Condensation

Published on April 13th, 2013 | by Henri

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Condensation problems in an older house

Q.  I have a house which is over 100 years old. It is a 1-1/2 story wood framed structure in eastern Iowa.

A couple of years ago, I had cellulose insulation blown in from the outside of the house. I have also replaced all of the windows with double-pane low-e glass. I added wainscot panels in the dining room the next year and the joints all started to come apart in the winter.

I panicked and installed a “whole house” humidifier. Now I am getting a lot of moisture in the unheated attic. I tried to insulate between the 2×6 rafters, which has helped a lot, but it is still getting moisture accumulation near the ridge in the attic. The house has NO attic ventilation, and it would not do any good to add it at the soffits if the rafter spaces are full of insulation (if I were to finish the attic).

Do you have any suggestions? I could add ridge vents or other roof vents, but there would be no air entering the attic at the eaves. Would foam work better? I was also thinking about putting ceiling joists across and stopping my insulation there and ventilating above the ceiling. What do you think?

I would appreciate anything you could add.  Also, I am concerned that I have created a moisture problem in the walls with the cellulose and no vapor barrier. The walls still have lath and plaster, but have been painted many times over the 100 year history of the house.

A.    If you haven’t done so, please check the relative humidity (RH) in the house during the winter. It’s easy to do so with an instrument called a hygrometer, which you should be able to buy in hardware stores. (One type of hygrometer is in the picture.)  If the RH exceeds 30 percent, you should consider adjusting or shutting off the whole-house humidifier.

If you had no condensation in the attic before installing the whole house humidifier, it is likely that your house was too dry. But it is also possible that the joints started opening on your wainscoting if the panels were not properly acclimated to the ambient air by stacking them around the room for a week or so before installation.

Condensation in attics is most often due to convection of warm, moist conditioned air into the cold space through ceiling fixtures, including fans; joints between different materials; cracks in plaster or separated drywall tape joints, etc. You should investigate and correct any of these potential convective paths.

Spaces around ceiling fans and light fixtures can be caulked with polyurethane caulking. You should also install closed-cell weatherstripping under receptacle and switch plates on the walls, as they can be a source of convection into an attic, especially if your old house was balloon-framed, commonly done in the old days.

If you are using the correct term for the 2×6 “rafters”, it would have been best to insulate between the floor joists instead of between the roof rafters. That would have allowed you to check for any convective paths, which could have been sealed with canned foam or regular caulking, depending on the size of the spaces.

If and when you plan to finish the attic, you will probably build knee walls, which are usually four feet high, from the rafters to the floor. These knee walls are the places to insulate, leaving the triangular spaces behind them at both front and back of the house cold.

Install collar ties at the appropriate height to still give you a decent ceiling height, and insulate between them with fiberglass. This would allow soffit ventilation to be coupled with an externally baffled ridge vent for an effective sheathing ventilation as long as you made sure that any insulation installed between the sloping rafters is kept a minimum of 2-inches away from the roof sheathing.

This can be done by nailing 1×2-inch wood strips to the sides of the rafters, 1/2-inch below the sheathing. Tack 1-inch thick rigid extruded polystyrene (XPS) insulation to the bottom of the wood strips, caulk all joints between the boards and the rafters and the boards themselves with polyurethane caulking, and finish insulating with fiberglass.

The advantage of this system is that it provides a smooth surface for the air to flow without robbing from the fiberglass, which is only a filter holding air – the real insulator – and which is easily degraded by moving air currents. It also adds R-5 to the roof insulation.

Staple 6-mil plastic to the bottom of the rafters to provide an additional vapor retarder and apply whatever finish material you choose.

Closed-cell foaming sprayed against the roof sheathing is an alternative, but not only is it expensive, it will also preclude any ventilation, which may void any warranty you may have left on the roof shingles, whatever that warranty is worth.

If your plaster walls have no cracks and have the many coats of paint you mention, there is a very low risk that moisture has affected the cellulose. The only ways to test this are by inserting the long pins of a special moisture meter around electrical boxes or using a sensitive moisture meter that can read through the plaster, or by having infrared thermography performed, which will show significant variations in the effectiveness of the insulation if it has become wet.

You may also provide an effective vapor retarder by painting the walls with B-I-N followed by the finish paint of your choice.

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