Funcrusher - are you still out there? I a'waiting with my six-gun!

Godd - because I said

Posted By: fostertomActually, now I do understand Funcrusher - and he's right - and my calc results agree with him! So fostertom's not refuted! I think.

You said:
WE KNOW: t1>t2>t3>t4 - I agree
(t1-t2) < (t2-t3) < (t3-t4) - I agree
as you proceed through the layers from the hot side, not only does the temperature decrease, the temperature fall accelerates - I agree
but not by much -

"despite the proper application of fourth-power, [T(n+1) - Tn] comes out at 3K +/- 10% for all foil pairs. That *is* "near enough" linear, as a useful approximation within the limited range 272K to 293K.

Still, never mind that, the point is that the foils' steady-state temps are "near enough" linearly (evenly) spaced, *not* as Biff, Martian et al have been saying, all bunched up at the hot side, diminishing "near exponentially" (funcrusher) to insignificant for foils 2-6."

Dont do it. Foil doesn't work! You have the room, stuff in loads of fiberglass!
No theory or tests, just used the stuff on half the house and guess what, its the cold half!

This is a pain in the ass! I would really love this to work. I don't follow all the maths yet - can we just do an independent test? how hard would that be? I'm game

Posted By: joe90have a look at this:-

http://www.foam-insulation.co.uk/insulation/multi-foil-insulation.htm" >http://www.foam-insulation.co.uk/insulation/multi-foil-insulation.htm

talk about the pot calling the kettle black!

Sorry to bring the level of the discussion briefy downwards for the benefit of someone outside the trade.....
I have tried to absorb some of this very long thread whilst looking for references to super-thin insulating material (2-3mm?) that would be suitable for the internal faces of our external walls. We live in an 1850s house with solid walls approx 250mm or more thick, good windows and well-insulated loft. But is there anything cost effective I can put on the internal (plastered) faces of the external walls to keep the heating bills down? The approx 25mm thick internal cladding one generally sees advertised would mean real problems with skirting boards and ceiling cornices. Any help or suggestions here would be really appreciated...

Unfortunately not no. If you want to address heating bills I'd look at other avenues first. You need to insulate your roof as much as possible and look at windows and doors. You want at least double glazed units all round, with really good draught proofing on all doors and windows, internal aswell. then you need to look at the heating systems that you use. Most of this is going to cost a bit to sort out, but long term savings will become apparent.

Grants area available for some of this, so contact your local council. You can also visit these guys

http://www.sustainable-energyacademy.org.uk/

They have show homes you can visit in most areas to get ideas and support, I'd recommend it anyhow...

HI Duncan,

Can I get a bit more info? what is your roof covering? what sort of felt do you have, are there any windows?

You do have a couple of options - you are correct about the ventilation gap being important but you can work around this quite effectively. You already have some insulation in the floor so you have what is called a 'cold roof'. what you are intending to do is add a 'warm roof' style insulating layer at the roof pitch. I would recommend using 50mm insulation board between the rafters, leaving your 50mm air gap on the external face and then finishing the interior with insulated plasterboard. This means you will only lose about 20/50mm on the internal face of your rafters. If you are going to board off the eaves you can put extra loose insulation in there too.

If you don't have windows I would definitely recommend installing a couple of velux - as well as providing natural light you will also have good ventilation in the summer, which you will need. If you don't want to/can't do this then get a roofer to stick some vents in so you get some air movement.

Drop me an email if you want and I'll be happy to help

On a micro scale, what if the 'scrim' surface consists of lots of little fibre ends that make v small (negligible) point contact with the foil? Would that be conducive to conduction(!) or would heat have to transfer from the foil to the solid fibres mainly by radiation and a little convection?

<blockquote><cite>Posted By: betterroof</cite>HI Duncan,

Can I get a bit more info? what is your roof covering? what sort of felt do you have, are there any windows?

You do have a couple of options - you are correct about the ventilation gap being important but you can work around this quite effectively. You already have some insulation in the floor so you have what is called a 'cold roof'. what you are intending to do is add a 'warm roof' style insulating layer at the roof pitch. I would recommend using 50mm insulation board between the rafters, leaving your 50mm air gap on the external face and then finishing the interior with insulated plasterboard. This means you will only lose about 20/50mm on the internal face of your rafters. If you are going to board off the eaves you can put extra loose insulation in there too.

If you don't have windows I would definitely recommend installing a couple of velux - as well as providing natural light you will also have good ventilation in the summer, which you will need. If you don't want to/can't do this then get a roofer to stick some vents in so you get some air movement.

Drop me an email if you want and I'll be happy to help<img title="" start="fileopen" height="15" alt="" isMap="false" hspace="0" loop="1" src="http://www.greenbuildingforum.co.uk/forum114/extensions/Vanillacons/smilies/standard/bigsmile.gif" width="15" vspace="0"></img></blockquote>

Joe - thanks for the input - I have sent you an email but just in case please see below:-

The property is a semi detached brick built 3 bed semi. It has a newish roof (replaced approx 5 years ago)



Roof construction is pitched with roofing felt and Marley tile over. Currently no windows in roof and we don’t really want any! Eaves are extremely well ventilated on all 3 elevations. This is achieved by an air gap between wall plate and felt. Being an older property this gap varies between 10mm and up to 65mm! – hence I have recently evicted a pile of sparrows (due to non payment of rent/wild parties etc) – Gaps now sealed with wire mesh to maintain ventilation.



My understanding is that given the current high airflow I would not require any vents in the roof or ridgeline…



My ideal solution would indeed be to

1/ create a warmer loft less prone to extreme heat in summer (?lol!) and extreme cold in winter

2/ increase the heat retention for the house

3/ maintain the current ventilation but restrict it to behind the insulation up the roof and over the top and down the other side – if you understand me

4/ not further reduce the loft space too much – as I said long term I would like a model railway up there and currently the useable bit is only about 3m x 3m (not as big as I would like!!!)



You mentioned insulating the 50mm and then overboarding with insulated plaster board. – I take it you are not impressed with multifoil either!!!



What material would you suggest for the insulation? How would I maintain the airgap if it is a flexible material?

Thanks

Duncan

I doubt it :)

That article started out at http://www.building.co.uk/story.asp?sectioncode=748&storycode=3143234 but it doesn't seem to be on the DCLG website yet.

There's only one true multifoil thread. The other is a young upstart, a mere imposter.

Surely it will cause condensation if you put a multifoil on the underside of the rafters (or PIR boards between the rafters) with glasswool at ceiling level.

You would be putting a vapour impermable layer across the cold side of the wool, just as you would have had with the old felt, but that had ventilation underneath it.

You should use permeable stuff like wool. If you want to use something on the rafters then again something permeable, TLX Gold is the only one I've seen so far, so that water vapour can still pass through into your ventilated rafter space.

The point is, everyone including the manufacturers knows that multifoils give pathetic results under BR443 methodology, which deliberately substitutes artificial steady-state conditions for the real-world dynamically-varying conditions under which multifoils do work.

Any multifoil manufacturer that claims good hotbox-derived performance for multifoils is puffing.

Having said that, very special testbed-special multifoils have been produced, that are custom-designed to do well when hotbox tested to BR443, and hang the real-world performance, just to prove the point that this is how conventional insulations are designed - but these are far from commercial, tho may be a pointer to the future.

In respose to http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4283&page=1#Item_5

Quiet! Moi!

I just didn't think your answer made sense and was too polite to say so.