Heavy frost in attic

I found this this in a home this morning. The owner said that he noticed how wet the attic seemed when he went to get Christmas decor. from attic 3 weeks ago. He also stated that he had some plastic ornaments that appeared to be melted form the summers heat. The home had gable and ridge vents prior to upgraded siding and had soffit vents installed and the gables removed and sealed off. There is about 6-8" of fiberglass on attic floor, some of it is compressed from plywood on bottom truss chord used for storage. Shingles are 5 years old. The humidity was about 90% in attic area, most of the sheathing was soaked.

What type of issue usually causes such heavy frosting on the underside of the sheathing.

Am I correct in telling the owners that there is a moisture issue and a possible venting issue as well. It seems logical that the addtion of soffit/ridge ventilation would have actually improved this problem.

You bet they have a huge concern .
No idea of the venting but more important to me is how is this much moisture getting into the attic .
Is the attic door gasket-ed and insulated heavy enough to seal it correctly . Is the Bath venting into the attic Is the kitchen venting into the attic.
Is there a plumbing stack venting into the attic.
Some where they have to find out these air leaks
can be via many electric cables Chimney race open to basement.
Plumbing stack open from basement
Good luck please keep us posted .
… Cookie

Vapor barrier on wrong side…should be down. From what I can see of the insulation shoots only 1/2 of what should be there is. Trusses look to be 24"oc & the insulation shoots have been cut in 1/2 & they only used 1 per space between rafters. There should be 2 between each rafter. That would cut the ventilation in 1/2. May not be enough air flow to completely dry out the attic.

The vapor barrier is actually correct. It appears that way due to discoloration. I mentioned that the baffels were not quite enough.

The access hatch is non-gasketed and a single panle of plywood.

I only had 20 minutes to get a few shots of this. I was helping a friend and I want to go back to locate the moisture.

Yea, what they said… also it looks like the soffit vents are blocked by the insulation…

I feel more attic ventilation could just bring in more air from the home.

It is obvious there is air leaks and it sounds like you have found a huge one .
I expect Fixing the attic entrance could make a huge difference .
This is the simplest and cheapest place to start.
A 3/4 piece of melamine 4 or more inches of foam on top and a gasket Cost about $20.00 for material .


Cookie that is exactley what I said before I left the house.
The soffits only had 1/2 of what should have been used.

Are there openings (or sufficient vent openings) in the soffit itself? Sometimes old wood soffits may be covered over by newer vinyl soffits, or not place adequately. Any source of moisture generation will only be significantly increased by lack of ventilation.


I also notice that the majority of the frosted areas seem to occur within 4 feet of the roof edge over the exterior walls.


Brand new soffits will ample openings.

The rear roof is south facing and the majority of the frost was on the front or northwall. I think that the moisture in the home from all sources is getting into attic and frosting, then in the am the south roof heats and vapors then attach themselves to the colder surfaces on the front side.

The HVAC company told the home owner to leave the furnace fan on all the time to gain better efficiency. I have never heard that before. Sounds like someone wants to install a new components soon.

Many manufactures do say to leave them running low speed.
This does tend to balance the temp through the home and save fuel cost’s.
How is the humidity level in the home .Is there an HRV and is it running and properly balanced .
I still agree with you some how the home is venting into the attic.
… Cookie

no hrv and readings I took were about 50-56% on the 2nd and ground level. must be a leak from somewhere

I agree keep looking Wished I lived closer . Please let us know how you make out . How about going to the incomming water and almost shut it off you might be able to hear if there is water running noise at the entrance . if yes then try same thing at how water tank trying to narrow it down to an interior leak.
Is there a humidifer on the furnace?
If so shut it off and see if the Interior levels drop.

… Cookie

As it turns out, there was a leak around the flue pipe flashing. Appered ok form exterior and interior, well, the interior was so wet that it was very difficult to tell exactly where the leak was. Problem solved for now.

Good for you and thanks for the questions I am sure we all got some great ideas from all the posts.

I would look at some of the lights in the cielings. If the electrical boxes are not properly sealed or bagged with 6ml plastic and caulked to the vapour barrier they could leak in moisture into the attic. Also is there a bathroom vent? If it has joints in the piping the moisture could be leaking out into the attic as well if the joints are not taped. I would also install a turbine vent especially if he had heat problems that melted some of the ornaments as you mentioned above. Definately it looks like he needs to open up his soffit ventilation more to get more flow through air. It is just sitting there and not exchainging enough.


The insulation is upside down. The paper side always faces the conditioned space (the room where the heat is trying to escape from). When the paper is up, you trap moisture rising from the living space.

Thanks for the update.


NACHI members helping NACHI members at it’s finest!!!

Good find Michael!!

Have seen the phenomenon in the pictures too many times to the point that the north facing roof sheathing rotted out over time while the south facing was still OK due to drying from the sun!!

Too much frost on the roof sheathing to be from a leak only!! The house needs an interior ventilation and moisture assessment…why the high rh’s???

Take care of the interior moisture first and then airseal the ceiling from the attic at all penetrations (wiring, lighting, plumbing, chimney, attic hatch, tops of partition walls, pot lights [be careful here if there are any], any other air leaks). This is an energy saver as it stops warm air from leaving the dwelling uncontrolled as well as reducing/stopping moisture from leaving the house via the attic…fans should be doing this!!


The world of the roofer is changing quickly. Only a year or so ago, customers blamed the roofer for ice damming and wet-attic problems. The solution was simply to add attic ventilation, in many cases beyond building code requirements. Why? Because it was the roofer’s only choice. In order to provide the homeowner with a full manufacturer’s warranty, ventilation had to be installed according to the requirements of the local building code: typically 1 ft2 of ventilation for every 300 ft2 of insulated attic space, and twice that amount for low-slope roofs. Unknowingly, the roofing industry was making the wet-attic situation worse. How? They were following the correct procedure for ventilation, but solving only part of the problem instead of the whole problem. Ventilating a previously unventilated attic has the effect of making the attic colder. If nothing is done to stop warm, moist air from entering the attic space from the living space, condensation on the now-cooler surfaces is a certainty. Mold, mildew, and eventually leakage into the living space will probably follow.

Insulation contractors, armed with the same lack of information about attics as the roofers, have caused similar problems. Insulating the attic floor makes the attic colder in the same way as adding ventilation.** But, if contractors don’t seal as well as insulate, they don’t stop warm, moist air from entering the attic and causing big problems. **

Now, thanks to public debate, reeducation, and the publication of Attic Venting, Moisture and Ice Dams, a report by Canada Mortgage and Housing Corporation (CMHC), roofers know much more about what’s happening, why, and what to do about it.

For the rest of the article, see:

For the CMHC article, see:
https://www03.cmhc-schl.gc.ca/b2c/b2c/init.do?language=en&shop=Z01EN&areaID=0000000026&p roductID=00000000260000000011