Loft Insulation. Correct procedure?

[Liam]

I am ready to lay 200mm of Rockwool at loft floor level in an 1896 terraced house. Ventiation has been taken already been taken care of (Eaves to ridge) but I am unsure as to whether to fit a vapour barrier underneath the insulation or not? Ceiling underneath is about 30mm. lath & plaster. I have had the following suggestions so far: Don't lay any barrier as it will rot the joists. Use barrier but in a U shape between joists so joists can breathe. Lay barrier so that the all the joists are completely covered. Can the group give me their opinions please.

[Vass]

insulation just fitted (via a gov't grant ) this weekend in my 1961 house they layed 100mm between the joists and then 150mm perpendicularly to that The loft now looks like a huge quilted cushion ;-)

[Christian McArdle]

Wood needs to be either ventilated or warmed to survive. Rockwool insulation needs to be protected against vapour to prevent interstitial condensation.

Wood: will be warmed, but not ventilated, as it is insulated above. OK. Insulation: not protected against moisture, possibly leading to poor performance or staining on the ceilings. Poor.

(method 1 = insulation only between joists) Wood: will be warmed below, ventilated above. Will cold bridge, leading to poor insulation and lines on ceiling below. Wood should survive OK. Insulation: insufficiently thick, but will be protected. OK.

(method 2 = insulation between AND over joists) Wood: will be warmed, but not ventilated, as it is insulated above. OK. Insulation: will largely be protected, except some vapour travelling up through wood. Marginal.

Wood: will be warmed, but not ventilated, as it is insulated above. OK. Insulation: will be entirely vapour protected. OK.

I would use the last method.

Christian.

[Colin Wilson]

I`ve heard the phrase mentioned before on here but never known what a cold bridge is - any chance you can exaplain it please ?

[PeTe33]

It's a connection between the warm-side and the cold side which can transfer heat/cold bypassing the thermal insulation whatever it is.

e.g. Aluminium window frames on older DG units, or I suppose, a windowsill that bridges the cavity/insulation so connecting cold outer wall with warm inner wall.

[Liam]

On Tue, 29 Nov 2005 14:51:34 +0000, Liam wrote

Thank you all for your replies. As suggested I will cover both loft floor and joists completely with a vapour barrier, lay 100mm Rockwool between joists which are 100x50mm and then 150mm Rockwool over that at right angles. Total of 250mm. of Rockwool should help with the heating bills. Looks like being an itchy weekend!

[Nick H]

Spelling - OK Ease of understanding - Marginal Usefullness - Poo

-- Nick H