Our roof is pretty good

The snow melts round the chimney first - it definitely stays on my roof longer than on next doors (which is still council) & accorss the road - which is a mix of private/council but the roofs are east west & mine is north south

My south slope melts before my north one - but I don't worry about that

Congratulations on that honour.

Since the first slope has insulation at ceiling level then the temperature in the void above will be closer to that of the outside. Hence slow melting of the snow

I don't understand the construction in the second roof, Is there a void between the warmcell and the PUR between the rafters?Is it ventilated?

It semms the PUR may be soaking up the heat energy from the slates on sunny days and keeping your slates nice and warm.

This may put a sprog in the theory that this is a reliable method for comparing heat loss.

Mike,
the PUR is installed to give a 50mm air gap/void between it and the felt/typar. The idea was to allow blowing in of warmcell after the false ceiling was installed, the PUR stopping it reach the felt, allowing ventilation to the underside of the felt. I argued with the builders about the 50mm air gap ! Does it really need that much ? I agree that the felt can sag, but surly 25mm would be enough, allowing more room for insulant. The warmcell guys used a hole saw and cut dozens of holes, they blew it in starting at the lowest row of holes, blanked them off, then moved up. seems to work a treat. Sorry if that's a long winded answer, I lack the building terminology required to explain it better, a drawing would be easier, but how do you put an acad drawing into a comment? or for that matter a sketch?

Can PUR store heat ? Looking at other threads on this site the, consensus would seem suggest that internal mass is the way to store heat, if PUR can do it short term (24 hour period say) then....

I'm sure your right that the slates will get hotter on a sunny day.. interesting point.

tom

Tom, all materials 'store' heat to some degree. It is called Specific Heat Capacity and is related to a materials 'mass' Not a bad definition here http://en.wikipedia.org/wiki/Specific_heat

Heavy materials need more energy input to raise therir temperature and this is why some promote heavyweight buildings, arguing that this ability to store heat will smooth out air temperature extremes.

So can PUR store energy - Yes it can, though not anywhere near the capacity of say concrete.

What you also have to remember is that convective heat loss is not instantanious, If the temperatutre inside is 20C and -5C outside then you have a 25degree temperature gradient accross your roof construction. When the sun is on the roof, this temperature gradient can be partially reversed with heat energy moving both ways until equilibrium is reached.

Living in a ground floor ex council maisonette in a city centre where it didn't (and rarely does these days), I can safely say that I have no snow on my roof; owing to the absence of both snow and roof....



J

Show off...

J

first october snow in london since 1934 apparently.

no doubt three flakes caused gridlock for the shandy drinkers...!

This is a classic all the heat going up the cavity melting the snow round the edge of the roof.

I noticed in my street the worst houses where those with new loft conversions, all done in the past 6 years so much for new regs.
(other than mine, which I was pleased to see held out til the rain cometh, I do have the party wall problem and a couple of small section just
eitherside below the veluxs where the double rafters meet the purlin wall) .

Hmm, loft conversion have a greater tempertaure difference across the roof structure because the space is heated. Therefore much greater heat flow by conduction than say a cold roof, which will ventilate the heat away through air losses at eaves and/or ridge rather than conduction through the fabric.So I don't think the level of snow melting is an infalible method.

I agree with tony's examples though.

in my street there 1950s unfelted roofs ,so plenty of ventilation so that makes sense
with the loft conversions, perhap its just orientation ,mine slightly SE whilst the others are S facing
there I was thinking it was my attention to detail , hells teeth
interesting the house that lasted the longest , got single glazing , I guess all the heat went out that way?
cheers Jim

And another one that looks like all the heat is going up the cavity again? Could a chimney effect draw lots more heat than we realise up cavity walls?

i do have a melted spot over the new HW cylinder - see other thread on how to resolve the matter!

Two loft conversions next door to each other with reference semis.