worried about insulation...

jasperok

Ok so i have a few questions about insulation and i'd be really grateful for some pointers. I am half way through a self build which has blockwork finished and roofed and slated.
there is a downstairs and an upstairs (one very big open area) with the roof being the ceiling upstairs. I am very concerned about getting the insulation right on the roof as it will really have a huge effect on the overall temp of the house.

1) we have a 150mm cavity for the block work - i was going to use warmfill but have read that rockwool can give better performance and wondering are there any companies that blow rockwool into cavities or is it not possible to get in that form?

2) the roof - (we did use breathable felt), I was thinking something like the vc foil crowd reccomend (but toe honest its the only thing i have found with a decent picture so maybe its juest a method i find it easy to understand!! as opposed to being a good method. Any suggestions on how to improve on this method or an alternative would be great. i have asked a local insulation business and they recomend acoustic insulation not rockwool and a foil but no mention of insulation accross the rafters only the insulation between?

ircoha

cant comment on wall issue:
re roof see http://www.rockwool.co.uk/graphics/RW-GB-implementation/datasheets/Rockfall.pdf for another sketch.

The sketch shows the cross rafter insulation added outside, but u can do the same inside.

ardara1

The picture/calc you show shows 90 PIR between the rafters and another 50mm below the rafters. WIthout any other magic materials/pixir dust it will achieve 0.17 - what is the VC stuff - What is the gain - your PIR foam has foil on it any way.
What htese nuggers have done in th egraphic is to include resistances abive nad below the PIR layers - do the dsame thing with the PIR and you get the same result - but remember - with all breather felts you MUST have a VCL to the under side.

I just can't see how this VC material benefits things?

jasperok

http://www.glidevale.com/downloads/VC%20Foil%204pp%20.pdf

rhats the spec sheet, Its just a VCL isnt it?

Mellor

ardara1 wrote: »

I just can't see how this VC material benefits things?

I can't open the spec sheet, but my reading of it was that it is a low E layer as well as a VCL. The low E characteristics improve the surface emissitivity/resistance

ardara1

Mellor wrote: »

I can't open the spec sheet, but my reading of it was that it is a low E layer as well as a VCL. The low E characteristics improve the surface emissitivity/resistance

But the PIR board has a foil facing on it already - which already gives you the impoved cavity R - adding another foil to the other side does not improve the Resistance of the cavity to any noticeable extent

250882

The foil isnt for insulation benefit, its a vapour check

Mellor

ardara1 wrote: »

But the PIR board has a foil facing on it already - which already gives you the impoved cavity R - adding another foil to the other side does not improve the Resistance of the cavity to any noticeable extent

I think the way they are looking at it is that the low E VCL in the a second cavity, giving another air layer resistance.

250882 wrote: »

The foil isnt for insulation benefit, its a vapour check

Its actually both

sydthebeat

Can i ask the opinions of others here regarding this type of construction, as it seems to be getting very popular.

It appears that a 'vapour permeable underlay' is incorporated below the roof finish.
Why use a breathable membrane here when the construction under cant breath?

This construction allows moisture into the structural components of the roof, namely the joists. These joists are surrounded in non-breathable, basically waterproof material, so all the moisture (which would be formed on top of the foil faced insulation) is directed into these timber joists. There is also the real threat that the 'vented' air cavity over isnt sufficient to evaporate this moisture.

I know PU insulation has been accepted by IAB to be used in this manner, but i still have my doubts.

Would it not be a much better construction to keep the joists totally within the 'warm roof'... ie batten over and incorporate insulation over and between, rather than under and between.

jasperok

so would this way of doing things be ok? or would rockwool be better (or much beter for the price)
From using the search function I see that rule of thumb seems to be:

as much as you can between the rafters providing you leave the 50mm airgap between the breathable felt and insulation.

then when your insulation is flush with the rafters you for want of a better word slab accross the rafters with another insulation creating like a wall of insulation.

then onto that wall you screw in lats of timber so you can slab your foil backed plaster board on to that.

I am a little hazy on what should be foil and what shouldnt and what products to use - Is it not a case of one thing is scientificly beter thsn the other rather some people think this and some people think that?