Frost in Attics: why is there Ice in my roof? 

When a warm sunny day arrives after a long cold stretch and it seems like you have a roof leak when there isn't a cloud in the sky, it's probably Attic Icing, otherwise known as Ice Dams or Ice Damming, and the surprising thing is that it's very unlikely that the roof covering itself is the problem.

Frost and ice forms in attics when warm, humid air accumulates and condenses on the underside of your roof sheathing. Depending on the temperature, and the insolation (ie. can sun get to the roof) you will have either frost or water collecting.  A little bit of frost forming is not unusual after really low nighttime temperatures, and it isn't a huge problem if it is able to melt and evaporate so that the wood can dry.

However, it becomes a more serious problem when the quantity of frost isn't able to evaporate before more starts to accumulate. The longer this goes on, the greater volume of ice you will have building up and the greater volume of water you may have leaking down into the ceiling and wall when it finally warms up enough to melt.

If you're further baffled by frost forming on one side and not the others, you're probably looking at the north side. The sun will sometimes warm the south side enough to thaw and evaporate moisture but leave it still coated on the north side - again, not a problem unless the ice starts to build up.

Many homes in the northern parts of the US and Canada suffer from ice accumulation in attics, but homeowners often ignore it unless it causes a huge problem - mostly because they aren't aware it's happening until there's an issue - a lot like the ice monster that forms in the top of the freezer if the seal is defective - ice dams are caused by warm and relatively humid air leaking from inside a home into an insufficiently ventilated roof space, condensing, and forming ice. 

What causes ice in attics?

Attic ice problems happen when there's a combination of 3 issues with a home's building envelope - and the thing that most often alerts home owners that a problem exists is a wet patch in the drywall on the edge of ceilings or the top of walls during the spring thaw. Most homeowners assume they have a roof problem like a leak, that maybe their roof covering has been damaged over winter, but it probably isn't if the problem wasn't there before. Ironically, in our experience, ice often forms in the attic after DIY roof insulation projects where new insulation covers up ventilation access. If you discover you have ice in the attic, the 3 things to check first are:

  1. Humidity levels in the Home - do you have high humidity and condensation problems?
  2. Attic ventilation - Is your attic ventilated correctly and sufficiently?
  3. Air leakage from the ceiling - Is the ceiling vapor barrier intact and doing its job?

There are 3 different issues to check for and to deal with to fix the problem of ice buildup in an attic, and you really should work towards accomplishing all of them.

1. High humidity in homes

First of all, if the relative humidity of air in your home is too high, work to lower it. Anything over 50% RH is a bit much; 35-40% or so is good for humans and houses alike. The more humid the conditioned space of your home is, the more condensation you will probably have in the home, and also have in the attic.

You can reduce humidity by remembering to use bathroom and kitchen fans when showering and cooking, run a dehumidifier in the basement if needed, and don't hang laundry to dry indoors, unless of course your humidity is down around the 30% RH level then go for it. It will save some dryer energy and a little more humidity will make for a more comfortable living space. If you have high humidity problems in the home, here's the guides on how to fix it and if condensation on the windows is a problem and needs fixing, here's the guide for that.

2. Attic ventilation:

Attic ventilation is essential to prevent ice in roofs, be certain that your roof ventilation is able to do its job.


Next, roof ventilation - make sure your attic is properly ventilated. Check the attic to make sure that insulation isn't jammed down on the edges and blocking soffit vents. Proper ventilation - proper air flow - in a roof keeps it in good dry and sound condition, so make sure there are air baffles on the roof deck so that air can pass through from the soffit, and a simple way to check this is while wearing appropriate safety equipment for handling insulation products, carefully put head and shoulders into the unconditioned attic space with the lights off, shut eyes tight for 30 seconds, then look around the lowest edges of the roof. If there is light coming through from all around the edges of the roof, the ventilation is probably fine. If not, chances are that when someone added insulation, they pushed it down all the edges and blocked air circulation - not good! There should also be a good vent at the top. And right at the top, either a ridge vent, or another we like is the 'Maximum' vent (or any similar design); it has really good draw.

Vents on gable ends are not great as they leave the very peak neglected. If gable vents are what you currently have, you're better off sealing them and adding one at the top. And be sure to seal them; if you forget and leave them open, they will limit the effectiveness of better ones you just installed.

Attic insulation air baffles ensure soffit ventilation works properly © InterNACHI

Some builders have expressed concerned that ridge vents aren't sufficient because they can be blocked by snow, but snow isn't an air barrier. It will slow the air, but as long as the vent runs the length of the roof there should be enough air flowing through. you can apparently survive (for a time) after being buried in an avalanche if you don't panic and breath slowly. Since roofs aren't prone to panic attacks, by that logic they should be fine for a bit too for adequate ventilation if covered in snow. 

3. Ceiling air barriers:

Lastly, but importantly, you should try to deal with any air leakage if you have ice forming in the attic. Electrical fixtures like diy recessed lighting or pot lights are notoriously leaky (and this may be better left to qualified electricians) but there are airtight boxes available that should be installed and that also hold insulation off areas that may get hot if still using incandescent bulbs. The best solution all around is to replace old, leaky and incandescent light fittings with modern slimline LED fittings - carefully sealed against air leakage and saving energy too. 

Another solution that isn't very complicated (or much fun for that matter) is dealing with these leaks from above. That would be you on your hands and knees in a dark attic crawling between trusses with a headlamp on, and if you've any sense all the safety gear like masks and goggles. Stumbling around, likely letting out the occasional curse word as you bang your head, cut your scalp on the nails hanging through the sheathing or jam your knee on a wooden block (or worse still a stray framing nail) while blind from the perspiration running into your eyes. 

This step will require removing insulation too, so personally - having "been there, done that", I leave this job to insulation professionals - but if you're up in the attic at night with lights on below, anywhere that light comes through, air will be coming through as well. Seal those holes any way you can - tape, caulking, spray foam, acoustic sealant, etc, but do bear in mind that you really must prevent excessive heat build up in light fittings, junction boxes etc. Also worth noting, we wouldn't even suggest touching anything up there if in an older home that might still have knob-and-tube wiring - get this replaced first.

But, if you have no air barrier at all, you really do need one. This requires lifting insulation, laying down a polyethylene vapour barrier over the whole surface (it can go up and down over joists) then reinstalling the insulation. Make sure the poly is well-taped at the seams and on the sides as well, it will serve little purpose if it allows air to flow up at the exterior edges. Pay close attention to not tear it while you're dragging it around, and tape it up if you do. Good luck with all this - we've seen enough old attics not to envy anyone the task.

Important to note as well, because it will likely come up if engaging contractors to help, that spray foam will also work to seal air leaks and add insulation. In the unfortunate situation of having no air barrier but a truss system that makes a poly air barrier impossible to install from above, spray foam may be the only solution. 

We've generally steered our readers way from spray foam products in the past due in part to health concerns, but mostly to the very high Green House Gas emmissions from spray foam blowing agents. 

However 2020 has seen great news about the reduced global warming impact of spray foam, as several manufacturers switch to less harmful blowing agents, read more here

Now you know all about frost in attics: what causes attic ice & how to fix it, find more info pages about ice dams & ice damming as well as sustainable home improvement & resilient green building techniques here : 

Also find all about sustainable home construction in the EcoHome Green Building Guide pages