Roof Sheathing

[Kid]

My house is 31 years old. We have lived in it since 1989. In the late 90's I had a second layer of shingles installed. I am in the process of deciding if the time is right to have the old layers removed and a new roof installed. I had a roofer come out who said that in his opinion the roof has at least five years left on it and he saw no reason to spend the money yet. I appreciated his honesty. Besides, our plans are to move in 2-3 years.

When we moved in 21 years ago it was brought to my attention that there was some darkness in the attic on the sheathing that it on the north side of the house. I know that this is caused by moisture, so I had some additional vents and an attic fan installed as well as additional insulation blown in. Over the years, from time to time (on really cold winter days) I sometimes see some condensation on the sheathing only on the north side of the house. (We live in northern Illinois.) In fact, the sheathing doesn't look much different than it did 21 years ago. There is nothing at the top and and it gets a bit darker as you look down towards the eave. It is structurally sound.

My question is.... The roofer said that the darkness is a type of mold. He said it was nothing to worry about, but when it's time to reroof, the sheathing should be replaced. I have no problem with replacing it when the time comes, and I'm not freaking out over the "m" word. However, since it's been this was for probably the life of the house, I'm questioning his diagnois. Any thoughts?

[pushkins]

It may well be mold. When you add roof venting you MUST add soffit venting, the ratio is approx. 1":1". that means for every 1 square " of exit air you need 1 square" of entrance air, without this you could cut a hole 3'x3' in your roof and never get any better airflow than a hole half the size. As the hot air leaves it needs to draw in air from the soffit to replace.

It could well be that the area your seeing mold is not getting a very good airflow, you should NEVER see condensation in your attic.

You may have inadvertently covered your soffit vents with insulation, there should be baffles between insulation and underside of the roof deck to stop insulation covering air paths.

[Kid]

Hi Pushkins! Thanks for the reply. When we moved in 21 years ago I added 6 soffit vents to the side of the house that I'm referring to. (It's about 24 feet long.) I also have 3 square rooof vents and an attic fan on that side near the ridge. The insulation contractor at that time also added baffles, although they are abot 12" wide and the rafters are 24" on center. To my knowledge the vents are not covered, because when I see the condensation on cold days there isn't any directly above the baffle openings. Maybe I should add wider baffles? (A tight job, but it wouldn't killl me.) More soffit vents? If I take the time to add wider baffles now they will just have to be redone when/if I replace the sheathing in a few years.

The house is a rasied ranch, and the other 3 roof sides are fine. It's only the north side that gets condensation.

[pushkins]

The baffles should be as wide as the rafter/truss spaces, if not insulation (especially blown in) can work it's way past and down onto the soffit, see if you can squeeze on down there and look just to make sure this isn't the case.
Usually you can see light coming up from outside between the rafters/trusses where the vents are, if you can see this then they are usually clear of obstructions.
A way to test for airflow is to take an incense stick up there, hold it up closer to the underside of the roof deck and watch where the smoke trail goes, it should be fairly rapid and obvious in direction from soffit to upper roof vents.

The baffles wouldn't need to be changed when you redo your roof they are basically wedged between rafters/trusses right down at the bearing edge of the wall to rafter/truss, all they do is keep insulation from covering your soffit vents while keeping it away from the underside of the roof deck thus allowing an air passage.

Do a quick mathematical calculation, add up the square inches of soffit vents (for example a 10"x4" vent = 40 square " then divide this by .80 = actual square ") The /.80 allows for screening and louvers etc...) and x this by the number of soffit vents.
Then do the same for the fixed roof vents, then look at the powered vent they range in size from about 1100 sq. F to 1750 sq. f / unit.

What you are looking for is 1 sq. foot of intake (therefore 1 sq. foot of exhaust) for every 150 sq. foot of area.

The biggest problem with adding powered vents when standard vents already exist is that if the intake volume isn't equal to what the fan is pulling out then the fan will force air to be drawn in via the upper standard vents as well, this kinda circumnavigates the airflow from lower to upper.

[manhattan42]

"you should NEVER see condensation in your attic."

Careful.

That is not true.

Even properly vented attics can still see condensation and do.

Condensation is a function of relative humidity of the air and surface temperature.

It is quite possible (and in fact happens all the time) where properly ventilated attics have occaisional problems with condensation and mold.

Why?

Surface temperatures are not constant in attics and dew point can be reached with as little as 1-2 degrees F variation from ambient air temperature depending on relative humidty.



See the Dew Pont chart:

Google Image Result for http://www.eaglecoatings.net/content/DewPoint.jpg

The reality is that condensation occurs all the time in attics regardless of how they are ventilated.

Keeping attics well ventilated can merely reduce the chances of condensation forming over long continous periods.

[Kid]

Thank you both. (I was out of town or else I would have thanked you sooner.)

Someone else also told me that it can sometimes happen, especially since it has always been only on the north exposure. Regardless, I am going to replace the bafffles with larger ones. (I'm really looking forward to that job. )

Pushkins, based on the 150 square feet formula, I could use more vents, so I'm going to add some in the soffit. (I have about 580 square feet in the attic in question and, by my calculations, I currently have 2 sq ft of vents in the soffit and the same on the roof, including the opening for the fan.) I doubt I'm going to mess with more roof vents now, since the roof will be replaced sooner more than later. I'll do it then.

The power fan is a summer (heat issue) and it works well.

When we bought the house 21 years ago, it had zero soffit vents. In addition to the ones on the north side, I also added some on the lower side (east and west slopes of the raised ranch). While I'm sure I could use more, I have never seen condensation there other than one very small section. It's only the north (25% of the roof) that is driving me nuts. (Yea, I know, my wife says I'm nuts too. I just like things to be right.)

In any event, thanks again to you both for your thoughts. I value your input.

[Kid]

Pushkins and Manhattan 42... You were both very helpful with your input, so I thought that I would give you an update.

Last month I removed, inspected, and cleaned the soffit vents on the north side of the house. While doing so I could plainly see that there were no obstructions to the current baffles.

We've had some very cold days here in northern Illinois lately. Below zero at night and highs only in the teens. Weather report said we had 70% humidity last night. It's been sunny during the day, but the sun isn't high enought to hit the north side, so it is still snowcovered.

Today I went up into the attic. It was very, very cold up there, so I'm convinced that warm air from the house is not escaping past my R38 (or more). Once again, I saw condensation on the sheathing only on the north (no sun) side.

Lets see... Clean and unobstructed soffit vents, unobstructed baffles, more than enough insulation, extremely cold attic, humidifier not yet turned on, and no showers taken in the house in 3 days. (Only two of us and we shower where we work out in the morning.) I give up!!!

I was thinking about adding wider baffles, but upon looking again, I think only a skinny 15 year-old could do it. Besides, I'm not sure that it would help, as I can see condensation on the sheathing within the current, unobstructed baffles.

Oh well. The house is 30 years old and the conditions have always been this way (or worse before I moved in.) In any event, when I reroof I'll replace the square roof vents for a ridge vent. Perhaps I'll also put a humidistat on the power attic fan. Think that will help?

(I'm really not as obsessed as I sound.) Manhatttan42: I'm thinking that you're correct when you say that even properly vented attics can get condensation.

Thanks again guys.

[Kid]

By "condensation", I mean frost.

[pushkins]

I was just thinking about a couple of things you might want to look at.
1. Are you sure nothing is venting into the attic space, like a bathroom fan or dryer etc... ?
2. Have you insulation over the top of any attic access hatch or pull down stairs or on the reverse of a door access to the attic?

[Kid]

Hello Pushkins!
1) Regarding the bathroom fan... I checked this originally. It's vented to the outside and is unobstrcuted. The dryer is elsewhere.
2) The access hatch to the attic is insulated. As I mentioned, I was quite surprised when I discovered how cold it was up there.
Thanks for the ideas/input.

[pushkins]

Have you considered adding another powered vent (with a humidistat) upwards on the roof section in question, how close to the section in question is the existing powered vent ?

[Kid]

The existing power fan/vent is on the roof side in question. It's temperature controlled for summer ventilation. Can I add a humidistat to it or is a second fan needed. Thanks.

[pushkins]

You can add it to the existing fan, the link I've added shows a thermostat/humidistat combo unit specifically for attic fans. They are not cheap and you may well find one elsewhere a little cheaper. They wire in just like your existing thermostat.


Lomanco Online Parts: Thermostat / Humidistat

[Kid]

Thanks. I am definitely going to look into this. It was cold again today (low 20's), but the humidity must not have been as high. When one thinks of humidity, it's normally associated with summer/heat, but in Chicago the average morning humidity level during December is 80%, and lower in the afternoon. It just isn't as noticeable because of the cold. I took a look in the attic this evening and saw almost no frost at all. Thus, it must be an outdoor humidity thing.

[pushkins]

Like manhattan mentioned in a previous post it's all about the "dew point" inside the attic would/should most likely be warmer than outside temps. even with perfect attic insulation restricting house heat getting into the attic space.
However IMHO I don't find constant attic condensation acceptable, installing a humidistat would force the fan to work when the moisture levels in the attic reach a preset level, this would force draw air in from outside.
I guess on the good side (if there is one...lol) seeing ice on the inside of the attic proves you have insulated the attic space well with obviously very minimal heat loss from the living space below.