A combination of a poorly sealed attic floor and/or inadequate insulation heats the roof deck and melts snow, or eliminates frost.

Even different units in a row housing behave differently.

Here's a commercial site that illustrates and explains what's going on.

I bet there are still people out there who grow all warm and nostalgic when they recall their childhood homes covered in wintry icicles.

Christina, if everyone were as observant as you, our homes would cost us all less in energy while keeping us more comfortable. I believe you pretty much nailed it. Frost and light snow often creates these conditions where we can easily witness the inefficiencies between homes.

Melted snow or frost can be from the thermal bridging of framing or structural members but it can also show areas of generally warmer air or inefficiencies in the attic air-sealing or insulation.

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These were taken minutes apart on my street with simalar orientations. The first shows a SIP roof with thermal bridging of the 2x6 connections between panels. Not a good thing, but much better than the results a few homes down, showing a poorly weatherized vented attic. Air leakage and thin insulation, resulting in melting snow above the inside floor space but also the outdoor attic space above the porch.

These same characteristics are making it more expensive/uncomfortable in the summer. If those air leaks are combined with humid air, bad things happen behind the finished surfaces.

The same thing is going on with walls and part of the reasons for the newer code required best practices of insulative sheathing.

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Thanks so much for the explanations. That helps me understand the heat loss a little better. Brian, those pictures where exactly what I saw on those roofs. I must admit I am a little dissapointed to see that is still a possibility with a SIPs roof. Thank you for those examples, I'm always impressed with all the knowledge shared on here!

Wow, Brian, dramatic examples! A picture (and a short caption) is worth a thousand words.

I'm definitely going to be checking my roof during the next frost. Thanks for the good info, I would never of thought to look at the frost on my roof to see how well our insulation is working.

However, I will never look at icicles the same way again!

Glad yall liked it! Not all frosts or winter weather create this "snow ghosting" effect and I think they show up more with warmer conditions typically on northerly facing roofs. I also think that asphalt shingles are the best finish roof material for this interesting form of nature's thermal imaging.

I wouldnt be too discouraged by the SIPS thermal bridging if its an option that youre considering. Most panels do a better job in this regard. Many versions almost completely eliminate thermal bridging in the field or uninterrupted areas of panel. For cape styles, timberframe or roofs with a full vault, SIPS are still my favorite application for roofs that arent too complicated.

For walls of typical stick framing that do not have insulated sheathing, thermal bridging reduction factors are often calculated at 25% of total wall area. That's a lot of thermal bridging! Most SIPS have around 8% and we often use a company claiming only 3-5%.

For those building new or looking to improve the weatherization of their existing homes, hiring a home performance contractor, or third party energy rater/certifier can help guide design, construction or renovations. Many use thermal imaging cameras, sometimes combined with blower door testing to better find and help fix the gaping energy holes.

What is really fun is to find the ones with obvious pot lights in their upper floor ceiling. Little round frostless circles all over their roof.

I always look at roofs when I am driving around after a snow- it's a true education to see which homes are well insulated and which are not.
You can spot new additions and porch conversions pretty easily.

I am happy to report that my metal roof is covered in snow right now except for a small area directly adjacent to my chimney, under which burns a fire of greatest intensity.

Wow almost 25% loss on a stick frame. Brian, what is the SIPs manufacturer that you use that is 3-5%? In our area the nearest distributor/builder is almost 3 hours away so we might not have a huge choice in the matter if we went with SIPS. I would be interested to see that same thermal imaging on a ICF. And thank y'all for making me feel like I'm not the only odd person for looking at houses and frost!

The math can get tricky and that high % is mainly associated with walls not roofs or foundations. Those elements have their own problems that are more associated with air-leakage, the biggest energy performance killer to be aware of.

Even that 25% number could be small potatoes compared to the air leakage coming through the window or between the window unit and rough framing. If you think the can foam that people use in this location is effectively stopping all air-leakage.. think again.

Eco-panels is our most local manufacturer. They are one of a few SIP companies to use Polyurethane foam. Not only does this foam have one of the highest r-values per inch, they inject it between the OSB sheathings and this process lends itself to a different attachment system which reduces thermal bridging. One of my favorite things about them though is that they are available with the huber zip exterior sheathing upgrade which makes it easier to tape the exterior seams. Did I mention air-sealing is more important than thermal bridging?

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3 Hrs away is actually pretty close for a manufacturer. You dont use the same criteria for windows do you? Eco-panels ships all over the world and interestingly, the owner told me about a project they are working on for an aid organization in N Korea! The bigger problem is how close is the experienced installer? Builder comfort is important for these systems too.

Iam not really a fan of ICFs, mainly because the pre-cast concrete panel company in our area eats them for lunch in almost all criteria. For energy performance, most of them dont meet 2012 IECC minimums for cold climates. A few do and its the exterior foam thickness that's of most importance. The interior foam does not seem to contribute much to its performance but worst of all, it blocks easy access to all that expensive thermal mass of the concrete!

Here is a link that might be useful: Eco-Panels polyurethane SIPS