Membrane for underfloor insulation

[Kaymo]

I have a 1905 house with suspended timber floor which I am preparing to insulate. I think I would have preferred solid insulation but B&Q selling fibre insulation for £3 a roll had made my decision for me. The joists are just under 180mm depth so I plan to staple netting to the undersides of the joists and the 200mm depth insulation should fit quite snugly in there.

I understand from reading other posts etc that I should fit some form of vapour barrier and this is the part that I am unclear about. Some of the rooms have carpets, would it make sense in this case just to lift the carpet and put the vapour barrier under the underlay? For the rooms that have wooden floors should I stick the vapour barrier to the underside of the floor before fitting the insulation and for tiles is there any need to fit a vapour barrier at all?

Thanks in advance.

 

[WoodYouLike]

Don't use DPM is this situation, it will block the normal ventilation and can cause condensation and even rotting of joists.

Type of underfloor is type of underlayment

 

[Kaymo]

Thanks for the response. Are you saying that all I need to do is install the insulation from underneath and forget about the vapour barrier for all the rooms?

I had read a whole lot of stuff in forums and on the internet suggesting that a vapour barrier was needed to prevent warm moist air from the house meeting the insulation and condensing.

 

[mointainwalker]

A

re you saying that all I need to do is install the insulation from underneath and forget about the vapour barrier for all the rooms?

Yes.

Vapour barrier required for walls and ceilings because warm air rises.

In any case your fibre wool will include a kraft paper ( waxed or poly-coated) vb.

Having previously said you don't need one, it won't hurt to install your insulating roll with the paper directly under the floor-boards.

 

[Kaymo]

Thanks for the response mointainwalker.

I have to say that I am still a little confused. When I initially started thinking about this project my old readers digest DIY book said to suspend the glass fibre between the joists as I suggested but also said to tape what they called reflective builders paper above the floorboards/below the carpet to eliminate any draughts coming through and reflect heat back into the room.

When I researched things a bit more on the internet I read many people suggesting that a sheet as above should be installed as a vapour barrier to keep warm moist air in the house and away from the cold insulation where it could condensate. This makes a certain amount of sense to me in theory at least.

From your reply above mointainwalker you suggest fixing paper below the floorboards but what would be the difference between doing that and putting a barrier above the floorboards?

 

[freddiemercurystwin]

There is no need for a membrane or VB in this situation. Warm air rises it does not travel downwards.