30 years ago R 11 was considered good... the price of oil was so low that was all that was required in most residential structures. The insulation that is already in that home is considered very good by today's standards... that should not preclude you from adding more. You do reach a point of diminishing returns and as you seem to understand you can get too air-tight... ventilation is an important key
Regardless of the insulation issues you nee to be sure that the mold is 100% gone before you invest in any additional improvements. You can not just burry the mold under new walls. As long as there is live mold it will continue to grow and become more and more of a problem and health threat as time goes on.
Mold is a potential problem in any structure and needs to be avoided or removed when it occurs.
"Molds are part of the natural environment. Outdoors, molds play a part in nature by breaking down dead organic matter such as fallen leaves and dead trees, but indoors, mold growth should be avoided. Molds reproduce by means of tiny spores; the spores are invisible to the naked eye and float through outdoor and indoor air. Mold may begin growing indoors when mold spores land on surfaces that are wet. There are many types of mold, and none of them will grow without water or moisture....
Molds have the potential to cause health problems. Molds produce allergens (substances that can cause allergic reactions), irritants, and in some cases, potentially toxic substances (mycotoxins)."
Read more from the U.S. Environmental Protection Agency Mold Basics
- Why is mold growing in my home?
- Can mold cause health problems?
- How do I get rid of mold?
Mold Cleanup Guidelines What to Wear When Cleaning Moldy Areas
- Who should do the cleanup?
Moisture and Mold Prevention and Control Tips
- How Do I Know When the Remediation or Cleanup is Finished?
- Actions that will help to reduce humidity
- Actions that will help prevent condensation
- Testing or sampling for mold
As far as the roof.... You really dont want to start redesigning the roof truss system unless you have extensive engineering experience. Changing the size or shape of the trusses will change the stress loading which could require other structural changes down to the foundation. I'm getting ready to add insulation to my roof. I'm going to install a 50 year designer metal roof. Instead of installing the new roofing directly over the old shingles we will frame the edges with 2x4s and put 3 inches of Styrofoam between the old roof top and the new metal pans. If you have not done this before there are pre-designed systems that can be easily installed. Take a look at the example at http://www.structall.com/content/pdfs/RoofOverGuide.pdf
You can also just add insulation to the existing roof see the 3 VIDEOS we have online "Adding Insulation to Your Manufactured Home" for two methods used by the U.S. Department of Energy's Weatherization Assistance Program (WAP) for installing additional insulation in a manufactured home roof. You can find that at http://mfdhousing.com/portal/stories.php3?nid=5604