enter

I want to insulate our concrete attic floor with glass wool. I have scoured the web and would like to know, do I have to lay plastic sheeting first on the concrete then lay the insulation. If so is there a special sheeting to lay to prevent moisture.

heli2010

Hi dont lay plastic sheeting on concrete as any moisture permiating through the concrete will get trapped and in time weaken the concrete, I dont know your situation but a better solution is to insulate the ceiling below with expanded polystyrene or foam boards, glass wool has a low insulating value unless its 300mm+ thick

enter

Thanks for the advice but the house interior is complete and insulating the ceiling is not an option. I can get polysturene 100cm reinforced tiles (usually pink) that you can walk on, would they be a good solution

heli2010

not really as polystyrene would act as a vapour barrier on top of the concrete if you are looking to walk on it you would be beter making a false floor using 2 x 8 or 2 x10 with T&G flooring and use glass wool to infill similar to this type of installation https://www.loftzone.co.uk/

fidobsa

I did this for one room (my bedroom) but the ceiling was a sort of adobe, not concrete. I chose that sort of insulation at the time as I tended to get snow blown in under the tiles. I thought this could wet any fibreglass and it could take a long time to dry out. Actually, open cell polystyrene does breathe, it is the closed cell type that could trap moisture. I think I might try the 100 mm polystyrene as ceiling insulation at my house in Croatia as that does have concrete loft floors. I think these are made up using a type of interlocking brick as the ceiling with concrete poured onto that. Any fixings for the polysyrene can be screwed to those bricks, avoiding the need to drill the concrete.

Peter_in_Hungary

I agree - don't lay plastic sheet due to moisture trapping.
However the insulating value of glass wool is very little different to expanded polystyrene (EPS). e-.g. 100mm of EPS has a u value of 0.35 whilst 100mm of glass wool is 0.37 which means that with a temperature difference of 20 deg glass wool will loose an extra 0.4 watts/m2 which can easily be made up by the fact that the glass hugs contours better than EPS.

The pink (or yellow, green or blue) polystyrene is usually extruded polystyrene (XPS) and is closed cell whereas EPS is open and allows the passage of water vapour.

An easier solution to fixing the EPS sheets would be to use the adhesive of the type used to fix EPS to walls for external wall insulation (EWI) otherwise use the 'plastic nails' used to provide mechanical fixing for EWI.

If you want to use the loft for storage then rather than use proprietary loft stanchions I would use 50x100 timbers spaced to fit glass wool between and lay alternate layers of insulation at right angles to mitigate cold bridges or gaps and then lay OSB boards over.

For information at a 20 deg. difference inside to outside a concrete ceiling with no insulation will loose about 65 watts/m2
The same with 100mm glass wool loses about 0.36 watts/m2
The same with 200mm glass wool loses about 0.19 watts/m2
The same with 300mm glass wool loses about 0.13 watts/m2
The same with 100mm EPS loses about 0.33 watts/m2
The same with 200mm EPS loses about 0.18 watts/m2
The same with 300mm EPS loses about 0.12 watts/m2
It is easy to see that with increasing thickness there is diminishing returns so there is a balance between cost and savings to be thought about. IMO putting in 200mm of insulation is a no brainer, putting the extra to 300mm is a lot more marginal.

Edited to add - With glass wool insulation there is an advantage to cover the insulation with heavy grade paper (NOT plastic) to stop wind wash which will reduce the negative effect of draughts blowing through the attic.

fidobsa

Hi Peter, yes I did mean the proprietory fixings for exterior wall insulation. I have never used these so was not sure if they were screwed or nailed in. I would guess that for a ceiling it might be possible to just tape and skim like you would with plasterboard.

Peter_in_Hungary

The plastic fixings used for EWI are hit in after drilling the correct dia. hole.
If you are using the EPS in the attic (on the top of the ceiling) then I would see no need to tape the joins but fill any cracks between sheets with squirty foam. If you use EPS in the attic you will have to protect it against mice because they love the stuff !! Which probably means putting a skim coat of EWI adhesive over the whole lot. If you are using it on the underside of the ceiling (internally) then I would first use the EWI adhesive as a skim coat the same as with EWI and then skim with what ever they use for plaster skim in Croatia. Why? I'm not sure that the plaster skim would stick well enough long term. (by the way I am not a fan of internal insulation)

enter

Thanks all, looks like EPS poly slabs is the way to go then or OBI sells YRSA sheeting which they term as breathable?

Peter_in_Hungary

YRSA ????

enter

URSA

Peter_in_Hungary

URSA being the manufactures name do you mean the USRA Terra which is rockwool by a branded name? Yes it is breathable. the only other sheet insulation I can see on the USRA website is XPS which is definitely not breathable.
Mice don't eat rockwool so probably a better bet than EPS.
Before you buy at OBI check out prices at your local builders merchants, rockwool in Hungarian is kőzetgyapot. google kőzetgyapot ár to get web prices as there are often offers available.

enter

Cheers Peter

Pilis Nemzeti Park Fella

Attic insulation: I got 20 rolls of Knauf Ekoroll üveggyapot 10 cm (9 m2/tekerc) a couple of years ago, HUF4200/ roll incl VAT + delivery for 20 rolls was HUF6000 (round trip 35kms)

Made a decent difference both summer & winter

Jack_Russells4ever

Just curious, we have an attic with a layer of dirt in it for insulation or so we were told, Would it be wrong to replace the dirt with some of the batts of rockwool insulation? I suspect it would take a substantial amount of stress off of the old roofing timbers and planking if we did so. Thoughts?