Roof Melt can tell you a lot

It’s the most wonderful time of the year.  The season of giving outward information about your thermal resistance.  Roof Melt season.  So what is roof melt season.  That is the time of year when we get light snow fall and the opportunity to visualize surface temperatures. 

What am I talking about.  Well, Insulation is the resistance to thermal energy transmission. Think about that practically. How does heat sneak out of our houses? It escapes the exterior surfaces of our house.  How do you tell that heat is escaping an exterior surface? It is warm. Or at least warmer than the surfaces around it. So you could use an infrared thermal imaging camera to take a picture of the surface temperatures, but those are very expensive and you get pictures like this.

Snow 1.jpg

So what do you learn from this? The chimney is warm. Windows are warmer than the wall, or specifically the siding.  Knowing the surface temperatures is great, but that alone is not enough to calculate the effective R-value.  It does give us an impression of where the greater heat losses are.  So does roof melt.

Now for the cost of going out and looking, you can learn about the insulation effectiveness of your roof.  Snow on a roof is an indication of surface temperature. Snow is most common between 20°F and 34°F.  Since we are only dealing with a few degrees between frozen and melting, the differences show up quickly.

This roof has 3 different stages of melt.  There are 3 things to learn from this roof.  The roof was clean before this snow. The snow fall was about 2 inches. 

The lower portion of the roof is un insulated.  The area physically under the roof is not heated but the ceiling underneath is heated.  That is melting some of the snow off the roof.

The next section of roof is probably a sloped ceiling that is insulated. None of the snow melted because the roof is the coldest in this range.

The top of the roof has the most melt.  This means that the roof is the warmest.  This would indicate a change in the ceiling to roof construction.  Given the window on the end of house, I would assume that there is a small portion of flat roof that connects to the dormer that extends out the other side. My guess is that the insulation is not on the ceiling.  If the insulation is against the roof and not air sealed from heated space the air will churn between the roof and the ceiling. Insulation is the most difficult and important at the transitions in construction.   

This roof looks good except for 2 specific spots.  The first spot is the where the lower parallel roof joins the main roof.

The spot outlined in orange is the warmest spot on the roof.  The heat is either coming from the ceiling transition or the end wall between the main house and the addition.

There is a small spot in the middle of the roof.  The 2 suspects for that is the alignment with the perpendicular peak of the lower roof.  Warm air pools at the top of a compartment, so any leaks will fill the top of the framing first.   There could be another source on the other side of the roof or just from the compartment below.  Chimneys are commonly located in the center of these houses.  A chimney could be heating this small portion and just not visible in this picture..

This roof has a few obvious issues. The roof compartments connected to the lower roof have the most melt.  There is a ling across the roof of isolated melt. The melt appears to be where the wall or ceiling of the interior meets the roof structure.  Also the framing is showing up in the snow, which means that the framing is colder.  The cold means that the framing is colder than the bays.  This means the bays are uninsulated or the insulation is compromised.

This roof have melt on the lower roof. This means that the insulation is well installed in the most of the upper portion of the roof.

This specific melt is due to leaks from the walls.  The heat leaving the wall hits the roof and melts the snow with an isolated warm spot.  This is commonly due to poor sealing of the ceiling to the wall or penetrations in the walls, like electrical wiring.  Changes, like framing where walls meet or around windows, can cause paths of heat to escape.  As with all snow melt, it is also an indication that the roof ventilation is insufficient to maintain outdoor temperature on both sides of the roof decking.

There is a quick sermon on using roof melt to diagnose you roof insulation situation.  Often fixing these is much more expensive on their own.  Noticing these problems should help give you ideas of what to pay attention to you when you are doing projects in these areas. Fixing the air sealing or insulation while drywall is removed is fairly low cost and low time commitment. Say a few hours and maybe $100 in material.  Always talk to a professional to get some “local” answers. A perfect wall in Alaska can be a nightmare in Arizona. 

I am Mick Lane and thanks for giving a watt.