Aluminium thermal insulation for loft

iamcornholio

vaio wrote: »

Two layers of the free stuff will have U value of 1.455 divided by 2 so about 0.7

Seven layers will be 1.455 divided by 7 so about 0.2 which is about the same as the stuff above except that 7 layers of the free stuff will only be about an inch thick rather than eight inches

No it does not work like that. You can't build up layers of this type of insualtion and then just divide the r-value to get a total u-value. The effectiveness of subsequent layers diminishes the more layers of foil are used

Foil reflects radiant heat, and is not good for the convected and conducted heat - which will be more prevelant at bedroom ceiling level. The r-value of foil can also vary depending on the direction and angle of the radiated heat, and so is not as consistant an insulator as board or fibre products are, in this situation.

Also, foil will suffer dramatically from thermal bridging, it really needs to be laid very flat/uniform into all nooks and cranies, and there is a significant chance of interstitial condensation. So in practice, it will not be laid optimally.

One layer of it may help in terms of reflecting radiated heat and limiting air leakage through the quilt (as long as the foil is all sealed at the joints), but the rest of the loft insualtion should be built up with traditional quilt

pmorrison_2

iamcornholio wrote: »

One layer of it may help in terms of reflecting radiated heat and limiting air leakage through the quilt (as long as the foil is all sealed at the joints), but the rest of the loft insualtion should be built up with traditional quilt

So, assuming we lay out the rest of the loft with further quilt insulation, will it be worth putting up the aluminium foil insulation on to rafters (the slope of the roof) or is this a waste of time? Will it cause condensation?

vaio

vaio wrote: »

…….Two layers of the free stuff will have U value of 1.455 divided by 2 so about 0.7

Seven layers will be 1.455 divided by 7 so about 0.2 which is about the same as the stuff above except that 7 layers of the free stuff will only be about an inch thick rather than eight inches

iamcornholio wrote: »

No it does not work like that. You can't build up layers of this type of insualtion and then just divide the r-value to get a total u-value. The effectiveness of subsequent layers diminishes the more layers of foil are used

Foil reflects radiant heat, and is not good for the convected and conducted heat - which will be more prevelant at bedroom ceiling level. The r-value of foil can also vary depending on the direction and angle of the radiated heat, and so is not as consistant an insulator as board or fibre products are, in this situation.

Also, foil will suffer dramatically from thermal bridging, it really needs to be laid very flat/uniform into all nooks and cranies, and there is a significant chance of interstitial condensation. So in practice, it will not be laid optimally.

One layer of it may help in terms of reflecting radiated heat and limiting air leakage through the quilt (as long as the foil is all sealed at the joints), but the rest of the loft insualtion should be built up with traditional quilt

Adding them is exactly how it works, doubling the thickness of insulation doubles the R value and because U value is reciprocal of R, double thickness halves the U value

I see your point about foil but don’t see what you mean about cold bridging, what’s doing the bridging? In any event it’s not just the foil, the air trapped in the bubble half also adds significantly to the insulation quality as does the air trapped between layers

These people seem to say that 5mm of the double bubble double foil stuff is equivalent to 100mm of conventional quilt http://www.roof-solutions.co.uk/

iamcornholio

pmorrison wrote: »

So, assuming we lay out the rest of the loft with further quilt insulation, will it be worth putting up the aluminium foil insulation on to rafters (the slope of the roof) or is this a waste of time? Will it cause condensation?

I'd say no. Current standards say you need another 150mm, so you can just buy another 150mm or 200mm roll and just roll it out.

Foil needs to be the first layer above the ceiling, so as not to cause condensation within the insulation, so it would be a lot of faffing about to remove existing insulation and then lay the foil, and put the rest of it back down.

If the ceiling is insulated, then you will get little value in foiling the rafters. You would need to keep the roofspace ventilated to avoid condensation issues, so you could potentially create more probelms than you are trying to solve

iamcornholio

vaio wrote: »

These people seem to say that 5mm of the double bubble double foil stuff is equivalent to 100mm of conventional quilt http://www.roof-solutions.co.uk/

If you work in construction, then you will be aware that no-one uses bubble foil wrap as it is just does not provide any meaningful insualtion, however much of it is used.

You'll also be aware of the debate concerning Tri-Iso and Superquilt and similar products - 35mm of foil and quilt fibre which is claimed to be equivalent to 250mm of fibre ....... but the claims are just not proven and the products are not authorised by most, if not all of the building control authorities. The testing methods of those products have not conformed to an acceptable testing method, and therefore the claims are not substantiated.

They perform differently (mainly good for radiated heat) and so overall do not perform the same as quilt. They also require 50mm of trapped air behind/before the insualtion to work as claimed.

homeenergise

pmorrison wrote: »

So, assuming we lay out the rest of the loft with further quilt insulation, will it be worth putting up the aluminium foil insulation on to rafters (the slope of the roof) or is this a waste of time? Will it cause condensation?

If you insulate the rafters with the foil you should leave a gap of 100mm at the eaves where the ventilation comes from. As you have no sarking felt, I would recommend fixing it as you have the material. Modern houses use 'jab boards' between the rafters for extra insulation. This is however expensive and the foil provides a cheaper alternative. As long as you leave a gap for the ventilation at the eaves condensation should be no problem.

angelavdavis

You can use the foil between your floor joists on your ground floor to insulate under your floors (cut into strips the width of your joists so the joists are left open to the air under the floors and don't rot). Staple onto the inside of the joists under the floor.

jonmp

homeenergise wrote: »

If you insulate the rafters with the foil you should leave a gap of 100mm at the eaves where the ventilation comes from. As you have no sarking felt, I would recommend fixing it as you have the material. Modern houses use 'jab boards' between the rafters for extra insulation. This is however expensive and the foil provides a cheaper alternative. As long as you leave a gap for the ventilation at the eaves condensation should be no problem.

Sounds good advice to me.But you would still have a 'cold roof' construction with just a single layer so ventilation within the roof space would still be needed - at least as I understand it.

I can't post a link but roof ventilation for a cold roof space might get you there.