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.

"Liam" wrote in message
...
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.

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 ;-)
--
Vass

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

Don't lay any barrier as it will rot the joists.

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.

Use barrier but in a U shape between joists so joists can breathe.
(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.

Lay barrier so that the all the joists are completely covered.

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.

Will cold bridge, leading to poor insulation and lines on ceiling
below.

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

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Colin Wilson wrote:

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

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.

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

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!

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

Don't lay any barrier as it will rot the joists.

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.

Use barrier but in a U shape between joists so joists can breathe.
(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.

Lay barrier so that the all the joists are completely covered.

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.

Spelling - OK
Ease of understanding - Marginal
Usefullness - Poor


--
Nick H