Loft insulation and Moisture

[AndySussex]

A hopefully straightforward loft insulation question.
I have a very small loft which contains my header tank with a jacket on it, and then maybe a 4m long and 3m wide stretch with rafters every metre or so.  There is a little bit of insulation between these rafters and no insulation between the eaves.
I am thinking of getting some acoustic insulation wool (very dense stuff) and wondered are there any issues with
a.) Laying this across the tops of the rafters, so at 90 degrees to the stuff that is between them.
b.) Insulating the eaves. I guess there could be moisture issues? If so, how does this moisture come into play? Where is it from and where does it go?

Cheers

Andy

[pb]

A hopefully straightforward loft insulation question.
I have a very small loft which contains my header tank with a jacket on it, and then maybe a 4m long and 3m wide stretch with rafters every metre or so.  There is a little bit of insulation between these rafters and no insulation between the eaves.
I am thinking of getting some acoustic insulation wool (very dense stuff) and wondered are there any issues with
a.) Laying this across the tops of the rafters, so at 90 degrees to the stuff that is between them.
b.) Insulating the eaves. I guess there could be moisture issues? If so, how does this moisture come into play? Where is it from and where does it go?

I'm not quite sure I understand what you're referring to there.  It sounds as though you might have some of your terminology a bit mixed up.

Just to be clear:

- the rafters are the sloped timbers which form the roof itself and carry the slates, tiles, etc.  For a traditional roof construction there wouldn't be any insulation on top of these and probably none between them either.

- the eaves are the areas around the outside of the building where the roof oversails the walls.

- the horizontal timbers which form the "floor" of the loft (and carry the ceiling of the room below) are joists.

In a traditional cold roof there should be ventilation around the eaves.  This allows any water vapour which might otherwise be trapped in the loft to be removed.  You don't want to stuff insulation right into the eaves because this will block the air path and might lead to condensation issues.

You can put as much insulation between, or on top of, the joists as you like.  The more the better.

You can put insulation between the rafters if you want, but its effect will be less because of the cold airflow through the loft.  Generally you'd do better to just pile that insulation on top of the joists, unless there are particular reasons why you don't want to do that.

Putting insulation on the tops of the rafters would generally involve stripping the whole roof which is a pretty major job.  If you did want to do this then you could convert it to a "warm roof" arrangement where the loft is inside the heated envelope.

[offthegridandy]

Andy,

word of warning, don't insulate under the tank it will increase the risk of the tank freezing in V bad winters.

Another Andy

[wookey]

Why do you want acoustic insulation rather than thermal insulation? Some materials can do both, but normally you want to maximise the thermal aspect.

It does sound like your loft desperately needs more insulation. It should be at least 300mm deep. (U=0.1w/m2K or better).

You asked about moisture: it comes through the plasterboard, especially above bathrooms, through holes in the ceiling (light fittings, plumbing/wiring holes), and in the outside air which gets in at the eaves. It leaves again through the tiles if the roof is not lined, or through the eaves or vents in the gable ends if it is. On more modern buildings there may well be roof vents at the ridgeline to let the air (and thus moisture) out.

[AndySussex]

Ha sorry for the wrong terms for things. I show my inexperience!
Ok, so my reading of this is
a.) No point insulating between the rafters as I don't need/want to keep the loft warm really
b.) Pile on the insulation on top of the joists, between the joists, all round the joists etc and this won't prevent moisture coming through from below and then exiting through the eaves in any way??
c.) Don't insulate under the tank (yep got that one :-) )

Wookey - I do want thermal insulation but the biggest problem with my house is noise - we're right by a big road and so whilst I am working on getting nice soundproofed curtains and windows, I'd assume that a fair bit of road noise would come in via the roof wouldn't it? Hence I'd like to insulate the bedroom just below the roof from as much noise as possible.

Also, since that bedroom is shaped like this: /-\ with some flat roof with loft above (easy to insulate), what can I do about the side bits, which I guess form part of the 'rafters'?

Thanks!

[KLD]

Also, since that bedroom is shaped like this: /-\ with some flat roof with loft above (easy to insulate), what can I do about the side bits, which I guess form part of the 'rafters'?

Thanks!

Andy

Lots of recent threads about that, try searching for the term "skeiling" on the forum.

Klaus

[pb]

Wookey - I do want thermal insulation but the biggest problem with my house is noise - we're right by a big road and so whilst I am working on getting nice soundproofed curtains and windows, I'd assume that a fair bit of road noise would come in via the roof wouldn't it? Hence I'd like to insulate the bedroom just below the roof from as much noise as possible.

If it's mostly noise that you're worried about then you might get better results by relining the bedroom ceiling instead.  That way you can soundproof the whole area and don't need to worry about blocking any ventilation paths in the roof.

You'd probably get a worthwhile improvement by just overboarding the existing ceiling with 15mm soundbloc or fermacell, assuming the joists will take the added weight.  Fixing the new boards on resilient bars rather than directly to the surface would get you a small additional improvement (but you'd lose another 16mm or so of headroom).

[AndySussex]

"Skeiling" Thanks! I'd never have come across that term.
So....looking at the threads, it seems that a good way is to aproach it from the room side, add insulation there and screw that into the beams and paint over.
this would be tricky for me as head room is limited already in these rooms. The house was made for midgets.

If I approach it from the other side, then am I right in my understanding that I shouldn't block the 'eaves' which is at the bottom of the \ and so if I do put celotex or something in, then it can be attached to the roof side or the house side but I must make sure I don't block airflow at all....therefore not do anything too tight to the bottom but I could put insulation maybe 3/4 of the way down the \

How does that sound guys?

Thanks