Insulating between joists in loft - vapour barrier needed?

Cato2

Hi all,

Part of my loft has old boarding on top of the Victorian joists, which are 100mm deep. The previous owner put about 50mm of vermiculite (loose fill) insulation under these boards, which have visible gaps between them.

I've bought lots of Space Board (Knauf's extruded polystyrene) and am planning to:

1. Remove the old boards and add some new insulation between the joists, on top of the vermiculite (I really don't want to remove it all even though it has a poor U-value) - about 50mm so it is level with the top of joists. This will probably be 50mm mineral wool as used in cavity walls, cut to fit the 330mm gap between joists.

2. Put 105mm of Space Board on top of the joists and 18mm of tight-fitting chipboard on top - not re-using the old boards.

3. Put 400mm of Space Blanket glass wool (2 x 200mm) in the non-boarded areas on top of existing 100mm mineral wool, since I got some very cheap at Focus (4 for 1 meaning £84 for 28 rolls i.e. £2.15/m2 - watch out for their next offer!). This part of the house doesn't have cavity walls, as it's Victorian, and is very cold sometimes.

The loft is quite well ventilated with gaps around the eaves, and the new Space Blanket mineral wool insulation is at least 25mm from the eaves. There is one bathroom under the boarded area - it has no window, but it does have an extractor fan with two inlets, venting via a pipe to the roof.

There's also another bathroom which is over joists that already have mineral wool, and I'm planning the same boarding over the existing mineral wool. Here, I could lift the mineral wool and put in polythene between the joists.

I don't want to create a new condensation problem in the loft due to the Space Boards. So....

- do I need a vapour barrier on the "warm side" of the new between-joist insulation (between it and vermiculite)?

- if so, is it enough to put polythene over the vermiculite (not quite the warm side but it's a poor insulator and quite thin) - between joists, but not over them.

- should I use a polyfoam / extruded polystyrene type board between the joists (over vermiculite) as that is also a vapour barrier?

- would it be enough of a vapour barrier to paint the bathroom ceilings with an anti-damp paint (and maybe adjoining bedroom for the ensuite)?

- should I drill ventilation holes in the Space Board and chipboard?

This is all costing quite a lot, so polythene or paint as vapour barrier would be preferable if they do the job. Or even no vapour barrier if it's a non-issue!

The mineral wool and vermiculite is not noticeably damp at the moment, but then it's not very cold weather at the moment.

Hope someone can help with this, I've done a lot of searching on various forums and come up with diverging views.

Gloomendoom

I read somewhere that modern latex (emulsion) paint provides quite an effective vapour barrier on its own. You might get away with giving your ceilings a few coats of new paint.

Cato2

Gloomendoom wrote: »

I read somewhere that modern latex (emulsion) paint provides quite an effective vapour barrier on its own. You might get away with giving your ceilings a few coats of new paint.

That would be a lot easier than laying polythene. I wonder if anyone else has an opinion on this?

David_Aldred

Hi Cato,
Thanks for the private message and reading through your post the following comments are made:
If you insulate at ceiling level the loft space above it will be colder than it was before and accordingly as the temp drops the risk of condensation increases. The roof covering may have enough ventilation to cope with this or it may not, especially if it has non breathable roofing felt fitted. If the loft void starts to suffer chronic condensation then you will have to increase roof void ventilation. Various options are available to do this at eaves and at ridge level.

The min air gap between top of insulation and underside of roof covering when traditionally insulated is 50mm.

The ducts from extractor fans should not simply terminate into roof voids because all you will do is allow the fan to discharge warm moist air into a cold roof void making chronic condensation. The fan ducts should be run through to the outside, if this is via the roof void the tube should be fixed / sealed to a puropose made roof vent tile / slate so it vents externally.

The polyfoam as you indicate is I think a closed cell foam and accordingly relatively waterproof.

The nitty gritty of your question is really whether you are going to hit the dew point in the insulation / boarding sandwich you are proposing if you don't fit a vapour barrier. I'd like to say no you won't but unfortunately experience has shown on occasion sometimes you might especially if the room below is a wet room (i.e. bathroom / kitchen) and the dynamics of the roof void / way the insulation is fitted add up to create such an oppertunity for it to occur.

There are coatings that will act as vapour checks as indicated which is certainly an easier way to do things without too much messing about. I would tend to not fix your boarding down but just lay it loosely and monitor the situation through the winter to see if interstitial condensation is occurring / you need to modify the arrangement before final fixing of the boards. As i say I'd like to be more black and white with the answer but unfortunately properties / roof voids / moisture within rooms below varies so much from one property to another that it would be reckless to state a definate specification to an existing property as opposed to a new build. Kindest regards, David Aldred Independent damp and timber surveyor

Cato2

Thanks for your very helpful post - much appreciated.

We haven't had condensation in this roof before, even at Xmas when it was quite cold - the gaps generally are enough for birds to get in and nest, though we have recently had some work done to exclude them from the southern side of loft - there's quite a big gap in various places around the loft, which is over 90 m2 in a T shape.

The extractor fans in the bathroom are ducted through to a slate in the roof so that should be OK at least - this bathroom is the one that is under the main boarded area, while the other one will only have partial boarding over it.

I will investigate the option of coatings as vapour checks on the bathroom ceilings - any pointers to what to look for in these, e.g. good types? Is it just a matter of a "bathroom paint" or something more technical?

I have quite a large area to get done, so I would not want to leave it all loosely boarded - however I'll see if I can do that in a few areas - maybe build a hatch that lets me lift the polyfoam+chipboard to get to the joist. Although that will tend to leave air gaps...

I will also get some 'loftcaps' to go over the bathroom downlighters, which should help reduce vapour getting into the rest of the loft including the insulation.