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Attic Ventilation, insulation and window condensation

Jeffrey410's picture

Hello Experts,

Just wondering the following:  can excessive attic ventilation and sub-adequate roof insulation (R25 in a Zone 5 northern climate) be the cause of our windows having condensation issues?   We have what are supposed to be "high end" double hung high energy efficiency windows but have had major window condensation issues (especially on the 2nd floor) ever since our addition was built in 2001.

Also, in terms of excessive attic venting, I mean there are two large gable end vents (16' x 30") at both ends of the roof, plus fully functioning eave/soffit venting all along both sides of the 30' long roof.   Any thoughts would be appreciated.

Cheers,

Jeff

Eastern Ontario, Canada 

Unlikely.  Window (post #207481, reply #1 of 39)

Unlikely.  Window condensation is purely a function of the air temperature on both sides and the indoor humidity level.  Poor attic attic insulation and "excessive" attic ventillation would tend to reduce indoor humidity.  Of course, it will also tend to reduce indoor temperature, but presumably the furnace would kick in to compensate.

R25 for zone 5 is below recommendations, but not outrageously so.

It sounds like your house may actually be too tightly sealed, or else you have some unusual source of moisture inside (presuming you don't run a humidifier).


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Your attic may not be (post #207481, reply #2 of 39)

Your attic may not be ventilating well. Despite open, continuous soffit vents, the gable vents may not be allowing lost heat to escape. Gable vents don't let enough air pass, it will take the path of least resistance and may only ventilate the areas near them rather than the entire attic the way ridge vents can. R25 isn't enough insulation to keep heat loss down. You should add more insulation R38 at the minimum and it has to be done well.

Do you have ice dams? What is your heat source? How many people and pets? What is the temperature in the attic and the RH? Hang a hygrometer and thermometer about mid way in the attic. The temp and humidity should be roughly equivalent to the outside, if not, you are losing too much heat to the attic and it isn't moving out. Very common with under insulated and poorly ventilated homes, it can even spawn the growth of mold, wet the existing insulation so it's much less effective and cause excess condensation.

Beat it to fit / Paint it to match

Jeff (post #207481, reply #3 of 39)

Even high end dbl glazed windows will show condensation on the bottom half inch or so with temps in the teens.  Most often-higher humidity in the room or lack of air wash across the windows.  Those with radiant heat often will find this happen.  Same goes for homes with concentrated areas of higher humidity-lots of plants in a room for example.  Not well vented bathrooms and kitchens are another.

What kind of heat in the addition?  Drapes or blinds fully covering the glass area?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Condensation is caused by (post #207481, reply #4 of 39)

Condensation is caused by excess moisture in the house. Attic ventilation and insulation have nothing to do with it.

I agree with the other (post #207481, reply #5 of 39)

I agree with the other replies--the condensation is not a result of attic ventilation or inadequate insulation.

Is the condensation on the inside surface of the windows? By that, I mean you're not seeing condensation forming between the panes of glass, are you? If that  were case, it would mean defective seals between the panes, which would allow moist air to intrude into the space.

Condensation on windows. (post #207481, reply #6 of 39)

Condensation will form on an average double glazed window when the temperature outside drops below 36F.

High humidity is caused by washing, cooking, breathing, sweating, fish tanks, indoor plants, animals, drying towels etc indoors.

You can deal with high levels of water vapor in the air by, opening a window, water vapor is programed by nature to always head for the nearest cold surface, or cold area - it is nearly always colder and drier outside. You can buy and use a dehumidifier, this brings a cold coil indoors that will attract the water vapor in the air, this will then condense into a container or may be routed outside by a gravity pipe.

Its a good idea to have exhaust fans in the kitchen and bathroom to expel water vapor, however, expelling this warm wet air means that cold dry air will be pulled from outside pushing up your heating bill - if you can afford it, buy exhaust fans fitted with heat exchangers, these will preheat the incoming cold air, taking the heat from the out going air, much reducing your heating bill.

I don't know how you can say (post #207481, reply #8 of 39)

I don't know how you can say that condensation starts at 36F.  It's 13F right now and the 35-year-old double-glazed window I'm sitting at has no condensation.

Water vapor isn't "programmed" to move anywhere -- it moves with airflow.  What does happen is when the temperature near a cold surface drops below the dewpoint of the air you'll get condensation on the surface.  The condensation is prevented by keeping the air temperature higher or the dewpoint lower.

Homes older than about  25 years generally are not tighly enough sealed to cause condensation problems unless there is something (a cook boiling down maple sap or a teenager taking 45 minute showers, eg) that is adding a lot of moisture to the air.  Newer homes, however, are usually more tightly sealed and may need active ventillation to keep humidity down when closed up.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Nonsense (post #207481, reply #10 of 39)

Perry525 wrote:

Condensation will form on an average double glazed window when the temperature outside drops below 36F.

I don't know............ (post #207481, reply #11 of 39)

davidmeiland wrote:

Perry525 wrote:

Condensation will form on an average double glazed window when the temperature outside drops below 36F.

 

Maybe in a greenhouse.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


This was a very common (post #207481, reply #7 of 39)

This was a very common problem in my area when we first started to increase insulation in attics but before continuous ridge and soffit vents were being used, 1970s. Homes were having excess moisture, condensation on windows, wet attics, mold growth. A lot of homes were using plastic sheathing for vapor barriers on the ceiling and many blamed this, pulling back insulation and cutting the plastic, to no avail. Insulation was often only 6" but additional to 12" still was having problems. Once we started to properly ventilate the attics and were more careful with sealing air leaks and going to a minimum of R38, the problems stopped. Studies at the time were showing how air in the attic wasn't ventilated by gable louvers, making for a warm attic where moisture vapor could condensate. Ice dams were also common causing additional damage to homes and much frustration for the owners.

You can drive around town a day or two after a snow storm and look at roofs. On many, you can see where stair cases, recessed lights, knee walls and other areas lack adequate insulation. You can see rafters since they have a little more snow on them than the area next to them. Ice dams are also common. At the same time, you see roofs that have much larger amounts of snow on them, no dams, no visible differences. These are the homes that are properly insulated and ventilated. Some attempts to cure the symptoms rather than the problems are also evident, particularly heat wires. Here is a picture of a particularly bad situation, Poor insulation, no venting and heat cables, makes quite a mess. This could easily be cured.

Beat it to fit / Paint it to match

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Condensation on windows is a (post #207481, reply #9 of 39)

Condensation on windows is a different problem from ice dams and ceiling stains.  Attic ventillation has very little effect on the humidity inside the house and is unlikely to affect window condensation.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Window condensation (post #207481, reply #12 of 39)

Thanks everyone for all the feedback.  Since we are constantly recording low moisture levels (25-30%) in rooms where we have heavy window condensation, I'm really scratching my head.  The only thing I can isolate as a possible factor is that these rooms on 2nd floor are always 2-3 degrees Celsius colder than rest of house.  These rooms are also the furthest away from our furnace room.  I'm stumped!

You get condensation when the (post #207481, reply #13 of 39)

You get condensation when the temperature dips below the dewpoint.  The temperature in the middle of the room may be 20C, but near the window it may be much colder.  (And, BTW, your standard dial humidity meter is incredibly inaccurate, and only really valid above 20C.)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Furthest from furnace? (post #207481, reply #14 of 39)

Try this.

Take an oscillating fan and put it in the room on low or medium.  Windows uncovered.  What happens?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Physics (post #207481, reply #27 of 39)

The reality is that your window glass is below the dewpoint of the air that's next to it. If you really want to measure the whole scenario, you can, but it will just confirm what is already known--the humidity in the air is high enough and the window is cold enough that you are getting condensation.

Standard moisture investigation for this sort of thing involves looking at several factors.

One is, where is the humidity coming from? Possible answers include: the basement (concrete in contact with wet soil), normal living (people breathing, sweating, showering, cooking pasta, line-drying laundry indoors, and more), an unknown water leak into a wall, attic, duct, or other building area, which is evaporating into the house, a humidifier on the HVAC system, a large collection of dogs, fish tanks, and plants, a ventless gas heater or a gas heater that is not exhausting to the outdoors, and so on.

Another is, what is the moisture content of the outdoor air and can it be used to dry the interior? In general, in cold climates, the answer is yes, the outdoor air is dry and it can be used to dry the interior, by ventilating the house. 

You look for these things by physical inspection and by using thermoi-hygrometers. It sounds like you have a thermo-hygrometer because you are reporting 25-30% humidity, but I doubt it's accuracy unless you bought a nice, professional tool like a Fluke or a Testo or something similar. I have seen hardware store hygrometers that were 10% off, so your measured 30% could actually be 40%.

I always want to know if people are using the bath fans and the range hood. I usually have to just ask and trust what they say. I also want to know if those work, so I measure the airflow they create. It is common to find bath fans that barely work. It is also common to find them ducted into the attic. It is common to find range hoods that barely work, or worse yet, recirculate instead of ventilate. Sometimes the dryer vent is not moving all the exhaust air outside.

I also want to know if the house is tight or not. For that I always do a blower door test. It is almost always highly informative. One thing to check during a blower door test is air leakage around the window sash and the window units themselves, which can help create very cold micro-climates right next to the windows.

The bottom line is that there are only a few things to do. You can remove sources of humidity if possible (repair roof leaks, wet basements, etc.). Source control is #1. You can remove humidity from the house by ventilating it or using a dehumidifier. You can increase the temperature of the surfaces that are now condensing by heating those surfaces. In the case of windows, you can sometimes keep humid air away from them by covering the interior side with an airtight interior storm window (check out indowwindow.com for an example). 

So, look all over for sources of humidity. Figure out if ventilation will help. If so, figure out whether you have installed equipment that will do it. You may have enough vent fans and you just need to run them more. You may need more air into the bedrooms from outside, in which case you could open a window slightly, install a passive air inlet, install a supply-air ventilation system or HRV system. My prediction is that you solve the problem with more fresh air. It might be as simple as running one bath fan on a timer for several hours per day.

This may help you understand (post #207481, reply #15 of 39)

Dew Point Calculation Chart (Fahrenheit)
   %RH     AMBIENT AIR TEMPERATURE IN FAHRENHEIT
       20     30 40 50 60 (70)8090 100 110      120
   90  18     28 37 47 57 67 77 87 97  107      117
   85  17     26 36 45 55 65 75 84 95  104      113
   80  16     25 34 44 54 63 73 82 93  102      110
   75  15     24 33 42 52 62 71 80 91  100      108
   70  13     22 31 40 50 60 68 78 88   96      105
   65  12     20 29 38 47(57)66 76 85   93      103
   60  11     19 27 36 45 55 64 73 83   92      101
   55   9     17 25 34 43 53 61 70 80   89       98
   50   6     15 23 31 40 50 59 67 77   86       94
   45   4     13 21 29 37 47 56 64 73   82       91
   40   1     11 18 26 35 43 52 61 69   78       87
   35  -2     8  16 23 31 40 48 57 65   74       83
   30  -6     4  13 20 28 36 44 52 61   69       77
               At Sea Level (14.696 psiA)

from the above you can see that at a temperature of 70F and a relative humidity of 65% the dew point
is 57F.
at 100F and 90% its 97F
at 32F and below the air is to all intents dry.

Perry (post #207481, reply #16 of 39)

It may be easy for you to understand,

but........

you've managed to confuse the heck out of me.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Help me to understand. (post #207481, reply #17 of 39)

Our house-Radiant floor heat-one great room ceiling fan on low.  Big glass on south side.

Humidity level inside-35%

Exterior temp-30F

inside-easily 70F.

Do the windows have condensation on them-

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Current (post #207481, reply #18 of 39)

Indoor-70F

out-18F

Humidity inside-32%

 

Cond. on windows?

Windows-Marvin '88 lowe dbl. pane.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


What's the temperature of the (post #207481, reply #19 of 39)

What's the temperature of the glass?  If the glass temp is below 39F you have condensation.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Well ok......... (post #207481, reply #20 of 39)

it must not be below 39 degrees, because there's not a drop.

 

So, a while ago I wrote that if your humidity isn't above 35, then the glass won't be wet or frosted on the bottom half inch unless it's real cold outside-and evidently that's below 18- that my science wasn't too far off?

while I think Perry said something to the effect that any glass would have moisture condensing on the surface in the low 30's outdoor temp.

Which one is correct according to all those numbers in the chart?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


The hardest part is guessing (post #207481, reply #21 of 39)

The hardest part is guessing what the actual glass temperature is, since it depends on the details of airflow inside and out, in addition to the characteristics of the window itself. 

Though there's also the issue that relative humidity is fairly hard to measure accurately.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I have an old Honeywell (post #207481, reply #22 of 39)

I have an old Honeywell humidistat that I keep around because it has this nice scale on the front, giving suggested humdiity based on outside temps.  I found that it worked fairly well for our house (1976, 2x4 walls, standard insulated glass), producing a setting that would sometimes result in a bit of corner condensation, but nothing serious:

temp humidity

-20    15%

-10     20%

0         25%

+10    30%

+20    35%

Above 40%


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Another factor that can lower (post #207481, reply #24 of 39)

Another factor that can lower the temperature of the inside pane of glass is if the seal between the panes is defective and allowing cold outside air to enter the airspace between.

Where is the moisture coming from? (post #207481, reply #23 of 39)

I'm pretty sure attic ventilation has little to do with this issue. The question no one has really asked is where the water is coming from. You say this is an addition, so I assume that construction is modern and has decent moisture control issues. However, what about the older part of the house? Tell us about the basement of both houses - Basments and crawlspaces are generally the only sources of moisture large enough to create ongoing issues in other parts of the house. And yes, I get that the condensation happens on the 2nd floor. I've seen roof sheathing that rotted from basement moisture.

Andy

Senior Editor, Fine Homebuilding

Condensation = again? (post #207481, reply #25 of 39)

Lets spread more confusion?

Every day the sun lifts millions of gallons of invisible

water vapor from the seas and land. We next see it as

clouds, rain, sleet, snow, fog, mist, frost.

 

Every day billions of people add their own water

vapor to the total.

 

We each add about 14 cubic feet of water vapor to

the air around us every 24 hours by breathing and

sweating - plus a lot more in cooking, washing,

driving etc.

 

We only usually see water vapor as steam from

boiling water, our truck exhaust or our100%

saturated breath on a cold day.

 

As you can see from my last post, water vapor

condenses on the nearest cold surface, that is below

dew point.

 

If you buy an infrared temperature gun you can easily

point and read the surface temperature of any object.

 

Water vapor sits in the gaps between  pieces

of air, slipping about moving towards areas of low

water vapor content. We are continually moving

about producing more water vapor wherever we go,

leaving a trail of high concentrations of water vapor.

Which is then moving, as it adjusts to the immediate

lower air temperature. (No air movement required.)

 

There is hardly ever a room with but one temperature

or a consistent level of humidity. Our body temperature

is normally a lot higher than that of the air round us,

the water vapor we produce is higher in concentration

so there is constant movement of both warm air and

water vapor away from us.

 

A window recess is often the coldest place in a room.

During the day when warm air is continually arriving

and dropping down the room side of a window, the

double glazed window (if fitted correctly)

usually stays above dew point.

 

At night when the heating is turned down or possibly

off and there is a body or bodies producing water vapor, the air

temperature drops, its ability to hold the same amount

of water vapor also drops and the excess water vapor

condenses on the now colder window.

 

If the curtains/blinds are closed this quickly removes

what warm air circulation they may still be in the room,

the temperature between the window and blind

drops further and you get heavy condensation.

 

A treble or quad glazed window rarely has condensation

especially if it contains one of the inert gases.

Any help!

Any help? (post #207481, reply #26 of 39)

If you addressed your post to me, then help-sort of.

I understood the sources etc and much of what you wrote above-understood that a while ago.

I'm just a dumb carpenter and I'm trying to bring this discussion back to the original question-not using science with graphs and tables, but using observation.

You wrote:

Perry525 wrote:

 

Condensation will form on an average double glazed window when the temperature outside drops below 36F.

 

I and Meiland responded that no, it doesn't. 

Then you posted the table with numbers, which I suppose is a scientific page from somewhere.  That it's all true is not my question.

I asked why above the temp outside is 18, the inside humidity is 30-35 and there's no condensation on the glass.

dbl pane '88 marvins with lowE and Argon (don't think I mentioned that originally).  Temp inside-70.

Now I guess you are saying to me that the glass is too warm.

 

Because, by observation-my glasses fogged up upon entering this aftn after I had been out for an hour or so.  They fogged up way more regularly at the job I was on today..........but the glass in our windows are is not wet.

So, I assume from what you are saying-my house is "dry" at 30-35 % humidity and the one I was at today is higher humidity because the glasses fogged everytime I came back in, being out only a very short while- retrieving from the van.

To apply this to the original post-would you not say that he has too much water vapor in the addition?  I think most here have said that.  The big question is from where.   None of the info about car exhaust and a crowd of people has anything to do with it.   There is no car in the house and tho he didn't mention it-I don't think he's talking about the night he had a party.

After all that, I don't even know what I just wrote................

 

Wait, I know what I'm saying.  The poor basturd(sp) has too much moisture in the addition and he wants to know where it's coming from-The why, is a given.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


Wait, I know what I'm saying. (post #207481, reply #28 of 39)

Wait, I know what I'm saying. The poor basturd(sp) has too much moisture in the addition and he wants to know where it's coming from-The why, is a given.

QFT

Andy

Senior Editor, Fine Homebuilding

Andy (post #207481, reply #29 of 39)

I'm no text-er.

What's QFT

Quit [JOBSITE WORD]'n talking?

Quid Fro Toe?

 

I'm stumped.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


QFT (post #207481, reply #33 of 39)

Quoted For Truth.

Andy

Senior Editor, Fine Homebuilding