Attic Ventilation Troubles.


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Old 09-19-17, 01:44 PM
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Attic Ventilation Troubles.

I'm banging my head against the wall so I hope someone can give me some practical advice. I've been in my tow story house for about four years. When we bought the home inspector provided pictures of the attic and the roof decking was spotless.

It was built in about 1963 and I got it from the original owners. After a snowfall our first winter I noticed that ours was the only house on the street with no snow on the roof. Sure enough when I popped my head into the attic I saw very little insulation... I also observed very clean wood. I called a local insulation guy with a good reputation and had maybe 16" of cellulose blown in.

Last summer I just thought to poke my head up and take a look. I was taken back. There was a fair bit of mold on the roof decking. I called a mold remediation guy that family and friends have used in the past. On inspection he said that he could spray the mold to remove it. He said however that the air was not moving up there so that needed to be addressed.

I have three mushroom type roof vents.

The house is about 30'x30' and the insulation guy put maybe 3 or 4 baffles on each side down to the soffits... I was told it wasn't enough. OK - I called the insulation guy back and he said that for my roof what he put in was adequate - but in the interest of good customer service he asked what I wanted - I asked that he put baffles down every second truss. He did. Following that the mold guy came back and sprayed the mold. I checked it out and it was clean.

Fast-forward to about four weeks ago. I popped my head up to check the attic and there was the damn mold again. I called the mold guy who came by and said "Air is not moving up here". Upon some inspection we found that whomever installed the (vented) aluminum soffits, did so right over the original plywood soffits.

As you can imagine I was furious. Every soffit vent that we poked into was solid underneath.

OK - I had a roofing guy over. He pulled back the aluminum and drilled twenty-four, 4-inch holes where the soffits have vents. He also replaced one of the mushroom roof vents with a whirley vent.

Now - Before I get the mold guy back in to once again spray the attic decking, I need to know that I have adequate ventilation.

I installed a thermometer / humidity sensor that has a probe up in the attic so I can monitor it without going up.

Currently it's 26C (79F) here but the attic is reading 44C (112F). The humidity is reading 72%. Outside it's 70%

This past Saturday it was about 25C (77F) outside and the attic was 41C (106F) and 60% humidity inside and out.

We have all kinds of intake and exhaust (apparently) now. These temps seem to be to be very high. I was always under the impression that while attic humidity should be close to what it is outside, the temp should not be much higher than 15F more than what it is outside.

A ridge vent is not an option for me. My roof is basically a pyramid and the ridge is about a foot long.

Both the roofer and insulation guy happened to be there at the same time and assured me that with the new intake holes I would not have future mold problems however I'm very nervous given the temps. I'm considering powered vents but that would be a huge expense for me right now... Wiring up an insulated attic is not something I want to do.....

I'm going to continue to monitor the climate in my attic and I suppose I will need to spray for the mold but would appreciate any suggestions out there.

I'm concerned that intake is still the problem however I don't know how to determine if enough air is going in....

I welcome all thoughts...
 
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Old 09-19-17, 01:52 PM
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It sounds like the blow-in insulation may have reduced the ventilation.
 
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Old 09-19-17, 01:58 PM
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The attic was spotless for about 50 years and the mold appeared after the insulation was blown in....So yes....
 
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Old 09-19-17, 04:39 PM
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Hi Joe, I'm working on your numbers but initial estimates are yes, short on low vent area. More than the zero you had but about 1/3 the recommended amount. Can you find an internet picture of something similar to your soffit vents. Looking for a "net free area" number.

Also, any vent area numbers for the mushrooms or the spinner.

Bud
 
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Old 09-19-17, 05:10 PM
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Did you identify where the moisture feeding the mold is coming from? This is not condensation mold season and it isn't coming in from the outside air.

You can't directly compare inside and outside humidity numbers unless they are at the same temperature. Since that is not going to happen naturally we use a calculator to adjust the readings to the same temperature and then recalculate the humidity reading. Calculator in this link:
Temperature, Dewpoint, and Relative Humidity Calculator
Example from your numbers:
Outside Temp = 79° and RH = 70% gives us a dew point = 68.4°
Attic Temp = 112° and RH = 72% gives us a dew point of = 100.8°
That's disastrous. The moisture content of 112° air at 72% RH is huge, think rain.
If the outside air were entering and being warmed up to 112° its RH would drop to 25%.

So the next question is, where is that moisture coming from? The answer is most likely air leakage from house to attic or an exhaust fan blowing into the attic.

Take some temp and RH readings from inside the house along with new readings from the attic at the same time. That will tell you if the moisture is coming from the house.

What is in the basement? Often basement air is leaking directly into the attic following chimneys or plumbing vents.

Although you may need more soffit venting I would not be concerned about that at this point. Highest priority will be to identify the source of the moisture.

Note, attics will always be warm/hot, 15F we wish. 40 or 50 F is common.

Bud
 
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Old 09-20-17, 06:40 AM
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Thanks guys. Let me get the extra info together and I'll get back
 
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Old 09-20-17, 07:37 AM
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I stuck my phone out the window and snapped these. Behind this screen are two, 4-inch holes. This happens 12 times around the house





The spinner and mushrooms both have 12" opening/stack

Here's a shot of the attic temp/humidity this morning....



21C (70F) and 68% but it's cloudy this morning

The only exhaust fan going through the attic is from an upstairs bathroom. I was on the roof and it's sealed up well on the top end. My next step is to pull the fan i the bathroom to see if it's sealed up well in there.

I had not considered the basement. The original owner in all his 50 years didn't do much. The basement is unfinished. all poured concrete. The dryer vent goes through the wall and out the wall. It's not a damp basement but not to say that air can't rise up from somewhere. Should I be looking into sealing around anything that goes up?

Thanks for all this guys.... I'm going to check on these things and report back. Please keep ideas coming.

Here's my outdoor temp humidity when I took the attic reading.

 
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Old 09-20-17, 08:52 AM
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Use metal flashing and fire rated caulking (or can foam) seal all penetrations through the basement ceiling. Judging the air flow headed up to the attic is temperature dependent. Example, no temp difference little to no air flow.

Take an RH and temp reading in the basement to get an idea how humid it is down there.

Those soffit vents are not very open and there are not many of them. Typical guidance would be 1 ft² of net few vent area (NFA) for every 150 ft² of attic floor. A 30 x 30 house would have 900 ft² of floor space and thus (divide by 150) should have about 6 ft² of NFA vent area in total. That should be divide half high and half low thus 3 ft² is the target.

Your 24 – 4" holes have a total area of about 300 in² or 2.1 ft². But we have to derate that because of the vinyl cover. Very common to derate by 50% or more and I suspect yours would be more. But at 50% you are left with about 1 ft². Better than the zero you had but in the future you might shoot for more. However, I don't think that is the major contributor to your mold.

The smoking gun so far is that attic temp and RH reading, Attic Temp = 112° and RH = 72% gives us a dew point of = 100.8°. There is more moisture in that air than can be coming from outside or your living space. Either a wet basement or some other water source is contributing to that.


Bud
 
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Old 09-21-17, 07:41 AM
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Thanks bud. What would you say is an acceptable maximum dewpoint in the attic?

My project today is to get a really good look a that bathroom fan.
 
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Old 09-26-17, 07:32 AM
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I was able to open up the bathroom vent. The conduit seems sealed up tight. I added some foil tape to the gap between the fan housing/box and the ceiling.

Everything is sealed up pretty tight. The walls and ceilings all have multiple coats of latex paint, the attic hatch is insulated and taped shut.

My last task is to seal up any gaps around stacks etc from the basement.

If there are any other ideas for where humidity might be intruding I'd like the input.

Would it be safe to assume that I'm in good shape if the humidity reading outdoors is close to that inside the attic? That would suggest to me that outdoor air is moving in.... I think
 
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Old 09-26-17, 08:42 AM
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A couple of points.
1. You can't compare RH readings unless the temp was the same.
2. Passive ventilation will never reach equal inside and outside temp readings unless the wind is blowing. It is the difference in temperature that creates the air flow.
3. From your 9/21 post you asked what an acceptable maximum dewpoint in the attic would be. That number varies. Example, during a warm day the attic can fill with warm humid air. When it cools at night the RH in the attic increases as the temp drops and as the temp drops the air flow can drop to minimal or zero. So, a target dew point might be your attic air lowered to the coolest night temp you expect, that's where that calculator comes in handy.

Did you take any humidity and temp readings in the basement?

Bud
 
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Old 09-27-17, 05:33 AM
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Thanks Bud. The basement seems to remain a around 24C (75F) at 60% + or - humidity.... However I've only been monitoring this for the last few days. We've had a recent spell of unusually high temps outside 31C (88F) and the air conditioning has been on so this might be drying out the air in the house.

I'm having a hard time understanding one thing so please pardon my ignorance. If dew point, temps or humidity are not necessarily indicators of problematic attic environments - What factors to you use to determine weather conditions are cause for concern?

I'm willing to take whatever measures are needed to avoid future mold, I just don't understand my target. Is there no way to definitively measure if I conditions are acceptable?
 
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Old 09-28-17, 04:35 AM
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So at a high level what is going on?

If the attic had minimal insulation both heat and water vapor easily pass into attic.

By adding insulation heat transfer is slowed but vapor continues just as it always did but now there is mold.

With the minimal venting it would seem that mold could grow in either situation.

So why now?
 
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Old 09-28-17, 05:27 AM
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"I'm having a hard time understanding one thing so please pardon my ignorance. If dew point, temps or humidity are not necessarily indicators of problematic attic environments"
They are primary indicators of both potential mold growth and the potential source of the moisture.

But only temp and RH are measurements, dew point is calculated from those numbers. We look at dew point when comparing measurements from two different places because temp and RH are inversely related, as one goes up the other goes down, same amount of moisture.

I'll be in and out today, shingling, so will see if I can summarize when I get more time to sit down.

We are getting the same warm dry weather so need to make roof progress while I can. At 70 I'm slow enough without rain.

Bud
 
 

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