(This is a re-post from 2 years ago but I felt it was timely and so here it is again!)
Ah the snow ~ There is nothing I geek out on more the day after a snow storm than driving around looking at other people's roofs. Last night we got 22" of snow in my home town. The way snow melts off of a house roof tells the story of the roof's insulation inside.Different amounts of snow tell the story in a different way. Heavy snow will tell the story more in icicles and what I call snow slumps-large swathes of snow falling down a roof.
Why do icicles happen? Well really what is going on are called Ice Damns. Ice damns happen when heat from inside the house flows out of the house- mostly through the roof (because heat rises). When the heat flows out through the roof (because there is not enough insulation) it melts the snow that is touching the roof. That melted snow turns to water and runs down the roof until it gets to the gutter/ eave and then it starts to drip down. However since the air is very cold (less than freezing) the water turns to ice and as more water comes dripping down the icicles grow longer.
What's bad about this is that at the gutter line a LOT of ice is forming! And behind that ice the dripping down water that has not frozen yet is creeping into and behind your roof shingles, sometimes popping them up and often penetrating through to the interior of your house and then causing water damage in the ceiling near the roof eaves. Bad!
The way this problem shows after a light snow fall is with stripes on a roof. Look at these 3 pictures. All 3 of these houses are neighbors, they face the same direction. ( and yes, sun and wind can often throw off the results!) They quickly reveal the levels of insulation in each house.
House #1 has very poor insulation. You can see the white stripes at each roof rafter. There is likely no insulation between the rafters which is why you can see the black asphalt shingles showing. The heat from the house has poured through and melted all of the snow off the roof. Where the rafters are, the heat gets though more slowly and thus there is still a bit of snow clinging there- un-melted. You can also see at the over hangs and the porch roof below there is still all snow in tact. That is because there is no heat flowing through there- just cold outside air.
House # 2 is less 'bad' but you can see the same patterns emerging. This house may be moderately insulated in the floor of the attic/ above the living space.
By the way if you ever see the opposite- broad white stripes and thin black ones where the rafters are- that means there is some decent insulation in the rafters- probably batt insulation- but it pulls away from the rafters, or is not well installed, so it is letting heat sneak out right along the rafter lines.
The third house (below) was recently built and I watched it go up and I happen to know that it was very well insulated with blow in insulation and sheathed with the Zip System which provides a great air infiltration barrier which inturn makes insulation perform to its highest potential. Good job! All of the snow is in tact! No heat is leaking through!
Here is an example of the detail for a decently insulated house:
Have a look at your roof today and the next few days as your roof melts! Do you need more insulation?
BTW: simply insulating your roof can cut hundreds and hundreds of dollars off of your heating and cooling bill per year. Payback times for this kind of work are usually less than 2 years! Why not !!??
Happy sledding ~ Elizabeth