Attic tin roof insulation options sought (Foil batts, wirly birds...)

I am looking for advice regarding the best way to improve insulatation of the upper floor of an Attic style tin roofed house from heat gain. The upstairs living area consists of a bedroom & study. Roof access can be gained from the study. The roof space that is visible is lined with a single sided foil matting directly beneath the tin, the ceiling space between the ceiling and roof of the bedroom/study area is only about 120mm and I cannot tell if the foil has been laid there. Some of the walls of the study/bedroom are insulated with batts. The floor of the attic also has insulation laid so I am only concerned with the heat upstairs.
A few thought I've had include adding a layer of Concertina Foil Batts and perhaps a wirly bird or two.

Insulation is primarily about getting a still air space between the heat source and the area needing protection - in crude terms the bigger the airspace the better the insulation.

A further consideration is the thermal conductivity of the insulating material. Foil (whether sisalation or batts) is highly conductive so should be isolated from other conducting materials (such as your tin roof), fibre batts are good so long as they are not compressed (ie: retain their still airspaces), and polystyrene is a very effective insulator.

Insulation material should not store heat so wood is good, bricks are poor while aerated concrete products can be used variously (eg: Hebel).

In your case the foil sitting on the battens/ rafters and in direct contact with the tin roof will be next to useless as insulation.

You could use foil batts or fibreglass or polyester batts in between the rafters. You need the highest R rating you can fit while retaining an air space above the batt and below the tin and existing foil. Depending upon the depth of the rafter maybe R2 fibre batts (about 80mm when expanded) and you make sure they are sitting at the bottom of the rafter so there is an airgap above them. You can use wires or purpose designed mesh to hold the batts in place - or the ceiling lining material (eg plywood or plasterboard or timber lining boards).

Another option is to use polystyrene insulating board - either cut to fit between rafters (again leaving an airgap above, although though not as critical as with other insulation), or fixed to the bottom of the rafters then covered with the lining board of choice. You can just fill and paint the polystyrene boards,but they are prone to impact damage of course.

If cost is factor you can use just foil insulation (silver side facing the roof) on the underside of the rafters and then cover with lining of choice (or even leave it as it is - tearing is possible, but with care this can be a quite effective and cheap solution.

To repeat - the key is always ensure that there is an airgap (of 30mm or more) and it is still air - all places for potential leaks should be sealed using gaffer tape, metalised A/C tape or urethane foam.

Do not make the common mistake of thinking "I'll use the thickest batts I can fit in the space" as compression reduces effectiveness and it is better to have a lower rated batt and a still airgap created when you fit the batt in place.

Thanks Bloss.
There seems to be a lot of single sided foil matting left over from when the previous owners placed the foil up against the tin roof - I assume this original foil may have only been laid to help with moisture build up?

Assuming there is enough left over I could staple this to the bottom side of the rafters to improve insulation. You mentioned foil side up would be best, whereas the existing foil laid below the tin roof is foil side down?

I also assume that the concertina foil batts might be of more use where the airspace gap available is limited e.g between the attic roof and areas where the attic also has a ceiling (due to the fact that they create their own triangular airspace).

If the foil was touching the tin roof then it was simply sarking and might have reduced moisture ingress to the roof space, but would have been ineffective as insulation.

You say "foil matting" - I assume you are describing insulation wrap ie: foil (sisalation) on one face with fibreglass attached?

If this is so you could re-use and you would have the batt side facing the tin roof with the foil facing the roof cavity. You can staple to the rafters. Joins and gaps can be sealed using adhesive foil tape although you would find that regular duct tape would last for a reasonable time (but it grips less well when stretched and the adhesive weakens in heat so not as good as the foil tape).

If the foil is just single thickness silver one side and blue the other of silver both sides then you can re-use that too with a shiny side facing the tin roof.

And yes the concertina foil batts would work well there, but again conduction occurs if the foil touches the metal so you might be better to use regular fibreglass batts. Just remember not to compress them, or at least not too much - just enough to hold them in pace.

I think you will find that there is little evidence to support the efficacy of 'whirlybird' extractors although they are common enough and are assessed as suitable to meet various sustainable design criteria. Key here is to make sure there is sufficient airspace and under-eave venting to get the rated airflow and to use the correct number for the roof space being ventilated.

The foil already used may have an anti glare coating on one side, this face is oriented out when installing as a safety measure I believe, & on this type of foil is no less effective at insulating radiant heat than the other side.

It depends on how far you want to go and how much roof space you have. The best way would be to use Anticon insulation blanket over the battens then insulation batts above the ceiling lining. Not the cheapest option but effective.