Over-insulating and condensation

[wbednarz]

Last night I went into my attic to bring something downstairs and I noticed that a lot of condensation had formed on the windows. That was an immediate red flag for me that something is wrong.

My house is a little over 70 years old. All the walls are plaster and the first floor exterior is stucco (the second floor is wood shake). I would guess is that contributes to a tight air seal for preventing air and moisture penetration through the walls.

This past summer I replaced all of the windows in the home. The job wasn't complete until yesterday though. Last year was the first year my wife and I lived here and we decided we had to replace the windows because of all of the air infultration through the windows. We replaced 26 of them, but the attic windows didn't get replaced yet. Yesterday I was placing additional insulation (fiberglass and expandable polyurethane foam) around the windows and soon I will be putting in the molding. The additional insulation made a tremendous difference on the air infultration.

Now I am wondering if I created too tight of an air seal on the home. My fear is that moisture may build up within the walls and cause the frame to rot.

I didn't find any condensation on the windows inside the living space, but the windows are double pane and the interior pane remains warm enough for vapor not to condense. However, since the attic windows are single pane, vapor will condense on it because it is cool enough.

We have had a lot of rain in the past couple of months (I live near Philadelphia). The roof is slate, and it wasn't leaking the last time I checked.

I guess I would like to know if I am over reacting, or should I be taking some action before some damage occurs or mold and mildew starts to grow on the walls?

[Randall]

>Last night I went into my attic to bring something
>downstairs and I noticed that a lot of condensation had
>formed on the windows. That was an immediate red flag for
>me that something is wrong.

You have windows in the attic? hmm, interesting! every attic I've ever see was alway a dark windowless cold cave.

Here is my take, unless you have some kind of finished former attic now converted to living space;

The attic is the area that needs to be vented to allow all the moisture that rises through the ceilings and walls to escape, so you will see insulation on the attic floor but not in th e roof rafters. There would also be louvered vents and eave vents, maybe even additional vents on the roof.
ALL of those vents need to be open, a common mistake is filling the attic with so much insulation those vents are blocked either by that or accumulations of dust and dirt.

Moisture condensing or frost on wood in the attic indicates you either are putting out a lot of excess moisture and/or there is not enough venting. SOME is normal but you shouldn't see rivulets of water or frost all over.
Look at reducing moisture (cooking, washing, laundry, hot tub/spa, showering etc all produce massive quantities as would a humidifier )

I believe the living space should run around 70% RH from what I gather, so if you find it is frequently say, 80% then that's too much.

>My house is a little over 70 years old. All the walls are
>plaster and the first floor exterior is stucco (the second
>floor is wood shake). I would guess is that contributes to
>a tight air seal for preventing air and moisture penetration
>through the walls.

You really don't want moisture going thru the walls ANYWAY, because it's the main reason paint pops off as well as wall rot.
What they find is if vinyl siding etc is put on and no venting is put in, the stuff acts like saran wrap, the moisture migrates thru the walls and can't get past the siding which is cold, it condenses and freezes and builds up. The result is your walls are saturated.

Same deal if you like cold air conditioning in the summer, and the barrier is on the inside, the hot moist air from outside condenses on the cooler interior walls but this doesnt appear to be as much of a problem as the winter.

I like to think of a house in terms of it's being a large bathtub full of water, except the "water" is air instead, if you start to think of it that way you can immediately visualise things.

For example, you fill a large tub or container that has pin holes and some cracks in it, and watch the water just flow out, it's no different for the house where there are holes, holes for utility lines, holes for outlets and switches, cracks in the walls, vents, the air drains out just like the water .
You can't see the air leaking out but just like the water in the tub or container with holes, it does.
So when I put up a vapor barrier inside, I treat the walls like it's a tub of water, so I use caulk and make sure cracks are filled and the plastic covers everything.

>Now I am wondering if I created too tight of an air seal on
>the home. My fear is that moisture may build up within the
>walls and cause the frame to rot.

I don't see a problem as long as the attic is properly vented, warm air rises, and the majority of that moisture will exit thru the ceiling, paint is NOT a moisture barrier, so it will go out that way.

>I didn't find any condensation on the windows inside the
>living space, but the windows are double pane and the
>interior pane remains warm enough for vapor not to condense.
> However, since the attic windows are single pane, vapor
>will condense on it because it is cool enough.

I would suggest checking the attic vents, again assuming this is unheated no finished space where you can see the rafters and the whole bit, if necessary you might need to add more or larger vents or even a small attic vent fan to remove and circulate the air.

My house was built in 1930, it has the plaster/lath walls and blown in insulation, clapboard siding.
What I am doing now because the inside walls are so bumpy and have multiple layers of wallpaper, is screwing on strips of lumber made from 2x4's ripped into 1-3/4" thick x 1-1/2" wide.

First I put a sheet of 1/2" insulation board up, then screw the strips on. I counter sink holes in them for 4" #10 square drive screws and screw them to the wall directly into the studs. Then I fill that 1-3/4" space with more insulation board, then the plastic vapor barrier, then 1/2" CDX is screwed to the strips, then 1/2" sheetrock.
I plan a 40" to 44" tall wainscotting and wallpaper above, so what I have now is a wall that has had it's insulation value DOUBLED, the wall is even stronger than it was, I have a vapor barrier, as well as a solid wall surface I can screw things to or put up heavy items anywhere, and it is also a sound/noise resist which helps when you have a large pipe organ in the house :)

I can put a palm on the sheetrock wall I just put on, and the other palm on the existing wall surface and I can feel the temperature difference between then immediately. I figure I added about R-10-12 to a wall that was about R-10

You don't want to make the house TOO tight, some infiltration of fresh air is necessary.

If you have central heat, it might be possible, maybe even a good idea to take part of the intake air from outside, so it heats and blows in warm FRESH air taken partly from outside, otherwise it just circulates the same dirty air taken from inside.

[woodchuck1954]

Condensation is caused by radical differences between the interior of your attic and the outside temperture. You actually will experience MORE condensation with summer heat than winter cold. The humanility in the air really is not that much of a factor. Like the previous response, vents are the answer. If its real severe, you may need an attic fan or two, to draw the air OUT.

[jmackay]

Something I didn't learn until recently is that the goal of attic insulation is to separate the heated space from the unheated space (or vice-versa in summer). So the insulation needs to be between the attic floor and the lower ceiling, with the vapor barrier on the lower floor's ceiling side. That is, of course, unless your attic is heated. Lack of ventilation may be the cause of the problem, improperly installed insulation may be another cause. Is the insulation in your attic above your head or below your feet?

[TDHofstetter]

Hey, Wbednarz.

For now, I think you should invest a little time & cash in plastic film for the attic windows. If you're getting condensation up there, that means your whole house is humid - and it's finding cold surfaces to settle on. "Tight-house" syndrome isn't such an issue with condensation unless your house is godawful humid during the winter (as it might be if you heat with ventless gas, which isn't a good idea anyway in a really tight house unless you enjoy the sensation of suffocation). "Tight-house" syndrome is more of an issue with carbon monoxide and fumes of urea-formaldehyde from plywood & carpeting & such.

Insulation in your attic belongs in one of two places: Either on the attic floor or between the rafters. If it's on the floor, the attic must be ventilated to the outdoors. If it's between the rafters, the roof must be ventilated to the outdoors - above the insulation. In either case, a vapor barrier must be installed between the heated space and the insulation. This, among other things, prevents your valuable insulation from getting soaked and matting up and losing about half its R-value. That typically means PAPER SIDE DOWN if you use paper, or PLASTIC FILM if you don't. Although a good latex paint job will serve as a viable vapor barrier, most latex paint jobs aren't effective enough because they don't get a good-enough coverage (because we're all too darned cheap to really do the job well).

If you've been getting a lot of rain, that'd account for the extra humidity in the air. After winter sets in really well, go up & check again to see if frost is forming on those same windows. Better, put up a good plastic film over one window and leave another uncovered. THEN when you go up in the dead of winter you can compare & see what I'm talking about. Have on hand enough clear film to cover the other window when you're convinced. :)

-- Tim --


Carpe Lignum

[wbednarz]

Thanks for all of the help.