Want to insulate under rafters with Multifoil - no membrane under tiles...

[Buenaventura Durruti]

Hi all,
I have recently bought a house (1920s/30s) and want to insulate my loft. Currently there is about 100mm of fiberglass between the joists and floored with chipboard. I really need the loft for storage so rather than add extra glass wool I want to use multifoil under the rafters - it looks relatively straightforward. Using multiple layers can achieve a U value without adding insulated boards between the rafters too (or so the manufacturers claim).

Anyway, my roof is old but seems solid, the loft is dry and very draughty so well ventilated, however, there is no membrane under the tiles - which is not uncommon among houses this age. The tiles back directly into the loft. Also the rafters are very narrow and shallow. Multifoil would also help with keeping my loft dust free as currently a lot gets blown in.

What I don't want to do when or if I use the multifoil is to cause problems with condensation or damp, between the insulation and the roof. They claim it is breathable but as there is always an assumption of a breathable membrane under the tiles during insulation I want to make sure I'm not causing a problem down the line.

Help and advice gratefully received

 

[oldbutnotdead]

Alas your cunning plan has a flaw in it. Multifoil may reduce the amount of heat lost through the roof from the loft but won't alter (much) the heat lost through the ceilings below into the loft- you'll still have a temperature differential and only 100mm of insulation. No idea on the price of multifoil (or its actual real world performance), have a look at the cost of jacking up the loft boarding and sticking another 200mm of insulation underneath.

 

[Buenaventura Durruti]

I am aware of that - but it will then become a 'Warm Loft', which should be fine. Ill be passively heating a loft space but that heat won't be lost. Jacking up the boards is not really an option - I'd need to do the entire loft. FYI I can use the multifoil over the joists to remove the loft/house differential but that is still quite a job lifting the flooring. I'd be doing this myself
- 25m2 would cost £250 in raisers not including wood screws and the time to do it, then insulation. It would work out about the same as the multilayer foil.

Apparently SuperQuilt can achieve a U value of 0.15 which is equivalent to 200mm of standard glass wool (their figures).

 

[^woody^]

Your theory of passively heating a loft is flawed, as the heat escaping from the house will not be enough to counter the heat loss through the loft foil. You'll get the worst of both worlds - colder house and cold loft.

Multifoil insulation performance is based on an air layer in front and some insulation behind it. Not just used on it's own or in its own layers.

There is a risk that the warmer air held in the new loft area increases the humidity and therefore the condensation risk. If the loft is no longer getting ventilated as you've sealed it all up, then any condensation will form on the foil and run down it or drip off it.
But whether that occurs in practice you wont know until you do the work, but it's a real risk.

The roof void between the rafters may well be ventilated enough via the natural tile gaps.

 

[Buenaventura Durruti]

Well its the rafters that I'm chiefly worried about, as there is no breathable membrane between loft space and tiles - I get the idea that the loft space becomes a sealed water tight box - however most of the suppliers have or claim breathability of their foils.

Hmm - some of the manufacturers claim that their foils, in particular configurations - say a double layer of quilts - have U Values close to 270mm of Glass Wool.

To use something else alongside the foil I would need to install batons to create a 50mm gap between tile and say insulation sheets, then affix further batons to hold the insulation in place (my rafters are very shallow so I'm not sure how I might do this), then perhaps affix a multifoil - although by that stage the sheets may be enough.

It may then be my best option is to indeed insulate with boards between the joists and foil or use a double layer of say Super Quilt - lift the chipboard flooring and reinstall. But still provides the current useable space within the loft without installing risers and glass wool, and keeps a super ventilated cold loft.

 

[oldbutnotdead]

I've not looked at the data in depth for these multifoil products but my nasty suspicious mind says their performance figures either come from gaming test parameters or non real world testing scenarios- physics is generally unforgiving in terms of energy transfer.
You want that loft cold and draughty- cold so low humidity, draughty so any water seeping in through the slates dries out quickly. There are plenty of rooves with no felt under the tiles/slates, many will have done 100 plus years by now.
By far your best bet (for bedroom comfort as well as roof preservation) is to lift the chipboard, get 300mm of rockwool between the joists & reboard (whether you use legs or bigger timbers is up to you, there's a lot to be said for putting 8 x 2 or bigger spanning next to the existing.

 

[^woody^]

Hmm - some of the manufacturers claim that their foils, in particular configurations - say a double layer of quilts - have U Values close to 270mm of Glass Wool.

Yes they do that but are they proven and acceptable? Air gaps are part of the requirement for may foils and in the not too distant past, the foils would not achieve the stated u-values unless "proper" additional insulation was used.

I'm not sure how a taped foil can be breathable, but maybe the materials have changed recently? The issue is that, as a risk assessment there would then be much warmer and much moister air in the loft and (assuming breathability) this could then migrate in greater quantity to the roof void and overwhelm the adventitious ventilation that exists between the tiles. In which case you may be better off with no breathability.