Does enclosed air space insulate?

[juliussee]

Hi guys,

I always get good advice here so I have another question for the knowledgable people here.

I have 2 bedrooms on a second storey of my house. The house is just timber framed clad with blueboard. The roof is metal deck with foil backed blanket under it. It sure gets hot up there as the interior is totally stripped (no internal linings atm). I will be using insulation batts in the walls.

For the ceiling, so far there is the foil backed blanket under the metal roof sheets. Then there is an air gap of 200mm (depth of rafters/joists) and an old thin plywood ceiling. From the existing ceiling I want to drop it again about 100mm using suspended metal track so I can install a new flat plasterboard ceiling and run wiring easily.

My question is should I put more insulation between the new plasterboard and the old ply ceiling? Or would the 2 air cavities work as insulation?

All advice greatly appreciated.


Regards

Adrian

[doug1]

There is a product called foil board which may be of use to you. Its a foil sided 12mm or so board of polystyrene, they have a website now forgotten, you should be able to search for it if this sounds like what you want.

[julianx]

Hi

has the existing blanket been layed between the roof battens and sheeting? If so, this is not the ideal way to lay bulk insulation as it is squashed between the two there-by reducing the R value (insulation value).
If you've got the opportunity to put more bulk insulation in I'd put it in, without access to the roof space you won't be able to put it in latter.
You should probably check with your electician first about running wiring under insulation.
regards julian

[johnc]

The two cavities would have some insulation properties, but nothing to write home about. For the small cost of additional insulation I wouldn't think twice about it, just pick the most suitable type. If you really want to heat up the upper floors in the summer months just instal an air-con downstairs and turn it on. It will chase hot air up the stairs faster than a rat up a drain pipe.

John.

[stolar]

The problem with air cavity is that air freely moves and with circulation it transfers the heat. It gets even worse with the open cavity. The whole point of bulk insulation is to keep as much air in one place without letting it move.
So to answer you question the air is a very good insulator as long as you can prevent it fro circulating.

If you can close off the cavities you are half way there but bulk insulation will take you further.

Regards

Branko

[Barry_White]

As Stolar says you cannot have circulation of the air but also critical is that that the airspace is no greater than 1 1/2". That is why double glazed windows work. The air space must be sealed.

[Termite]

If it wasn't for the insulating properties of air we would all be fried to a crisp by the sun. As has been said previously by my learned colleages, the air must be still and in small lumps.

[helga]

Hi
We have used Aircell insulation which is like an oversized bubble wrap with a foil backing so it is the sarking as well. Very easy to install and no fibres. the Bubbles have air in them that can't move! It has made summer just bearable.
We bought it from a shed builder in Casino which doesn't help you, but check for a website if you are interested. See ya

[juliussee]

Thanks for the advice guys, its always good.

Think I'll just put some bulk insulation up there, better than nothing.


Regards

Adrian

[Dion N]

According to the book Cool House/Warm House, your air gap between the two ceilings would have an R value of 0.46 (maybe less since your gap is 100mm). The higher the R value the better the insulation.

Bulk Insulation is usually R 1.5 or R 2. A Tiled pitched roof with sarking and R2 insulation will have R3 in winter and R4.5 in summer (with the shiny side of the sarking down).