Roof Insulation

[Roger Mills]

With ever-increasing fuel prices, I want to reduce the amount of heat which escapes through the roof.

My roof is a conventional purlin and rafter construction (not trusses). The pitch is only 30 degrees, so there's limited headroom - crawl only.

The upstairs ceiling joists at the base of attic are 3" x 2", and the the space between the joists is filled with fibreglass insulation (3" only).

There are two longitudinal binders on top of and at right angles to the joists. The width between these is approx 40% of the total roof width. This central area is boarded and carpeted, and used for storage.

I'm contemplating added another 6 - 9 inches if insulation to the outer section - between the binders and the eaves, laying it across the top of the joists.

However, I'm not sure what to do with the central portion. The usual suggestion would be to jack up the boards on stilts, leaving room for lots of insulation underneath, but I don't have the luxury of enough headroom to be able to do that. I'm wondering about taking up the carpet, laying 2 or 3 inches of rigid foam insulation on top of the boards, and then putting the carpet back over the insulation. Any thoughts?

If I do that, should the foam have a foil covering on one side (which side?), both sides, or neither side? There is currently no obvious moisture barrier, and I don't want to create a damp problem.

Any advice gratefully accepted.

[Tricky Dicky]

You could create a floating floor on top of the boarded section by as you say placing PIR foam on top however you would need a more rigid covering than carpet. I have installed such a floor in the mancave that forms the back end of my garage using 75mm PIR foam boards ie. Celotex, Kingspan etc. The floor is then made up of sheets of flooring grade chipboard ie. Caberboard these boards are simply glued together with PU glue to make one homologous floor surface.

Richard

[Chris J Dixon]

The centre area of my loft was already boarded with a mixture of materials that had accumulated over the years. Much of it was nailed to the joists, and could not have been lifted without risking damage to the ceilings below.

A few years ago I then covered this with two layers of Knauf space board insulation,(1200 mm x 500 mm x 50 mm) with the joints staggered. I didn't bother with any foam sealant or tape.

As I needed to hold everything down, and the insulation will dent from point loads such as bony knees and storage box feet, I covered it with hardboard (full sheets cut across into 500 mm strips {plus off cuts} at B&Q) and screwed it all down to the boarding using M6 x 120 mm woodscrews which were on offer at the time from CPC.

I was very pleased that the installation went exactly as planned. I did drill pilot holes, but the screws would probably have self-drilled.

The only slight problem, which became apparent after a few weeks, is that (despite the fact that the hardboard spent some of the autumn stored in the caravan on the drive, until I could get the job done) the sheets have expanded slightly and bowed a little.

It has now been in place about 12 years, and is still working fine.

Chris

[Theo]

I've been doing something similar: fitting 125mm celotex underneath the boarded areas, to replace patchy rockwool.

When I did the stackup calculations (ubakus.com), if I rested the boards directly on the foam it shows I'd get some condensation on the underside of the boards. Foil on the foam forms a vapour barrier, but the joists are a cold bridge and so moisture can travel through the joists from the room below. If the boards are on stilts this moisture can dry from airflow over the surface, but not if enclosed by foiled foam.

A possible solution I found according to the calculator was to drape a vapour barrier (in my case foiled plastic which is cheap as 'emergency blankets' in 1x2m pieces[*]) over the joists so that the vapour would not escape the top side of the joists. In other words, the foil on the foam and the foiled plastic over the joists form a continuous barrier with the joists on the room side.

However I forgot to install the first few bays of plastic, and I came to the conclusion this was futile because the boarded area is only about 1.5m wide, so any moisture is going to escape laterally through the joist anyway, to the other areas that don't have vapour barrier.

Now I have the 125mm of celotex (100+25) which makes the level slightly above the 120mm joists. I've then put some pieces of 25mm over just the joists to support the boards on - not enough to seal over the joists, but keeps the boards from putting weight on the ceiling.

It won't be perfect but I'm going to try that and see. Because there's a room-in-roof the detailing is quite complex (as well as a PITA to access) and it inevitably won't be as good as a normal loft, so I suspect the temperature won't be as low in there as the calculator assumes given perfect insulation.

Theo

[*] somebody here recommended 'turkey foil' as a similar barrier, but actually the calcs for that are much worse, since it's thermally conductive.