Still seeing frost in attic...!


#1

My roofer installed a new roof including new plywood decking last spring. He insisted a ridge vent would work on my hip roof. I’m really doubting that now as I still see light frost on the plywood decking and nails from inside the attic.

Here are specs. on the house:

  • 1200 sq. ft of attic floor
  • Hip roof
  • 12 soffit intake vents sized 16â€Â

#2

Other things to consider:

  1. Do you have gas appliances?
    If so and if the chimneys are not tight they will emit loads of moisture into the attic.

  2. Do you have a crawl space under the house? If so, does the crawl space have a plastic vapor barrier on the ground, or is there any standig water in the crawl space?

  3. Exhaust fans:
    Where does your dryer vent to? Are all the connections tight?
    Do you have bathroom fans? Do they really work well or are they old or weak? Do they vent out the roof with the exhust pipe being as vertical as possible? Does anyone in your house prefer hour long showers? Do your bath fans vent out the sofffit? If so the hour shower steam is sucked in the soffit venta and this can SOAK an attic. The moisture will condense on the plywood.

  4. How hot is your house? How many people? Is the house full of kids and people all day who like the place hot? Is it a modern house that is airtight with a heat exchanger fan? Are all the windows closed all winter so there is no fresh air flow? Is there a boiling pot of soup or water on the stove all day?

  5. Hip roof. Why not add 4 hood vents? Two below the ridge vent about 6-8 inches down and one on each adjacent hip at the top. I know the arguments against this, but the world is not as perfect as the engineers as their pencils wish it was. If your roofer is against this, I don’t want to disagree with him since he is there and I am not. Also,where you live, both regionally and locally can have an effect and I don"t have that information.

  6. Are any of your close neighbors, maybe with similar age houses, having a similar problem? Are the houses on a slab with lots of wet ground nearby?

  7. Are you overly concerned about a minor bit of moisture on a few nail heads, or is the problem truly serious?

Hope the considerations are some help. I’d like to hear the resolution
John


#3

Hi,

A lot of roofers think ridge vent is a cure all.

You may need a poser roof vent. Use one with a humidisat.


#4

Good points by mwbergin.


#5

Other things to consider:

  1. Do you have gas appliances? I have a gas stove/oven in the kitchen. It just vents into the kitchen. We crack the kitchen window if use the stove for a prolonged period of time. I see frost in the attic even if we haven’t used the stove for days.

  2. Do you have a crawl space under the house? We have a crawl space that takes up about 25% of my basement. It’s covered with thick plastic and vented with a pull fan through PVC pipe for radon mitigation. My sump pump has been running daily since we had some snow melt and this past weekend. The sump pits are covered, but not air tight. I have a hard time believing the sump pit is causing a lot of excess moisture when my basement RH reads 38% at 68 deg.

  3. Exhaust fans:
    Where does your dryer vent to? Are all the connections tight? The dryer is vented to the outside of the home and connections are tight. The RH level in my basement is 38%, first floor is 35% and second floor is 35%.
    Do you have bathroom fans? The bathroom fans pull well and are vented with flexible tubing straight up through the roof with a dampered vent. I had this corrected with the new roof. They were originally vented to the soffit. If I stand outside the house to view the roof while the shower is running, I can see steam coming from the dampered roof vent above the shower. We run these fans for every shower and/or bath. My wife takes long showers, but she uses the bathroom fan with the door closed. I only notice a tiny bit of moisture on the top of the mirror in the bathroom after a shower. It goes away within five minutes of the shower ending. We continue to run the bathroom fans for 10-15 mins. after the showers as well.

  4. How hot is your house? How many people? Is the house full of kids and people all day who like the place hot? Is it a modern house that is airtight with a heat exchanger fan? Are all the windows closed all winter so there is no fresh air flow? Is there a boiling pot of soup or water on the stove all day? My thermostat is set at 68 deg. and it’s 68-69 deg. in the basement and first floor. The second floor is 70 deg. I don’t run the HVAC blower motor continuously. Maybe that would help? My wife and two small children are home for half the day. The house was built in 1982 and is fairly tight. The windows are single pane glass with storms. I cracked open the storm windows to allow for some air leakage. I’ve also insulated the attic access panel in the ceiling and used weather stripping for a tight seal from the living area. I don’t use a humidifier either.

  5. Hip roof. Why not add 4 hood vents? Two below the ridge vent about 6-8 inches down and one on each adjacent hip at the top. I know the arguments against this, but the world is not as perfect as the engineers as their pencils wish it was. If your roofer is against this, I don’t want to disagree with him since he is there and I am not. Also,where you live, both regionally and locally can have an effect and I don"t have that information. I live in the Chicago suburbs. I also wondered about a power attic fan, but thought that might pull snow into the attic through the ridge vent. Can roof vents be added in the winter?

  6. Are any of your close neighbors, maybe with similar age houses, having a similar problem? Are the houses on a slab with lots of wet ground nearby? Nobody nearby seems to have a hip roof.

  7. Are you overly concerned about a minor bit of moisture on a few nail heads, or is the problem truly serious? Every single nail had frost on it last night and this morning. The frost on the plywood is light, but I can see it reflect off the light in the attic. I can also feel the moisture if I lay my hand on the plywood. It seems worse in the morning. I can only think that’s because its coldest at night and the wind speed is low. On some days, I can go back in the attic in the later afternoon and the nails will be dry. I could be imagining this, but it seems like the attic dries out if there is a decent wind speed that day. That what makes me think there isn’t enough ventilation.

Does 9ft. ridge vent sound like enough for 1200 sq. ft.? Thanks very much for your help. When I was obtaining roofing bids, three of the four roofers recommend the ridge vent. Only one roofing company recommended several hood vents instead of the ridge vent. Not sure why.


#6

Assuming you have adequate intake at the eaves to supply the power fan, that won’t be a problem. See the last video on the link/site posted above.

Yes. But the shingles can be brittle so try to have them installed on a warm/sunny day.


#7

Firstly, I don’t think that minor frosting, just in the morning is significant, as long as the air movement has an opportunity to continually dry it out.

I posted in your other thread, that I believed you needed to beef up your Total NFVA and the only recognized way of doing that, as per the most accepted ventilation theories is to increase the amount of intake ventilation.

Your home:
A) Firstly, did not have the vapor barrier, which you have since corrected.

B) Does not have a “Balanced” ventilation scenario.

Due to item B) you were required to have ventilation installed at the 1/150 rule, rather than the 1/300 calculation.

At this point, I would just keep an eye on the frost and see if it looks like it is permeating the deck sheathing.

Also, Coravent is one of the inferior ridge ventilation products. (Yes, that is just my Opinion, but most people and contractors value that highly, especially regarding ventilation) It does not contain the External Baffle, like the Shingle Vent II or the Cobra Snow Country Ridge Vent products, so the roof material price saved a few bucks on the material cost, but does not provide the most optimum performance. You should have been aware of that fact when you did your initial research.

Since you wound up with the weaker and cheaper version of plywood, it Will be more susceptible to future bowing and buckling, as you already have experienced. This will more than likely continue on for the life of the roof, unless you add a continuous soffit strip vent along the entire perimeter of the upstairs 2nd story portion of your home.

Also, due to the pitch of your roof, which if I recall, was about an 8/12, there is increased cubic footage of contained air space, which should be vented at an additional 20% to 30% capacity, regardless of the 1/300 or 1/150 formulas, to compensate for the additional air contained within the attic environment.

One final point about the option of utilizing a PAV, Powered Attic Ventilator…I do not think that you have sufficient enough intake ventilation to properly allow the fan to expel the air as required. I will try to look that up for you.

If you don’t mind, I would like to relay your thread on a Building Science Forum that I participate in and relay advice regarding proper ventilation techniques to other contractors and home builders.

Ed


#8

How does this help with total attic ventilation? Adding more intake only makes the situation even further unbalanced. Regardless of the amount of intake sq. footage, the exhaust is still limited by the 9 ft. ridge vent. Right?


#9

Total NFVA is the key.

Yes, a 50/50 or weighted 60% Intake to 40% Exhaust are traditionally prescribed as being the best for total balance, but Intake Vents also do operate at Exhaust vents and one of the more studious researchers has clearly stated, that given a choice, he would choose continuous intake ventilation with no so-called exhaust ventilation to suffice with.

That does not make sense at first glance, but when you add wind and fluid dynamics to the windward side, pressure is being exerted on those under soffit continuous vents, which will obviously need an exit portal, which would be on the leeward side of the structure.

By adding continuous strip soffit ventilation providing 9 square inches per lineal foot, the direction of the wind would never matter.

Ed


#10

Add some hip vent near the ridges if possible, this will give you more exhaust.

A different ridgevent like the already mentioned ShingleventII or GAF snow country should help also.

A power vent is an option, but you would need to remove the ridgevent for it to work correctly, and they burn out.


#11

Ed, not a bad idea. But that only works on a windy day.

Axiom, you don’t need to remove the ridge vent. Check the video.
roofingcontractorreview.com/Roof … ation.html


#12

[quote=“dennis”]Ed, not a bad idea. But that only works on a windy day.

Axiom, you don’t need to remove the ridge vent. Check the video.
roofingcontractorreview.com/Roof … ation.html[/quote]

Dennis just because I haven’t argued with you about your thoughts on ventilation (lately) doesn’t mean that I agree with you.

I think that you are wrong.
I feel that the link you post as proof is wrong.
My personal experience with ventilation issues is in line with the conventional wisdom.
Your ideas about ventilation apply to special cases only.
With all due respect.

I read it on the internet so it must be true. :roll:


#13

Just to support my previous comment regarding the usage of More Intake Ventilation, even to the point of excluding Exhaust ventilation, I think we will all agree that the generalized opinions from William Rose, of BRC, the Building Research Council at the University of Illinois, has some legitimate standing as one of the peers in attic ventilation studies.

Here is the link to one of the articles purporting Rose’s opinion regarding the significance of Intake Ventilation.

oikos.com/esb/30/atticvent.html

Below, is the pertinent excerpt from that page linked to, which comes just below the paragraph labeled Soffit Vents and the building attic diagram.

*Soffit Vents

**Rose considers soffit vents to be very important.

“If a roof had only one type of vent device, I would choose soffit vents,” he says, “because they work well as inlets and outlets.”**

There’s less problem with rain and snow getting in, because soffit vents point downward. To get maximum protection, locate soffit vents as far out from the wall as possible. If rain or snow blow into the soffit, it’s less likely to soak the insulation or drywall.

Soffit vents should always be installed whenever there are high vents. High vents, on ridges or gables, will pull air out of the attic. Without soffit vents, make-up air would be drawn through the ceiling, which increases heat loss and adds moisture to the attic.

Because of the importance of soffit vents, Rose believes that at least 50 percent of the vent area should be low on the roof.*

Now, there may be situations where additional exhaust ventilation must be obtained through the induction of hip vents on the structure. Several ventilation manufacturers will allow this per their specifications, but I would do this only with the understanding and caution that this goes against thew grain of precedented anecdotal conventional theory and the majority of specifications from most manufacturers. But, it can work in the right circumstances. Personally, I feel very comfortable with this being done on a vaulted/cathedral ceiling structure versus an open attic area space.

Dennis, I respectfully disagree, but I do give you credit for striving to prove your point and go through the effort to assimilate the theorized conditions. I do most respectfully believe that your model would not pass any scientific method as a typical modeling of a real life home, but dang, I just love that you are willing to go out on that limb and defend your perspective to such great lengths.

Ed


#14

Now I know how Galileo felt. :slight_smile:

We respectfully disagree.

You feel that my model and calculations are wrong, yet you can show me no evidence to support the “short circuit” myth.

Ed, I am aware of the report on intake ventilation. But how does that help the house in question on a calm day?


#15

[quote=“Mono”]

  • 1200 sq. ft of attic floor
  • Hip roof
  • 12 soffit intake vents sized 16â€Â[/quote]

#16

just a thought,was there underlayment installed with new roof?


#17

If by underlayment you mean felt paper, then yes, there was 15lb. felt paper installed under the shingles. There was also an ice & water shield installed on the lower part of the roof.

To DaveB - I like your comparison of the hip roof to an upside down funnel. That makes sense. I don’t see how adding a two or three static roof vents as close to the ridge as possible is going to ruin the stack affect. Good suggestion.

To all - I love the feedback and energy. Please keep it coming. We can figure this out.

To Ed - I respectfully disagree that simply adding more soffit intake is going to solve the problem without doing anything to the exhaust side. My issue seems to be worse on days with very little or no wind.


#18

Not enough venting or material problem


#19

Thinking about this a bit more…you are in cold country and the temperature difference between the attic and outside is not much so you might have a weak chimney effect even with more ridge venting? Maybe one of those solar powered ridge vents would be a good option? Seems like some mechanical help might be useful. I’ve never seen the solar vents in person so what do you roofers think? Are they snow proof? I live in California where it never snows so don’t need to know these things :>) …DaveB


#20

I’m glad you mentioned this. I have wondered how the stack effect works if my attic temp. matched (or was very close) to the outdoor temp. To make matters worse, we had very little wind lately. I don’t see how the attic air would turn over (and remove water vapor) from the thermal effect if you have good attic insulation. The heat may not get through the insulation from the living area below, but some moisture will always be able to migrate into the attic.