Increasing roof insulation

Hi,

Reading about HIPs has made me think about my loft insulation, so they're doing some good already.

In my victorian terrace the loft insulation is a bit wimpy - tired- looking fibre glass between the joists which looks like it was laid a good few years back, probably no more than 75mm. A lot of it is now under chipboard flooring which I don't much want to take up.

I'm thinking that to make it better I could put insulation between the rafters (I'm never sure about roofing terminolgy, I mean the lengths of wood that support the tiles and make up the pitched roof itself). What do you think? If this is a viable option would this sort of stuff do the job?

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is cellotex the way forward?

The design of the house means that the (original) top bedroom is partly up in the roofspace, and it can get uncomfortable up there on the hottest summer days, so I'm hoping doing this would keep the loft cooler in the summer too.

Cheers!

Martin

Reply to
Martin Pentreath
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I used 50mm Rockwool slabs between the rafters. Cheaper than celotex but you can still achieve the recommended 50mm gap under the roof tiles (otherwise I'd have used the standard fibreglass rolls). I also fitted lift-up panels as "walls" to save going right down to the eaves. Well worth doing IMO. P.S. About 3/4 of the way through the job I found the quickest/easiest way to cut slabs (or foam) for a tight fit. If anyone's interested, I'll elaborate.

Reply to
Stuart Noble

OK, I just found a source of cheap celotex on ebay.

Not quite sure what you mean by this, or what the advantage is.

Oh go on then. I also just looked around the celotex site and found this handy guide:

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recommends cutting the boards on an angle to get a wedge effect as you push them in for a tight fit, is that what you mean?

Reply to
Martin Pentreath

As the space close to the eaves isn't really useful for much, I used the existing vertical rafter supports as walls. It cuts the floor space perhaps by half but it gives you a room you can stand up in. Most loft conversions have this kind of arrangement. The "walls" in my case are just insulation in hinged wooden frames so I can lift them up and get access to the bit near the eaves. At the time I wanted a workroom up there and I was less concerned with overall insulation values. Now I might just insulate all the rafters and forget about walls.

What I did was to overcut the strip by half an inch or so and gradually push it into the gap whilst running a handsaw against the rafter. You need to keep the handsaw flat against the rafter but it's deadly accurate and takes account of the variations in width between rafters. Also you don't have to support the inevitable undersized strips.

Reply to
Stuart Noble

Just been doing some research into ventiallation requirements and I'm beginning to go off the idea now.

I was aware of the need to leave a 50mm ventilation space between the celotex and the sarking (which I'm fairly sure is non permeable), but I hadn't really thought about the need for slate vents or soffit vents to get air circuating through the 50mm void.

On the whole it may be cheaper and a lot less hassle (including the risk of rotting roof timbers) to pay =A3100 a year more on my gas bill!

Reply to
Martin Pentreath

So much for HIPs doing much good then :-)

Reply to
Andy Burns

You could chop out some of the sarking to provide ventilation.

Reply to
John Rumm

If yours is anything like mine you get plenty of airflow from the eaves

Reply to
Stuart Noble

I'd be very careful about it, and monitor it closely for a while after you finish

You need two things to avoid any problems one is a good airflow and the other is a vapour barrier

If you do not have a vapour barrier then the warm moist air passes through, or around gaps in your insulation, then it becomes cold damp air and condenses on your rafters. I wouldn't be convinced that a good airflow on its own would be adequate, it could potentially just make the airspace cooler. You also need to ensure there is ventilation at the top of the airspace and not just at the eaves to ensure good airflow.

I did the stuff insulation between rafter technique in our current house not long after moving in (7 years ago now), but a few months after doing this, and while installing a new bathroom fan, which went out through the ceiling I had cause to move some insulation and there were signs of damp underneath. There was a 50mm airgap behind the insulation, and moderate ventilation form the soffits, but no slate vents or vented ridge tiles

The lot came down immediately, but will be replaced in due course when I eventually convert the loft.

The only area that showed any problems was directly above the bathroom. But I've heard of a few people with older loft conversions, (and probably no or poor insulation and ventilation) complaining about the state of their roof, and letting water in. I now wonder if the problems are inadequate ventilation and condensation issues, rather than the actual roof.

cheers

David

Reply to
DM

Thanks for the advice. I'm interested by what you say about a vapour barrier. When I boarded the bit of the loft that is now boarded I was careful to take up all the old fibre glass in that part and staple and tape continuous lengths of heavy-duty polythene between and over the joists before putting the fibre glass back down and then laying the chipboard. So effectively part of the loft space already has a vapour barrier, but I boarded less than half of the total area, so that's not really going to help.

I guess I could do the celotex-between-and-over-rafters treatment and then put a polythene vapour barrier over the whole lot stapled and taped to the lower layer of celotex to stop any water vapour getting through the celotex?

Reply to
Martin Pentreath

The foil covering on the Celotex is a better seal than polythene just use aluminium tape to seal the joins.

Reply to
FKruger

Actually they are similar, but since it comes with the foil, and needs taping anyway, why waste money on polythene too?

Reply to
The Natural Philosopher

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