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295yards 11-26-2009 06:12 AM

Frost in the Attic
I have a friend who is complaining he has frost in his attic.

I'll ask him direct questions regarding this but what are some of the main reasons this would happen?

I'm assuming he is not venting directly into the attic or that any venting is leaking into the attic, but I wil find out.

I think it is possible he put too much insulation where it does not belong and is not allowing air flow to circulate.

What are the basics for attic insulation and air flow?

oh'mike 11-26-2009 06:41 AM

There is not proper ventilation in the attic. My guess is that the soffit vents are blocked by the new insulation.

could be the house was reroofed and not enough roof venting was installed.(or a combination of the two)

ScubaDave gave a very easy to understand explanation of attic ventilation in this long post-----
How hard to install insulation yourself? - DIY Chatroom - DIY Home Improvement Forum

ccarlisle 11-26-2009 07:22 AM

More insulation doesn't = more frost. Nor does less ventilation = more frost. Frost comes from humidity and somehow your friend has produced more than the attic can handle. Now the humidity can come from vents directly dumping moisture into the attic space instead of through the roof, it can come from an inadequate vapour retarder/barrier - or the existing moisture balance has been upset by blocking the ridge or soffit vents. It can also be caused by a roof leak...

Maybe he has done some electrical or plumbing work up there that has produced a leak of the warm inside air to the colder attic. Maybe he has put more insulation where it shouldn't be, but either way there is an excess of humidity in there that has to escape. The amount of ventilation coming in has to be balanced by the amount of ventilation going out and somewhere this equation has been put out of whack. In newer homes, the amount in square inches of, for example, soffit air inlet is very carefully balanced with amount of ridge vent air outlet, all other things being equal and depending on what microclimate he lives in.

You can easily treat the symptoms (frost) by increasing the ventilation. Put a fan up there...but that doesn't treat the cause. However, if you find that he has blocked the air inlets, and you find that nothing else has been done to cause more humidity, then that's the cause right there.

But IMO, frost means a lot of moisture is leaving the house. That can be solved by a look at his vapour management system (air retarder/ barrier etc)...

Gary in WA 11-26-2009 12:18 PM

"What are the basics for attic insulation and air flow?"
Be safe, Gary

295yards 11-26-2009 12:28 PM

Thank you!

295yards 11-26-2009 01:11 PM


Originally Posted by GBR in WA (Post 358082)
"What are the basics for attic insulation and air flow?"
Be safe, Gary

I love this stuff!

I am not only ever looking to just "finish a job" but I am looking to fully understand it.

From what I have gathered on this forum so far is that your information can be very useful. I definitely appreciate all your feedback.

A quick update:

I was at my friends today and I have quickly learned that the problem was "created" by introducing new inbalances that were not accidentally designed into the home.

This is good!!! You might appreciate this :

Someone decided because the upstairs bedrooms were colder than on the main level, they would just stuff more insullation in the attic.

While up there they thought the soffit vents were in effecient so they pulled them out and also stuffed heaps of insulation in the soffits :laughing:.

"Wow, it's so drafty up here" :help:

This was all just done recently coming into the winter season so there is very little surface dampness in the attic, but no sign of any rot whatsoever. :thumbup:

Also a couple of lights were added to the upstairs bedrooms since there was no main light fixtures in two of the three bedrooms. This was done through the attic legally and passed by an inspector ( I'm told ) but it is very clear it is not sealed off properly.

Ha, it's funny, they went into the attic with batts of insulation to stuff the soffits but didn't bother to even put any insulation back from where they moved it to do their light fixtures.

We're talking two big 3ft circles clear to the vapour barrier, forget about a proper seal!!!

A side note. I think the upstairs problem started when they converted their electrical baseboard heated house to gas.

They actually crammed 4" round duck between a 2 X 4 studded walls. The 4" ducting was bigger than the wall cavity so they flattened it "some", which may not be extremely terrible 'till I learned it is pretty much pinched off at both ends! Once going into that wall cavity, and the other going out.

Long story short.

Not only do I have some research to do to make sure when we restore his attic to the way it was, it'll be okay. I also have duct work, framing, drywall, taping, mudding, and painting to do.

All for free, I'm sure :eek: .

Oh yeah. His vent for the bathroom is surprisingly just a flexible "sock" that runs to the soffit in a hap hazzard path. I'm thinking of replacing it directly out through the roof with a solid metal duct and wrapping it with insulation.

Any thoughts?

Besides even though we are friends $500 for all that work he can't do is a great deal!

ccarlisle 11-26-2009 01:45 PM

"Oh yeah. His vent for the bathroom is surprisingly just a flexible "sock" that runs to the soffit in a hap hazzard path. I'm thinking of replacing it directly out through the roof with a solid metal duct and wrapping it with insulation..."

Where abouts are you located? Your plan for the bathroom vent is perfect, keep it as short as possible, and seal the all joints between pieces with self-sticking aluminum tape - not "duct tape".

I think the key here is 'to understand' the wheres and whys of insulation by literally getting up there and having a look around...sure, you can glean a lot of info from various websites and most of us can read, but there's nothing like getting some opinions from guys who have been there before. Confirm it if you will with stuff from the internet but I reckon all you get is at best 50% of the solution.:laughing:

Gary in WA 11-26-2009 04:33 PM

Use a turtle back roof vent with damper, screen and inside neck, or a specific one for your snow load/area. Tape all joints, as ccarlise said, including the individual elbow ones. (Tend to fall apart) Insulate and vapor barrier it. Run 2' of straight pipe, pitched down away from fan, before going to roof. (Catches any condensation to let it dry in pipe, not bathroom). Use 4" smooth metal, not flex which creates turbulence, and caulk between fan box/ceiling. Many sites for installation such as: Good time to check the fan/room size:
Sounds like someone will be busy soon! Enjoy.....
Be safe, Gary

batticdoor 11-27-2009 08:54 AM

How to Diagnose Common Causes of Attic Mold
Here is a related article:

It happens to countless homeowners around the end of the year – you make the annual visit to your attic to collect the holiday decorations and what do you find? Spots and blotches covering the bottom of the roof sheathing. Worse yet – it turns out to be attic mold!

What does energy conservation have to do with mold in the attic? Well if you take a step back and consider how the house behaves as system, they are often directly related.

Building science experts have long been using the “house as a system” approach to diagnose the cause and origin of building defects.

For example, ice dams. These are often caused by warm air seeping into the attic which causes the snow and ice on the roof to melt. The water drains to the edge of the roof (which is colder than the rest of the roof because it is an overhang and not warmed by the attic), freezes and creates an ice dam. As this process is repeated daily, the ice dam grows larger. Eventually water is forced under a shingle where it can seep into the house.

Understanding how the house behaves as a system and the various causes and effects is necessary to diagnose most building related problems.

But how about that attic mold? How did it get there?

Mold requires chronic moisture to form and to thrive, so source(s) of moisture must be present. Possibly the moisture came from outdoors. The roof is newer and a quick check of the roof shows no obvious damage or leaks.

Possibly the moisture came from indoors. During the heating season, the interior of the house frequently has high moisture levels, especially bathrooms and kitchens. A quick check shows that all bathroom fans, kitchen vents, etc. are properly ducted completely outdoors and not into the attic. The amount of insulation looks good and the attic is well ventilated.

Don’t give up – you are almost there! Remember the house as a system? You know that warm, moist air is in the house, but how is it getting into the attic?

By air leaks! Air leaks are the leading source of energy loss in most houses, and a frequent source of chronic moisture that can cause attic mold. Most homeowners are well aware of air leaks around windows and doors (especially old ones), but many overlook the numerous gaps leading directly into the attic!

Have a look around the attic and you may find large gaps around recessed lights and fans, holes where wires or pipes are installed, even large gaps around the chimney. And don’t overlook the whole house fan and especially the folding attic stair - a big, uninsulated hole in your ceiling that is often overlooked!

These gaps can add up to a large hole that allows warm, moist air from the house to flow right into the cold attic. The warm moist air condenses on the cold roof sheathing, creating chronically damp conditions that can lead to attic mold growth. And the energy loss – it can be like leaving a window open all winter long!

Seal these air leaks and you stop a significant moisture source. And just think of all the energy you can save and the cold drafts you can stop!:thumbup:

Mark D. Tyrol is a Professional Engineer specializing in cause and origin of construction defects. He developed several residential energy conservation products including an attic stair cover and a fireplace draftstopper.

Randy1974 12-06-2009 08:40 AM

i have just noticed frost in my attic also. it's strange though, 2 years ago i finished off my garage with insulation, drywall, etc. i put a storage attic above the garage between the rafters. the attic space above the garage is connected with the attic above the rest of the house. i have frost hanging off of the nails on the underside of the roof above the garage but not the rest of the house. i never did see this problem last year and i havn't changed anything in the house and this is the second winter i have had the garage finished. i do not have any vents above the garage because it used to be open rafters and the rest of the house has 2 turbine vents and a gable end vent on the other side. right now i have a fan up in the attic on oscilate but i dont know if that is helping, i had frost again this morning. what's the problem? do i need to install some vents above the garage? i was thinking of putting a powered gable vent. would that stop the frost? i have all vented soffit so i figured the attic would have enough ventilation. could someone let me know if a powered gable vent would do the trick.

ccarlisle 12-06-2009 08:53 AM

Well you have excess humidity in the attic causing condensation on the coldest spots and in this case it's the nails. Not a major problem but a sign of a problem. Excess humid air is getting in there either from the garage below or from a leak. But only excess humid air would cause condensation therefore,depending on where you live the addition of a vapour barrier or retarder may be all that's needed to keep the moist air out of there.

A fan up there dilutes the moist air but if it has no place to go, you're only moving it around. Better than nothing - but you really need ventilation + an outlet so look into vents at the top of the roof and soffits where air can get in.

Randy1974 12-06-2009 09:28 AM

well i have plenty of inlet from the soffit. like i said it's vented soffit the whole length of the house. if i installed a power gable vent at the garage end of the house do you think that would do the trick? how do i put in a vapor barrier? i insulated the attic when i drywalled it so i dont know what else i can do after it's all done now. thanks.

ccarlisle 12-06-2009 09:47 AM

For a properly balanced attic, there should be an equation where inlet air volume= outlet air volume depending on where you live. Some areas there may be 2x the inlet for the amount of outlet so it depends. But a powered gable vent would do fine.

Gary in WA 12-07-2009 04:27 PM

"rest of the house has 2 turbine vents and a gable end vent on the other side." ---- I would figure out the attic venting numbers you need, as ccarlisle said. With 2-say 12" turbines at 100sq,in.- NFVA each and one gable vent, size ? , that's only 300-600 of exhaust. If the house is 1000sq.ft. with continuous soffit vents at per foot times 140 lin.ft. = 1260/144 = 8.75 with exhaust at 600/144 = 4.17, not near enough. Soffit vents and gable vents and turbine vents all vary in NFVA:

" how do i put in a vapor barrier?" --- At this point I would use a v.b. paint primer over the garage to stop all the wet from the parked cars:
Be safe, Gary

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