Unless there is a roof leak the sources of moisture in the house are pretty obvious and include bathing (steam shower), clothes washing, cooking, overuse or malfunction of a humidifier, moisture penetration through foundation walls, floor slabs, or crawl spaces, and unvented gas or kerosene space heaters. Check these sources out first.

You mention knee walls but call the space an attic. How is the space finished and/or used? Is it heated? Are there windows in it?

Is there excessive condensation on the windows?

Are the attic HVAC ducts properly sealed (not taped) and insulated? Same for the bathroom and laundry exhaust ducts.

Is there a vapor barrier anywhere in the attic floor or roof assembly?

Are the eave exhaust vents adjacent to rafter vents?

If the problem is serious enough the owner could hire Lstiburek's firm, Building Science Inc., to look at it. This time last year their minimum fee was $2,500 which I'm sure would cover this situation. They would not only identify the problem but suggest a solution and probably address other issues in the house.

The third level (attic) is finished and there are knee walls and the front and rear wall (the side walls have windows). I believe there is some condensation on the windows. I doubt the ducts are properly sealed. The attic is heated, but I am not sure if there is a return. The space is a playroom right now, but they are planning to convert it to a bedroom.

No roof leak has been identified. It seemed to me that it was a matter of just too much moisture. But what do you think of the way it was insulated, with no insulation in the knee-wall wall? I knew that was fine if you use foam insulation, but what if you only use R30 batt insulation in the rafters?

There is no vapor barrier on the attic floor. Around here they typically use plastic over the insulation, but I'm not sure it was used here.

Thanks for the help. You brought up a lot of things I hadn't thought of.

I don't see how knee walls could be an issue.

Obviously, there is too much moisture, it's passing into the upper level, and it is condensing instead of passing through to the outside.

I suspect there is a vapor barrier at the bottom of he rafters (faced insulation?) and moisture from somewhere is collecting in the top floor but I don't understand why it is condensing unless the room is not properly heated and ventilated. The lack of a return duct and transfer of moist air from lower levels could be the cause.

After checking out the more obvious possibilities, I would have a pro check the entire HVAC system since it is common for modern forced air systems to be installed improperly with too many flex ducts, poorly sealed joints, too little duct insulation and poorly regulated humidification systems. I see these systems replaced often.

When you say vented at the soffit, I assume you mean vented in the eaves above the soffit. They tried that with us. I can't believe how many builders think that is acceptable. Soffits are intake vents. The incoming air just carries the moisture laden bathroom air up into the attic.

Do you know where the kitchen is vented to? If a cooking hood is vented into the attic, I would think it would be a major source of moisture up there.

It's possible the house needs a, what is it called?, an air exchanger. I know we were told we may eventually need one because out house is so tight (but there are only 2 of us here, not 4).

An air exchanger isn't just for moisture. It also helps remove offgassing chemicals from the house. The newer and tighter the construction, the more important that is. Its the major reason we specified an ERV in our build.

The kitchen is vented out the wall. I don't think that is an issue unless they aren't using the vent (contributing to the moisture issue).

I am confused about what the proper way to insulate in this situation is. I am sure they used kraft faced insulation, and may have used plastic sheeting. I know our town requires the plastic and he have it on all exterior walls and ceiling that are insulated. Isn't that the correct way to do it?

Could the R30 just be inadequate, not keeping the kneewall space warm enough (since the heat goes into the finished attic, and unless the kneewalls are leaky there won't be much heat in there, just the bit that might leak from the ducts), causing the moist air to condense when it hits the coolish air in the kneewall? And I know the ducts are flex ducts, that is what pretty much everyone does around here (we have a mixture, but mostly flex ducts).

You need to look at the construction of the attic ventilation system above the insulation. You indicated they put the channels in and left a gap for airflow, prior to insulating the rafter bay.

In order to get condensation in the attic, it takes two things, Presence of moisture and a condensing surface. Obviously, there is moisture present, and the source will need to be identified, but what baffles me, is that a conditioned attic (heated) remains cool enough to have a condensing surface on the heated side of the insulation.

Have you looked into the insulation and up onto the baffle that was installed? It is my guess that by installing the baffles continuous from top to bottom, that they possibly did more harm than good, particularly if the channels are continuous or sealed. They certainly have a ventilation channel, but you need to consider the reasoning for that ventilation channel. Attic ventilation first and foremost (at least in my heating climate)is to relieve the attic of moisture. The other reason is to relieve the attic of excessive heat in the summer time.

With the insulation at the rafter level instead of the attic floor, they will need to have a continuous(sealed) vapor barrier or a dual purpose air barrier, such as drywall (ADA Approch). Kraft facing is not an adequate vapor barrier.retarder, in this application. Couple this with warming the attic temp, should alleviate the problem, but dont do it by simply warming the attic without the sealed vapor barrier or you will just drive the moisture deeper into the insualtion system.

With the types of temperature differentials this winter has provided us, problems are going to arise, that didnt happen last year, irregardless of how many occupants were there last year.