Why Well Insulated Houses Get Ice Dams and Icicles

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In my last post, I dove into my house and the few icicles it gets.

Now for a little advanced discussion.

I have a detached garage:

My detached garage got icicles last winter - why?

The garage has no ventilation at all, so the heat from the cars while they cool after driving is enough to melt some snow:

Icicles and ice dams starting to form on the detached garage of my 1835 home

It’s a little tough to see because I’m too short to take a great picture, but the little indentations in the snow have ice in them. There is no heat or insulation in the garage, so this shouldn’t happen, in theory. 

Why is my garage getting ice dams?

My hypothesis for this ice on the garage is also why I think well insulated houses had issues in this ‘storm’.

A ‘storm’, by the way, is my term for a period where it snows and stays below freezing. It could be a few hours, or it could be months like the winter of 2010-2011 that gave plenty of people 2 story icicles and made gutter guys happy replacing gutters that got ripped off.

When the temperature went into the mid-thirties for a few hours on a few days combined with the fact that it was just below freezing for most of that storm, the houses with big problems dropped the biggest icicles off.

It was weird seeing my house with ice and then places in town that I know have nasty problems having no ice at all. Those houses get ice even when it’s in the low twenties because so much heat escapes into the attic they can get a 20 degree attic above freezing.

Newer houses that are well insulated, or well-insulated older ones, only get ice when the outdoor temperature is close to freezing, say 28-32 degrees.

The latest weather has been closer to deep freeze, so my house and garage haven’t showed problems.

Buildings with little heat leakage are affected by 3 other factors:

1. Heat from cooling cars, or the bit of heat that escapes through well insulated and air sealed attics.

Remember, insulation does not stop heat flow, it just slows it down, a lot like sunscreen. SPF 50 is strong, but if you stay out long enough while wearing it, you’ll still get a tan (or burned). Same thing with R-value, it just slows down the heat.

2. Snow on the roof was insulating the heat in.

Snow has an R-value of about 1 per inch. This 6-7″ of snow represents between R-5 and R-8. Just enough to slow the heat down and melt the snow. Essentially, the snow commits suicide. It unfortunately does not respond to counseling, either.

3. Ventilation can be more critical.

The garage has no ventilation at all. It doesn’t allow any cool air to wash the roof deck and keep the snow from melting. If the air could circulate, the temperature in the garage would not climb above freezing under most conditions.

The fact that the garage has no ventilation also causes moisture to condense on the inside of the garage windows. It turns to frost in very cold weather. A similar situation will happen in a poorly ventilated, well insulated and air sealed attic. Any moisture that gets into the attic is trapped and will likely cause moisture on the roof deck, then mold.

This lack of ventilation in the garage is another thing on my house that will get fixed this summer. Once the garage stays cold because it is well ventilated, no ice and no condensation. The same goes for your attic.

So there you go, that’s why well insulated houses still get icicles and ice dams sometimes. The conditions have to be just right, but it can happen. Those conditions are:

1. Temperature close to freezing.

2. Lots of snow on the roof insulating the attic.

3. Poor attic ventilation.

Those 3 factors will lead to some icicles and ice damming even on structures that are well insulated and air sealed.

BONUS! South facing sections will allow the sun to warm up the peak of the roof, melting snow and causing more ice in the gutter. South facing sides often have worse ice problems than the other sides.

Thirsty for more info? You info glutton, you. (I can call you that, because I’m one too.) Check out the following resources:

Related Articles:

Why do I get icicles and ice dams, and how do I solve the problem? 

Icicles and ice dams on my own house. Newsflash! I’m not perfect either.

Energy Smart Page on Icicles and Ice Dam Prevention 

Joe Lstiburek, patron saint of Building Science, wrote possibly the definitive article on icicles and ice dams –  ‘Dam Ice Dams’ – a great advanced, yet very readable, guide to ice dams and their causes

Mike Rogers’ writings on ice dams and icicles – he cut his teeth in Syracuse, they get a TON of snow there! 

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