loft insulation effectiveness??

nicke20

I have a very very difficult loft to insulate, none of the companies will go near it, access is horrible, working height is a max of 3 feet in the good bits!
anyway, I am insulating as much as i can to a depth of 300mmm with knauff eco rolls but the edges of my loft dissapear down just under the roof line as the edges of the cielings are vaulted if you know what i mean, there are also some very difficult to get at bits
So my question is :- Is it pointless only covering say 85% off the loft? will the heat just escape through the none insulated parts or will 85% coverage be 85% effective??

MX5huggy

Your right to be thinking about this. 85% coverage will not be 85% effective.

The 300mm has a U-Value of 0.13 uninsulated loft has a value of 2.3.

If you have 60m sq of loft, 50 insulated and 10 not. Then the average U-value is (50x0.13 + 10x2.3)/60 = 0.50 the same as 75mm over the whole lot.

The good news is that just fitting a little to the difficult to reach areas really improves things. If you can get 50mm (U-value = 0.68) this gives (50x0.13 + 10x0.68)/60 = 0.22 just worse than 200mm all over.

Because of this you must insulate the back of your loft hatch as well, the standard way of doing this is to staple gun the plastic wrap from the insulation to the loft hatch to hold a pillow of insulation.

nicke20

If thats the case I noted that knauff had a loose fill product in the national diy chain at £17 a bag (condensed) that did 4sqm at100mm ill use that to try to backfill all the bits i cant get at
did your calculation assume the ceilings/loft is level?as my lofts dip either side following the underside of the tiles so that there is a small gap say 100mm between the top of the lath and plaster ceiling and the underside of the roofing slates all at say 45 degrees these gaps plunge to a depth of about 1.3 meters
I have noticed that the walls render tight to the roof with no air gaps but as the roof is slate on batton with no felt and little lime mortar loft ventilation is not a problem!!

colalba

I had almost exactly the same shape of loft space. In the hard to reach sloping bits I used knauf building slab to insulate. These were stiff enough to cut to size and push down the slope but flexible enough to friction fit.


Rightly or wrongly I didn't want to use a loose fill product as I wanted to maintain an air gap to allow some ventilation at all parts of the roof space.


I had the same issue with working height and in my case the gap between every joist was different. So it was very time consuming but I reckoned I'd only be doing it once.


http://www.knaufinsulation.co.uk/en-gb/products/rock-mineral-wool-slabs/earthwool-building-slab-(formerly-known-as-universal-slab).aspx#axzz3ERuaijv7


It comes in different densities. I used R45.