Answer is to look at the area covered and start doing sums.
You can assess the heat loss from average inside/outside temps..
If the roof is say 30 sq meters, then heatloss at .15 a compared with .25 goes from 7.5w per degree C to 4.5W per degree C.
At an overall - say - 10C average temp differential, that's a saving of
30W, or, over a year 262KWh...at at least 4p per Kwh on any energy, thats about £11 a year :-)
Is the loft vented? Are the eves vents left intact?
How does this translate to energy saved/used.
The loft is habitable space and ventilated accordingly.
Above the insulation, from front to rear, there is soffit ventilation, 3 ridge vents supported by cross ventilation at the apex and then ventilation at the rear of the dormer to the soffit. There is at least 5cm to the ventilation channels. In addition, the sarking is breathable membrane.
Christian.
Well, the rafters had been insulated with a miserly 25mm of EPS. This had significantly worse performance as it used to be freezing cold up there. Clearly the Super 9 is doing something, although I have no figures to make a proper comparison.
Christian.
So you installed breathable membrane under the tiles and have eves ventilation that vents up the rafters following the roof slant to the ridge tiles. I read that only breathable membrane is sufficient as long as the is an air gap of 25mm; and no eves and ridge ventilation is required. The gaps between the tiles is enough to take away the water vapour.
Indeed. The ventilation provided is very belt and braces. Personally, I wouldn't have bothered with most of the vents and just relied on the breathable membrane. However, I'm certainly not expecting the wood to rot away any time soon!
Christian.
It has been proven that the breathable membrane is all that is needed, but Building Control sill insist on an air flow so the eves ventilation and ridge tiles have to go in. All they are doing is over cooling the house in many cases.
If they just accepted the membrane all could be done from inside the loft. Seal up the eves ventilation gaps into the loft. Rip out existing sarking with a Stanley knife. Install membrane between rafters stapling and using tape where needed,. Leave an air gap between underside of tiles and membrane. Another air gap on the loft side of the membrane and then insulation and plasterboard. water vapour working its way through the plasterboard and insulation will work its way through the membrane and out into the outside air via the gaps in the tiles.
That will work, but I doubt they would allow you to do it. Or will they?
Well Kingspan say it is OK even without any air gap at all. I don't know if it was the builder or the BCO who insisted on the vents.
Christian.
BCO, I'm sure. They like an uninterrupted air path. The cross batons that the tiles are hung off blocks the flow. The makers of the membranes say it is not necessary as the water vapour will just float out of the membrane and through the gaps in the tiles.
Our BCO would have been happy with a breathable membrane and no additional ventilation.
Personally I would not be happy with replacement sarking installed like that, since it will defeat one of the benefits of having it in the first place - i.e. as a secondary weatherproofing layer under the tiles.
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