I need to know the best way to solve attic venting problems on a 1-1/2 story, gable-roofed house.
I have a 1929 house with gable-end half-moon vents. There are no soffit vents and there is no room to install vent troughs at the eaves, since the roof joists are 2x4s. The house originally had wood shingles nailed to spaced roof boards which allowed the roof to breathe. It has since been decked over with plywood and asphalt shingles. Very little air movement has led to extensive mold growth on the roof decking now. What are my options?
What you are describing seems to require immediate and serious attention.
When homes are remodeled without an understanding of the original systems and details, such outcomes may be inevitable.
Considering the seriousness of this case, in my opinion you have two options, neither of them easy or inexpensive! There are typically two forms of roof system with regards to insulation and ventilation: cold roofs and warm roofs.
- The more common system, the cold roof, is based on isolating the "lid" (ceiling) from the attic above, by means of a vapor barrier and some form of insulation, most often blown-in fiberglass. Ventilating the attic space by a combination of eaves and roof vents is the second component of this system. In this method, the heat and vapor released from the lid will disperse through the ventilation process and keep the attic dry and free of mold and other adverse conditions.
- The other system, called a warm roof, eliminates the need for an attic ventilation system by placing the roof insulation, most likely of rigid or spray foam types, directly under the roof sheathing. In this process the entire house, including the attic space, is contained in one sealed and insulated envelope. In this case, you would want to make sure your exterior walls are also well insulated and sealed.
From your description of your roof system and the 2x4 rafters, it seems that the warm roof is your best bet. However, I have to caution you that considering that you have such shallow rafters and the amount of insulation that it would allow, especially at the rafter heel (the point directly above the exterior walls), you may still have some problems, including ice dams.
Ideally, you would want enough space on all locations for the adequate amount of insulation.
To do the cold roof system, you will need to remove the existing sheathing and sister the existing rafters with new 2x12s to create the space required for insulation and venting.
Last but not least, in either case you will need to remedy the mold issue, either by chemical treatment or removing the affected material. Mold is a highly toxic material and will pose health risks and adverse conditions for your indoor air quality.
For more information:
Read "How can I improve my roof's venting?" a Q&A answered by Mick Dalrymple.