I see a lot of lofts that have been insulated but not very carefully often leaving gaps and missing bits

This causes a disproportional heat loss through the uninsulated area

If the loft has 200mm of quilt and 5% of it was missing what would the difference between the heat lost compared to a properly done loft?

Posted By: tonyThis causes a disproportional heat loss through the uninsulated area

or

Posted By: SteamyTeaIts a weighted average

James

As it can be easily calculated it will be proportional to the differences, the U-values will not change, nor will the uninsulated percentage. The only differences will be in the temperatures and these can be assumed to be equal for both (unless there is some reason why not).


Edit:
Take a ceiling that is 10m by 10.5m with the U-Values of 1 and 2 and a temperature difference of 10C.

the first 10m is insulated at U=1 and the 0.5m (the 5%) is at U=2

Then:
10(m)*10(m)*1(U)*10(C)=1000

10(m)*0.5(m)*2(U)*10(C)=100

Or

10(m)*10(m)*1(U)*20(C)=2000

10(m)*0.5(m)*2(U)*20(C)=200

Conversely:

10(m)*10(m)*0.1(U)*10(C)=100

10(m)*0.5(m)*0.2(U)*10(C)=10

Or

10(m)*10(m)*0.1(U)*20(C)=200

10(m)*0.5(m)*0.2(U)*20(C)=20

Or was the question is the heat loss proportional to the percentage uninsulated?

Disproportionate then

disproportinate in the eyes of the person paying the energy bills.

Killa wot hours? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5434&page=2#Item_30

kWh

the above heat losses are in Watts (W) multiply by time to get totals of energy lost .

Hi, ah ha one I could get opened! There are three variables the %coverage m2, the thickness of each (hence the R value) and the temp difference.

Assuming a same temp difference then there is a simple relationship between the areas and the respective R values of the covered/uncovered area (high/low R). So for example for a given area if you have an area very highly insulated U=0.1 R=10) and an area very poorly insulated (U=10 R-1) then the affect is more pronounced than if the well insulated area were say 0.3 (R=3.33) and the poorly insulated area was say U=3 (R=0.33).
Therefore, the greater the relative difference between insulated/un-insulated the more acute the affect is.
Thus, depending on the actual levels of insulation the affect of the various area combinations 90/10%, 85/15% etc will alter. Not always easy to visualise these in ones head, so best to insulate as fully as possible and ignore the “a few bits missing doesn’t matter brigade�

Cheers

Mike up North