Thread: Frost in Attic...again

I’ll try to keep this short and just list the details to get the ball rolling. I am extremely frustrated and looking for potential issues I might have missed.

First identified issue: Fall of 2014, noticed considerable mold staining on underside of roof sheathing throughout my attic.I began to read about causes to determine how to fix it.In winter 2015, I saw extensive frosting on cold days, so I had my symptom and was determined to figure out the cause.

Summer 2015 – I identified a number of issues and took the following corrective actions:
Bathroom fan incorrectly vented through soffit. – Had it vented to the outside
Plumbing stack incorrectly vented into attic had it properly vented through the roof
Removed old (tight and not very permeable) ridge vent and replaced with new ridge vent
Additional Soffit vents added to front and back of house.
Removed all old blown insulation and had a company treat the mold stains. I boxed all exhaust fans.I then sealed every inch of the attic while no insulation was in there.Everything from a bulkhead that was covered, a chimney that dead ended in the attic, all electrical holes all wall studs, everything.I am very sure I got every little hole; I spent 3 days on my belly.
I had 16.5 inches of fiberglass insulation blown back into the attic.
I installed 2 hygrometer/thermometers, one on each end of the attic.
During the summer months the readings were not concerning and I told myself I’d check again in the winter when frost would be the most telling indicator if all this work was for nothing.


Today I went into my attic. Hygrometer reading were in the low 80%’s, but what I find very interesting is that the temperature in my attic is never more than 5 degrees colder than the outdoor temp and today it was 28 degrees in the attic and 34 degrees outside.
Can’t it be safe to say it can’t be warm air from the house, or wouldn’t it be noticeably warmer in the attic than outdoors?


Today there was more frost on the roof sheathing, nails, etc. spread uniformly across the attic and the top of the insulation was slightly damp. Where could this be comine from? Could outdoor air get trapped in the attic? Outdoor air is generally in the 70 – 80% RH range where I am in PA throughout the season leading up to the first frost.

I am king on trying to load some photos













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How old is the house?
What insulation is in exterior walls and is there a vapour barrier?

It is possible for interior room air to be pulled up into attic via the exterior walls, via baseboard gaps, lack of vapour barrier, attic floor vapour barrier, electrical outlets/switches.

Is dryer vented to exterior, range exhaust to outside? Are exhausts used during showering cooking?
How many occupants?
Is there a humidifier on the furnace?

Plants, aquariums .. ?

During winter months the 'stack effect' can come into play, interior air can be drawn up into attic from lower floors due to air leakage into attic, which may be pulling air up into attic via exterior walls where they terminate in attic.

The house is approximately 30 years old.

I haven't been in the walls, but when I have drilled holes to hang pictures, etc. I've noticed yellow fiberglass batt insulation in the walls and On the outer side of the walls between wall and siding is a foam barrier. I was able to air seal up to where the ceiling drywall meets the wall drywall/top plate, but that's as far out towards the eve as I could get. There is not concentration of frost over the exterior walls as I would expect if that was where it was coming from, but more so towards the peak of the roof. I do suppose air could get into the walls under baseboards, electrical outlest, etc. but would be surprised the that could cause this. I suppose the only way to capo off the top of those exterior walls would be to remove the fascia and go at it through the soffit from the outside.

There is no vapor barrier in the attic and it was advised against by the insulation company in my climate zone (Western PA).


Dryer, range, etc is all vented outside. It was not until I did this last year, but the frost persists. I thought venting the fans and appliances would be the smoking gun, unfortunately it hasn't solved the problem. We do always use the fans when showering and often when cooking. The Rh in the house is generally 40% - 50% in the basement and 30% - 40% in the first floor living area (lower once the weather is consistently cold outside)

Two adults and two children live here.

No humidifier on the furnace, no plants, aquariums, etc. Since I first discovered the issue, I have made painstaking efforts to keep things dry in the house to be on the safe side.

I understand the stack effect, but can't imagine with all the air sealing I did where the air could be pulled from. It is extremely cold in the attic, often colder than the outdoor air, so that tells me the insulation is good and there can't be much indoor air getting up there. I wonder if the problem is airflow and the damp air from outside (live in a heavily wooded area which gets a decent amount of rain throughout the fall) gets caught in the attic and isn't escaping through the ridge vent.

It really sounds like "stack effect" with indoor air entering the attic. Depending on how the walls are built it could be air drafting up the wall cavity and into the attic. This would be more common on older homes, which sounds like what you have. The chimney that ends in the attic could be a source of warmer air from whenever it ends in the home.

What makes no sense to me is that the air temp is colder in the attic than the outside air.

Originally Posted by Scott Patterson

It really sounds like "stack effect" with indoor air entering the attic. Depending on how the walls are built it could be air drafting up the wall cavity and into the attic. This would be more common on older homes, which sounds like what you have. The chimney that ends in the attic could be a source of warmer air from whenever it ends in the home.

What makes no sense to me is that the air temp is colder in the attic than the outside air.

Thanks, yes. That baffles me as well. It's not always colder, but it's never more than 2-3 degrees warmer in the attic so far this year.

Does anyone know how to approach sealing the exterior wall cavities? I can't get to them through the attic.

As far as the chimney, I have sealed all entrance points in the basement and capped it off with sealant and metal flashing in the attic. Is it worth wrapping it in an insulated sleeve in thae attic? Can moisture get through cinder blocks?

It sounds like you managed to correct several mechanisms to remove unwanted moisture in an attic space.

Matt, if I may hypothesize, by the sounds of it, the roof deck, wood, may be susceptible to saturation.
If I am not mistaken, your post omitted roofing application and decking material.
1: What type of roof covering.
2: What type of roof deck as well.

If there are old asphalt shingles, but not necessarily seeing there are ongoing class action suits, the shingle mat can become susceptible to saturation.
Then think of conduction and loading.

Also, if there is no selvage paper on the roof deck under the shingles the deck may becomes susceptible to saturation.
Repeated saturations can cause a deck to rapidly degrade and the attic space humid.

On that note: The ceiling and vapor diffusion.
1: Is your habitable space humid. Over 40 to 50%.
2: Is there a vapor barrier under the insulation?
If so, what type?

Awaiting a reply.
Any exterior / interior Pics would also be helpful.

I am wondering if the house sits in the shade for most of the day or gets sun for part of the day? As this could have some affect on attic temps as well as the dew point within the attic during different times of day?

Originally Posted by ROBERT YOUNG

It sounds like you managed to correct several mechanisms to remove unwanted moisture in an attic space.

Matt, if I may hypothesize, by the sounds of it, the roof deck, wood, may be susceptible to saturation.
If I am not mistaken, your post omitted roofing application and decking material.
1: What type of roof covering.
2: What type of roof deck as well.

If there are old asphalt shingles, but not necessarily seeing there are ongoing class action suits, the shingle mat can become susceptible to saturation.
Then think of conduction and loading.

Also, if there is no selvage paper on the roof deck under the shingles the deck may becomes susceptible to saturation.
Repeated saturations can cause a deck to rapidly degrade and the attic space humid.

On that note: The ceiling and vapor diffusion.
1: Is your habitable space humid. Over 40 to 50%.
2: Is there a vapor barrier under the insulation?
If so, what type?

Awaiting a reply.
Any exterior / interior Pics would also be helpful.

Thanks, Robert. Could saturation contribute to the overall humidity level, not just surface condensation? There is also some frost on non-roof sheathing, like lower on trusses, etc.

The living space humidity is about 40 - 50% generally and in the winter it's lower than that. I run dehumidifiers in the basement year round. There is no vapor barrier in the attic.

I have added some pictures and will try to find exterior pics. Its a 44 x 25 sf area with a single peak roof, but a gable on the front over a porch and the back over a porch.

Originally Posted by Raymond Wand

I am wondering if the house sits in the shade for most of the day or gets sun for part of the day? As this could have some affect on attic temps as well as the dew point within the attic during different times of day?

Heavily wooded area, gets some sun, but mostly shade. Very little summer sun, more in the winter when the leaves are lost, but it's low sun.

Hello:

I have been trying to reply to posts, but am not seeing them when I refresh the thread, so I have answered all outstanding questions below:

The house is a ranch style built in 1986. The attic SF is 44 x 25. It has a gable covering a porch on the front and the same covering a porch on the back.

There is no vapor barrier in the attic, just 16 inches of insulation on top of the ceiling drywall. It was advised I don’t do a vapor barrier.

The exterior walls are filled with yellow fiberglass batt insulation. I was able to air seal up to where the ceiling drywall meets the exterior wall cavity, but I couldn’t see the wall cavity from the attic. I did see a layer of foam insulation on the outside of the exterior walls under the siding.

Dryer, range, exhaust fans, etc. are all vented outside the house. I have no humidifier, we use fans when we shower, etc

2 children and 2 adults live here.

The chimney goes from the foundation and dead ends in the attic, but I have sealed all entry points and the top of the chimney with sheet metal and sealant. Could the cinder block be permeable to moisture? Should I wrap it in an insulated sleeve?

The air temp in the attic is sometimes cooler than the outdoor temp and has never been more than a degree or two warmer than outdoors which leads me to believe the insulation level is fine.

The indoor humidity is 40% - 50% in the basement and I run dehumidifiers down there. The main floor rh is in the 40s and gets into the 30s throughout the winter.
I have not had the roof checked, but didn’t think I needed to. Could a saturated roof contribute to the humidity level or just the condensation on the roof sheathing? There is some frost on lower areas like the trusses , etc.

The house is in a wooded area and gets little summer sun, but some winter sun, albeit low sun.
I have added some pictures

Just a couple of thoughts. The exterior air temperature is not the same as the roof temperature due to radiant cooling on clear nights, the mechanism for frost. Also true if roof is snow covered.
Just like the top of the roof can frost (form ice) when the air temperature is above freezing, the same mechanism, radiant cooling of the roof deck, can create frost on the bottom of the roof decking inside the attic. Once the roof deck warms, the interior frost should dissipate and be washed out through normal attic ventilation with little adverse effect.

You may have corrected the offending issues. You may just want to wait and check a bit more through different weather cycles.

I'm NOT saying there is no other problem and ignore it; just that further observation and understanding is warranted.

Also see

GM-1001: Attic Air Sealing Guide and Details | Building Science Corporation

Download document (bottom right), and see related articles to right.

This caught my attention -

There is no vapor barrier in the attic, just 16 inches of insulation on top of the ceiling drywall. It was advised I don’t do a vapor barrier.

That could be part of the problem, I think in your area a vapour barrier would be required? However since vapor can move through the drywall an up through the insulation, this could be creating your problem. In lieu of a vapour barrier there are paints available that will act as a VB.

Thanks for the update.

Mr. Patterson and Jim are of one mind on this, as am I.
I have not read Rays post and link but I can bet it's within the scope of the issue you are having.

Personally, I do not like blown in particle fiberglass. Poor insulator for the depth and air goes through it like a sieve.
Expensive, almost triple the price and not as dynamic as cellulose.

Having no barrier under the fiberglass insulation makes little sense but whom am I to disagree with the professionals that recommend doing without.
Let the proof of what is occurring tell the tale.

Note: You run a dehumidifier 12 months a year you mentioned in an earlier post.
Matt, I suspect more to this than meets the eye.
As Mr. Patterson said, all the likelihood of stack effect but at times, there are other forces at play.

Originally Posted by Matt Scicchitano

Thanks, Robert. Could saturation contribute to the overall humidity level, not just surface condensation? There is also some frost on non-roof sheathing, like lower on trusses, etc.

The living space humidity is about 40 - 50% generally and in the winter it's lower than that. I run dehumidifiers in the basement year round. There is no vapor barrier in the attic.

I have added some pictures and will try to find exterior pics. Its a 44 x 25 sf area with a single peak roof, but a gable on the front over a porch and the back over a porch.

Sorry Matt, I missed the post.
Let me start out by saying after reading everything;
Adding insulation will make the attic colder.
The colder it is in the attic, the greater the potential for frost accumulation.

Now to this post.
Frost on the lower cord or upper cord near the sheathing?
If frost is on the lower cord, that might indicate an air leak in the attic from the ceiling finishing assembly. Openings in the dry, switches, outlets.

I read; noticed considerable mold staining.
Wood is cellulose and absorbs and expels moisture. Without adequate attic ventilation conditions remain for regrowth.

1: I would have the insulation contractor vacuum up and save all the insulation that is clean. 1b. Seal all the openings, 1c Add vapour a barrier, 1d. Blow the insulation back in.
2: Purchase an HRV. They scrub out excess moisture and particulate, recover heat and do a great job at maintaining a set relative humidity.

Sounds like stack effect.

My first guess is that you did not seal everything as well as you thought. Do you have a pull-down stairway? They usually leak a lot of air.

Also, 40% RH inside the house is too high for cold winter temperatures.

Also check the RH outside and compare to the attic.

Do you have baffles in the rafter cavities to allow air in through the soffits? Hopefully yes.

Old houses did not have this problem because heat from below kept the attic warm and dry. Now with super insulated ceilings, you need to minimize air transfer from below, but you also need to have some fresh air movement to remove moist air. Sometimes this backfires, when moist air enters at the soffits.
If the frosty areas are down low on the North side of the attic, the moisture would be coming in from outside. I have seen this on the West coast with a sudden drop in outdoor temp.

But in your case, lack of a poly vapour barrier may be the the cause of the moisture.
If the blown in insulation can be moved aside, a 6" layer of cellulose insulation could be spread between the ceiling joists and then the loose fiberglass could be spread over that. Cellulose forms a denser blanket when it compresses. What Robert said.

Make sure your attic hatch is sealed with weaterstrip and insulated, what Mark said. Add weight to a drywall hatch cover with a second layer of drywall or 1/2" plywood. This will compress the weatherstrip seal.

John K. and Mark R., I support your hypothesis and recommendations.

Make sure your attic hatch has self-sealing on the lower hatch cover or the frame base to which it sits.
Self-sealing one side adhesive foam sealing tape (EPDM) rubber is what I most frequently observe.
adesive foam sealing tape (EPDM) rubber.JPG
The hatch cover can be 1/2" plywood covered with closed cell polystyrene.
8" inches will do.n Nice and light when you have to mount the attic.

I concur poly plastic barrier and blown in cellulose between the joists.
Atop, the salvaged new loose fiberglass. Waste not, want not.

Have an installer "dam" the hatch entry and upper truss cord's past the height of the insulation.
Height; 24" is enough unless you are using an insulation that is not dense.

Recommended: Rigid damming material.
8' x 4' 1/4" ply can be ripped at 24" to get 16' feet of damming for the upper roof truss cord.
I would also recommend the same material for the attic entry.

I was amazed at the energy loss created by air leaks in an attic hatch or pull-down ladder.
I was equally amazed at the energy loss of an inadequately insulated hatch or pull-down ladder.

As previously mentioned I would check to make sure the insulators didn't block the baffles from the soffit or eave vents. If you have a raised floor I would also put some 6mil plastic sheeting over the exposed earth. Sounds like you covered all the other bases here. Vapor barrier or plastic in the attic is not required when the attic is ventilated.

- - - Updated - - -

Originally Posted by Mark Reinmiller

My first guess is that you did not seal everything as well as you thought. Do you have a pull-down stairway? They usually leak a lot of air.

Also, 40% RH inside the house is too high for cold winter temperatures.

Also check the RH outside and compare to the attic.

I agree...
Looks to me too much air being drawn out of the attic then comes into the attic from the outside .
I think the needed air is coming from the home check all electric boxes Box lights Bath room fans .
I think you do have moist air being drawn from the home.
A Little less discharge air from he attic might be some thing to try too .

Roy Cooke

- - - Updated - - -

There should always be a vapor barrier in the attic facing the warm part of the house no matter where you are. It is there to protect the structure from moisture. I think your insulation man is not a good source of info on this.

Warm air carries moisture with it and it can carry it through sheetrock and insulation without a vapor barrier. If the moisture content of the roof structural members gets above 19 percent that is not good. Wood destroying insects cannot live in wood below 13 percent and thrive in wood above 19 percent and you can get mold/mildew issues with persistent RH humidity above 60.

Generally speaking there should be 1 square foot of vent area per 300 feet of attic floor area. It should be noted that louvres or screens reduce air flow by 50% so if they present, it would be 2 feet per 300


R38 is recommended for northern climates for ceilings below attics.


The other piece is the type of roof you have, the roof sheathing, the pitch etc etc. that may be a factor.

Does the house have a basement or crawlspace? If so, there are vapor barrier requirements there as well. Hope this helps, thought I would chip in my .02.

Good luck!


Edit: You can get a semi-permeable vapor barrier installed (vapor retarder) instead of a barrier which let some vapor into the attic.

Understanding Vapor Barriers | Building Science Corporation

I am reviving this thread because I am having the exact same issues. After reading all the replies in this thread, there are a couple things I haven't addressed yet. Poly sheeting the crawl needs to be done and I also do not have a vapor barrier in the attic. My attic is open to the attached garage as well. I need to tape and mud the drywall that is backing the living space still. I have a ton of tree coverage as well.

I had black mold covering my attic that I needed to remove before I proceeded with my loan to buy my house. I have done everything I could do within my budget to keep heat and moisture out of my attic. I have recently sided and had all windows replaced in my all electric house with baseboard heaters. I had the siding guys cut in more holes for new soffits and installed baffles at every vent. I don't have a ridge vent but I have plenty of box vents and an attic exhaust fan.
After pulling out the old insulation in my attic, sealing all holes and blowing in new insulation, I installed a range fan and bathroom fan (vented through the roof). The roof is about 8-10 years old with one layer of asphalt shingles. It recently snowed and I went up in the attic to check on things and noticed all of my sheathing is damp and at the peak of the roof there is active condensation drips. Is there anything else I can do to fix this issue or at least lessen it?

Has there been any update with the original post? Has the problem been fixed? I would appreciate any thoughts or guidance.

Try reading this.
Principles of Attic Ventilation

Originally Posted by ROBERT YOUNG

Try reading this.
Principles of Attic Ventilation

Robert, that is a lot of useful information. Thank you. I have some knowledge of the venting system for attics. I have nearly finished my HI training and have done 10 practice inspections so far. I know I have much to learn about the trade and I am using my home and all of it's issues to help out. The two inspectors that I had come in didn't inform me of the downfalls of an unfinished crawl and lack of exhaust ventilation. Nor did he catch any of the electrical issues. Reverse polarity and open grounds throughout the home. It was a huge indicator of a half @ssed inspections. After looking back at their reports, it makes me wonder if they sold their soul to a realtor.

A ridge vent is definitely the best way to go. However that is not in the cards right now. From the literature, it sounds like my best bet is to rely on a fan. Getting the correct size, with a humidistat should be a lot easier now that I have some calculations to go off of. I am hearing some conflicting info pertaining to powered roof vents. Some say not to use them at all but I think that my be my only option right now. I don't think the box vents I have now are providing enough air flow. I know that I still need to track down and eliminate my moisture issue to solve this. I plan on selling this house in 3-5 years. As long as the roof stays in decent shape I don't think I will be installing a roof vent.