that this is one of the reasons why a breathable insulation like rockwool is more suited to attic and suspended timber floor insulation.

10-10-20

Plasterboard screws, with a Black Phosphate finish, usually, straight into the rafters.

https://www.goodwins.ie/products/drywall-screws-150mm-coarse-thread-yellow-box-100.html?filter_set%5B%5D=3057,3060,7018

Check the screw diameter, 3.5mm should self-tap, larger won't.

But yeah, never heard of using it direct on rafters though.

Shoog

https://www.boards.ie/discussion/comment/122650396#Comment_122650396

If you already have good eves ventilation then using a suitable spacer should keep the airflow adequate. It's only really at the wall plate where serious condensation develops and good air flow here will keep it in check.

So don't just stuff whatever up to the face of the roof. If using PIR then angle it at the eves to maintain a constant gap. If using rock wool then use eves boards to maintain the air gap.

https://www.amazon.co.uk/Refurbishment-Eaves-Panel-Vents-Rafter/dp/B09MJ7B8ZK/ref%3Dasc_df_B09MJ7B8ZK/?tag=googshopuk-21&linkCode=df0&hvadid=697339313975&hvpos=&hvnetw=g&hvrand=15609802992613472713&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=1007850&hvtargid=pla-2196107207425&psc=1&mcid=41719af25fbc33498f446b05d7efb699&gad_source=1

Rock wool is going to be a fraction of the price and massively easier to install. You need a vapour barrier preferably just above the plasterboard ceiling and ideally a second one over the top to stop wind wash of air drawing heat away.

I have this job to do to boost my 150mm to 300mm. A horrible job to do in a confined trussed roof - but less work by a country mile than what was suggested to you.

Another alternative, if you have the ceiling height, is to drop your ceilings. Frame out, stuff with rock wool and add a taped vapour barrier to the bottom before plaster boarding. By far the best job if you have the space.

gnolan

The house was built in the 1930s and I'm honestly not sure how much ventilation there is in the eaves; I know that I can't see any vents in the fascia or soffit. The attic is felt-lined (with tears that I need to resolve another time) and draughty.

There's no affordance to drop the ceiling height, they're already very low. I've attached an image that I should have attached in the first place.

Shoog

https://www.boards.ie/discussion/comment/122650835#Comment_122650835

Tricky.

We had similar situations in our old house. The roof came down and met the wall plate with zero air gap for ventilation. We never really resolved it befoe we sold on. However the only really viable solution would have been lifting off the whole roof to create adequate ventilation at the eves.

I spent 20 years trying to work out a solution. In the end we dropped the ceiling which stopped the moist airflow into the loft.

I can see why the installer suggested the approach he did - but it's a hard solution to implement. There is also the very real potential that adding the extra weight to the ceilings will cause significant structural issue to the rafters which will undoubtedly be minimally specified. Expect sagging and cracking at the joints of the plasterboard.

10-10-20

Expect sagging and cracking at the joints of the plasterboard.

Good point. Might be better to look at non-conventuonal options, whatever they are.

Doolittle51

https://www.boards.ie/discussion/comment/122649986#Comment_122649986

If you're removing the lath and plaster on the sloped part of the ceiling, then surely there are many options available?

You could counter batten the rafters with 2x2, PIR in between the counter battens and then fit 50mm insulated plasterboard to the new battens.

Or build a new frame in 2x4, attached to the existing rafters, insulate in between the new frame and fix plasterboard as normal to the new frame.