Tri Iso, or Kingspan / Celotex

[poo at Paul's]

OK, we have a 1970s house with a boarded loft, that has about 4 inch of fibreglass under the boards, (ie aboe the ceiling in the upstairs). IN the loft, it is very drafty and cold in winter, and the upstairs is tricky to keep warm.
The rafters show from the inside of the loft, you can see the rafters, some longitudingal timbers also (8inchx8inch?) and roofing felt, then there are concrete tiles. So, the only insulation seems to be the underboard 4 inch glass.

So I was going to buy loads of celotex or Kingspan and cut it tight to squeeze between the rafters, againstt the felt. But someone reckoned it would be better to have the "airgap" that is under the felt, ie fit the celotex etc directly over the rafters, leaving about a 60mil gap to the felt. He said, and it loads easier, less cutting......but he forgets it needs to go in the loft hole, so cutting will be required regardless. I cannot find anyone who will tell me the "best" way to use this stuff in my particular roof, (I dont want to remove the tiles!)

Today my plasterer recommended Tri Iso Super 10 and said it is in rolls (easy accesss, and can be just stapled over the rafters in lengths and joined with foil tape. It looks high spec, equivalent to 210 mill of glass they reckon, and is only 30mil thick. So sounds easy to use, but again, the useage sheets show it battened and integrated under the tiles...

So anyone have any ideas if this stuff will work as the platerer says? Ie just tack it on over the rafters with staples and leave the airgap underneath to the felt? ANy other ideas, its a fair size roof, about 120m2 in all, and at about a tenner a metre, I dont want to use this stuff unless it is good and will wwrk like this. At the same time, it would seem the easiest of the options and I should be able to do it in a day pretty easily.

Any ideas. I dont really want to lift the floor and make that thicker, and this will also keep the loft warmer for all my spare car parts!

[TooLateForAName]

Tell the plasterer to stick to plastering.


The main benefit from Tri-Iso and the like is that it will act as a barrier - you could as easily use any poly sheet - It will stop the airflow just as well (watch out for condensation)

There is a bit of reading here: (get yourself a coffee or something first)

http://www.greenbuildingforum.co.uk/forum/index2.p...

I'm not up on the latest story but I think that at one point they had their certification withdrawn and the claimed performance has been knocked right back.

[satans worm]

Tri iso wasn't certed last year when I was building our house, that said your not looking for building reg approval , however I would avoid any insulation in the rafters as this can cause them to rot if they cannot breath properly. Best just buy(cheaply on govt scheme) top up on the standard ichy glassfibre stuff and lay it across the floor. IMO

[mrmaggit]

The problem between the rigid boards and the multi-layer stuff is that they were tested in different ways. Test either in the way the other is and they fail, badly. The manufacturers have come up with a fudge (as usual) meaning that you have to use them together.

The rigid boards need to be fitted to allow at least a 25mm air gap between the top of the board and the underside of the first roof covering (whether that be lath, felt or sheet material. The multi-foils are easier to fit, just being stapled/battened to the underside of the rafters, but because they move with any airflow, they MUST be taped together, so there are no gaps. Over time, there is no guarantee that the glue on the tape won't perish, then the heat will escape. We sell both, the rigid sheets are MUCH more popular.

You pays your money, etc.

[mk1fan]

No point insulating between the rafters as it appears that you have a cold roof construction. If the voids between the ceiling joists under the loft boards are fully filled with insulation then why not over lay the loft boards with rigid board insulation?

[Sam_68]

mk1fan said:

No point insulating between the rafters as it appears that you have a cold roof construction. If the voids between the ceiling joists under the loft boards are fully filled with insulation then why not over lay the loft boards with rigid board insulation?

You would get some benefit from rafter-level insulation, as it would turn the loft into what we call a 'semi-exposed space'; the amount of air circulating through it from eaves level ventilation is fairly limited and since warm air rises, the majority of heat would be retained by the volume of air under the pitch of the roof, in much the same way that a tent works better than you'd expect, despite the ground level vents.

Be aware that in reducing heat loss by additional ceiling level insulation, in particular, you might cause much bigger problems, though: it's very difficult to predict, in practice, whether you will run into issues with interstitial condensation in older properties.

There is a calculation methodology, in theory, but I wouldn't trust it as far as I could spit.

If you do go ahead with it, keep an eye out for problems developing and be prepared to remove the insulation if you run into trouble.

Some relevant discussion here, though the topic is mainly concerned with 'dew-fall' condensation caused by over-insulation of new houses, rather than interstitial condensation on old ones.

[poo at Paul's]

mk1fan said:

No point insulating between the rafters as it appears that you have a cold roof construction. If the voids between the ceiling joists under the loft boards are fully filled with insulation then why not over lay the loft boards with rigid board insulation?

It would be beneficial if we can increase the temp of the loft space a little, hence looking at the rafters. Only toruble with rigid boards on the floor of the loft is the need to reboard over them and all the pipes and wires etc running on them will need access.

Also, getting rigid boards trough the loft hatch is an issue!

[mk1fan]

Lift existing loft boards. Cover any 'recessed' lights with fire caps. Lay down vapour barrier over existing insulation and joists. Note down position of electrical runs. Over lay joists with rigid boards (cut narrow enough to pass through loft hatch) ensuring eaves ventilation is expossed (assuming there is some). Re-fix loft boards. Fair bit of work but much more benificial than insulating between the rafters.

[blackcab]

My advice is get an expert in - Try colin Illstone at TRS for a bit of free advice - get insulation wrong and you will end up in the crap 5 years down the line with damp and condensation issues