Frost in attic

[Brian A. MacNeish]

Guys:

Read this document from Canada's housing agency CMHC:

http://cmhc.ca/en/co/maho/gemare/gemare_001.cfm

Here is an excerpt that may shock you:

"What To Do About a Wet Attic

There are many signs that an attic is wet. Prolonged wetness will rot out the roof sheathing. Often this is first noticed when re-shingling. If you have ceiling leaks only in the spring, it may be that ice has been forming on the sheathing all winter and it suddenly melts when a warm spell arrives. You may see water stains or evidence of mold on the sheathing, rafters, or trusses when you are inspecting the attic. You may find the insulation has been packed down or stained by water or ice. The smell of a moldy attic will enter the house under certain weather conditions, usually in summer.

The usual response is to increase attic ventilation. This is the wrong approach. In some cases, adding ventilation will actually pull more moist house air up into the attic and make the problem worse. (I have worked on houses like this) The best way to fix a wet attic is to stop air movement, or leaks, from the house. Once this is done, the existing ventilation is usually more than enough to keep the attic dry."


My own instructions are:
(1) clean up moisture sources such as damp basements, don't store damp firewood indoors, don't dry clothes indoors, etc
(2) Vent all areas/appliances that produce moisture to the exterior (kitchens, baths, clothes dryers)
(3) Fully airseal at the attic floor level. (this is an energy saver also!!)
(4) You now probably won't have to add more attic ventilation.

A small bit of frost on nail points/shanks is not a large problem and won't cause problems.......perfection is not needed but management of the moisture. I don't like adding attic ventilation if there is already some there, I have seen too much snow blow into attics.......something that was mentioned in the last week by another poster.

[Tim Spargo]

I would say that ventilation has got to be problematic to some extent.. possibly compounded by warm air rising up (insulation?)

I prolly would have poked that sheathing with an awl (curiosity) as it is delaminating..
moisture content would be almost irrelevant at time of inspection as it is actively wet.

Roofs/attics rarely have enough proper ventilation.. It's a tough balance but ultimately can save a structure such as this one.

[Brian A. MacNeish]

Originally Posted by tspargo

I would say that ventilation has got to be problematic to some extent.. not if you've read some of the INDEPENDENT research on the topic and truly understand how the house works in HAM (heat, air,moisture) movement

possibly compounded by warm air rising up (insulation?)

Other than odd cases of fans discharging into attics or rain/ ice dam leakage, warm air (containing vapour) leakage from the house below is the predominant source of moisture ending up in the attic. Vapour diffusion, which unsealed vapour retarders greatly slows down, is only 1-2% of the total moisture movement from the house upwards.....the other 98-99% is air leakage.

Airsealing or a sealed vapour retarder is needed to stop this leakage and substantially reduce moisture movement to the attic. If you have a wet/ damp house below the attic, a ceiling loose to air leakage and a fairly tight attic upper envelope (attic roof /walls above the insulation), you have a situation where moisture will end up condensing/freezing in cool/cold climates. To add attic venting without dealing with the interior moisture sources and loose ceiling may increase the attic moisture.

By adding vents, you have just opened up the tight attic upper envelope, letting the house air (that has dropped its moisture as condensation) out......... to be replaced with more damp house air rising by stack/chimney effect or wind/mechanical forces. This house air exfiltrating at the ceiling level brings more moisture to the attic (as well as heat being lost in the exfiltrating air) to increase the condensation!!

My story to the man-on-the-street:
Imagine the house turned upside down with the roof analagous to the bottom of a boat. You have water (moisture) from a leak gathering in the bottom of the boat. The way to get rid of it is not to cut a another hole in the boat to let the water drain out!!! Stop the water (moist air) from getting into the boat in the first place.

I prolly would have poked that sheathing with an awl (curiosity) as it is delaminating..
moisture content would be almost irrelevant at time of inspection as it is actively wet.

Roofs/attics rarely have enough proper ventilation.. It's a tough balance but ultimately can save a structure such as this one.

Regards

PS: BTW, insulation only slows down and filters air rising to the attic!!!

[John Allingham]

Originally Posted by jcampbell

hows the insulation... inadequate insulation allowing heat to leak up through will cause condensation and freezing/frost... I see this often in the areas of the scuttles/access where the heat leaks up through...

What am I missing. What does insulation have to do with it? Lack of vapour barrier - yes, lack of ventilation - yes, but how does lack of insulation lead to high humidity in the attic.

[John Allingham]

Originally Posted by jallingham

What am I missing. What does insulation have to do with it? Lack of vapour barrier - yes, lack of ventilation - yes, but how does lack of insulation lead to high humidity in the attic.

Anybody?

[Jeffrey S. Campbell]

I think we address most of the possibilities in the post... without seeing more photos of what is going on with insulation, ventilation, VB, bath exhausts, scuttles, etc... we have a pretty good ideal of what could be the problem...

[Ed Fako]

Originally Posted by jallingham

What am I missing. What does insulation have to do with it? Lack of vapour barrier - yes, lack of ventilation - yes, but how does lack of insulation lead to high humidity in the attic.

The lesser amount of insulation would allow more warm air, which also contains an RH content to invade the attic region.

Although the tightly sealed attic possibilty is a viable, yet costly undertaking on an older existing structure, it certainly could be one of the recommendations made.

Alternatively, I saw no notation of the Intake Ventilation, other than a casual comment that the ventilation"Seemed" sufficient.

Was ther any blockage of the soffit vents from the insulation?

Were there any insulation baffle vents, which would allow the Intake Air Flowage to progress into the attic?

Since this was a Hip Style Roof, what types and amounts of Exhaust Ventilation were provided for?

Commonly, on a hip roof, people rely on a Powered Attic Ventilator, which usually only contains a thermostat for interior heat detection.

A working PAV for the winter months, would also have to have a Humidistat hooked up for proper vapor exhaust and good moving air flowage.

Ed

[nwagner]

Scott, is that sap I see along the rafter? If so, it looks like there may be a ventilation problem. Baking of sap out of rafters is often an indication of the attic overheating during the summer ...

[John Evans]

CertainTeed offers a free "Principles of Attic Ventilation" online course that is very informative.
http://216.83.181.215/elearning/cert...rces/index.jsp

[Scott A. Hand]

It does appear to be sap...frozen sap.

[jwarner]

Scott,

This is an indication of both poor insulation and poor ventilation. Unless you see bathroom vent that is not directed the exterior and discharging or leaking into the attic area.

The warmer conditioned air in the interior rooms below is escaping into the attic where it is met by the colder air in the attic and it condensates. The condensation then freezes on the nail tips and anything else it settles on. Looking for rusted nail tips in the summer inspections will help determine this problem when the frost is not present. Hope this helps.

[John Allingham]

Originally Posted by jwarner

Scott,

This is an indication of both poor insulation and poor ventilation. Unless you see bathroom vent that is not directed the exterior and discharging or leaking into the attic area.

The warmer conditioned air in the interior rooms below is escaping into the attic where it is met by the colder air in the attic and it condensates. The condensation then freezes on the nail tips and anything else it settles on. Looking for rusted nail tips in the summer inspections will help determine this problem when the frost is not present. Hope this helps.

Why poor insulation?