Priming insulation mushroom fixings/washers?

SodiumCooled

Hi All,

I am seeing a lot of issues being discussed online with mushroom fixings rusting or whatever and showing through skimmed and painted ceilings. I am seeing conflicting info though around whether they need to be treated or not with some people saying they should be primed, others saying it’s not needed and a few saying they primed them and then the plasterer complained that he isn’t happy skimming over the primer.

What also confusing me is metal beads (angle and J beads) are used and there is no mention of needing to prime them and I’d have thought that should suffer in a similar way?

I’ll be having a word with my own plasterer on the mushrooms but as always I like to hear some options/feedback from others too. Specifically in my case it is insulated plasterboard slabs on the upstairs ceiling that are held using screws with mushroom washers.

Thanks for any pointers.

10-10-20

What about mushroom caps?

Nevermind the insulation claims, they have practally zero insulation resistance, but the cross-section of the bore is small anyway.

10-10-20

What also confusing me is metal beads (angle and J beads) are used and there is no mention of needing to prime them and I’d have thought that should suffer in a similar way?

Beading can also suffer from rusting and discolouration but it's typically from surface condensation/damp environment or an excessively long drying-out duration after the room was initially plastered. But the main reason why mushrooms experience discolouration is down to the thermal-bridge effect and probably excessive indoor humidity. So just cap-off the mushroom head (as above) or paint it using a suitable primer (probably a shellac) and manage the RH.

SodiumCooled

Thanks for the pointers, I don’t think the caps can work in my case as it’s only a mushroom washer with a drywall screw going through the middle so no opening for the cap to slot into.

Can’t see thermal bridging being a big issue from the cross battened ceiling/9x2 joists that the screws will be going into which are packed with attic insulation and a layer across the top also of the joists also. Maybe I’m wrong though.

Biker1

https://www.boards.ie/discussion/comment/122226154#Comment_122226154

If you have say 200mm insulation between the joists and another 150mm across the top then there is no need for insulated slabs underneath. Overkill and expensive.

10-10-20

Can’t see thermal bridging being a big issue from the cross battened
ceiling/9x2 joists that the screws will be going into which are packed
with attic insulation and a layer across the top also of the joists
also. Maybe I’m wrong though.

That's a good point - I can't seen the delta-T being significant enough to cause condensation anyway. To avoid it then maybe the primer mentioned is sufficient.

Shoog

A few things to be aware of with mushroom

-ensure the split is facing down to allow condensation to drain out of them rather than to the inner or outer surface.

• f They are relatively cold because they pass all the way through the insulation so they will always been the point at which condensation will form. It's a good idea to spray foam down the shaft to stop air circulating from the inner to outer surface of plasterboard.
• It's a good idea to cover over the heads with airtightness tape - the same as you should be using on all seams.
• So far with these measures we have had no signs of rust.

SodiumCooled

https://www.boards.ie/discussion/comment/122226645#Comment_122226645

200mm between joists and a 50mm insutated slab was specified by the Architect - for the sake of not a massive extra cost we went 300mm in the attic + the 50mm slab. Extra insulation is never any harm to have. I had people telling me I should be putting a much thicker insulated slab + the 300mm of attic insulation too but I stuck with the above. It's done now anyway so no changing it.

SodiumCooled

https://www.boards.ie/discussion/comment/122226730#Comment_122226730

As I mentioned above these are just the disc head like a mushroom fixing but held with a drywall screw as it is for insulated plasterboard so there is no open channel like full mushroom fixing. The ceiling is cross battened and all these fixings are below the airtight membrane so should be no issues with air leakage - there shouldn't be condensation forming either.

I'll go with the primer anyway and that should be sound, my own plasterer is happy with that anyway also said its normal.

Thanks all for the help as always.