I do not know what type of unit that you have, but I can tell you that you can use the attic as a fresh air intake, if there is adequate venting (ridge vents, gable vents, eave vents). If you have a heating unit that requires outside air for combustion, there should be two vent pipes going outside, one intake and one exhaust. Those two pipes have to be a minimum distance apart. Another possibility is that the return air plenum is not insulated properly, and could have condensation building up on it. If anything, you should call several legitimate HVAC contractors and ask them to come out and take a look at your issue and give you some advise on what to do, or an estimate. Shop around, don't take the cheapest price, but rather the best advise. If you plan on doing something yourself, at the very least that will give you a direction to go in. As far as your attic, extra insulation can't hurt, just make sure that your eave vents have a channel to vent above the insulation, so that there is adequate air flow. I would attach some type of insulation to the attic door/access. Today’s houses are built much tighter than houses in the past. Proper sealing protects against air leakage, keeping conditioned air inside the house, and unconditioned air outside. In general, a tighter house means more comfort and lower energy bills. However, homeowners today rely much more on air conditioning rather than on natural ventilation, often keeping their homes closed up year round. In older, leaky homes, this didn’t present much of a problem. But in today’s homes, some negative side effects of a having a leakage-free house are beginning to make an unpleasant appearance. Odors from pets, cooking, or even new carpeting or paint, can become trapped inside a tightly sealed house. Likewise, without proper ventilation, pollutants, radon, or vapors from household chemicals may build to unacceptable levels, causing health concerns. Homeowners themselves may even contribute to problems by burning scented candles to mask odors, which leads to soot deposits under doors and in other unlikely places where the air flows by. Also, remember that your air conditioner is a giant dehumidifier, so watch the humidity levels in your home. Forced-air heating systems are more popular today than baseboard electric, hydronic, or any other static system. Distribution fans for forced air systems typically move 800 – 2000 cubic feet of air per minute. A properly designed and sealed duct system allows efficient distribution of the heated air to various parts of the house. But leaky duct systems – which, unfortunately, includes most installations – can cause severe pressure differences between different areas of the house. This can lead to increased infiltration of unconditioned air into the home. Forced-air systems today are often designed with only one return-air inlet located in the main body of the house. This design reduces the complexity and cost of the air delivery duct system. However, it can often result in severe pressure imbalances within the home, when not taken into consideration as part of the house system. With only one air-return inlet in the home, closing an interior door can suddenly change the entire pressure balance of the house system. Since supply air delivered to the closed room has no path for return, the room becomes pressurized. Air in this pressurized room is then forced out through any holes it can find, such as electrical outlets, gaps under doors, or around window sills. With one room in the house pressurized, the rest of the house becomes depressurized. All of the air lost from the pressurized room must be replaced by air entering the house through other openings. So the house draws air in through the largest available holes with the least resistance, which may be the chimney, the vent stack for the hot water heater, the dryer vent, or other holes. In addition to drawing in unconditioned air, the air-handler or furnace is now competing for air with the vent fans, the fireplace, and other combustion devices. And since the air-handler or furnace is much stronger than any of the other draft sources, those weaker sources often become starved for air. To provide increased energy efficiency, homes today are designed with higher and higher levels of insulation in the walls, ceilings, and floors. But poor installation techniques that leave gaps, voids, or compressions, drastically reduce the effective R-value of the insulation. Improper installation of the insulation in a home can actually encourage mold growth, moisture damage, and higher energy use. Properly installed, the performance value (P-value) of thinner insulation may actually exceed that of higher R-value insulation that was installed poorly. Effective installation of the insulation helps to maintain the temperature of the conditioned air inside the home, while poor installation can reduce the overall efficiency of the home system. With older, leaky homes, the simple solution for heating and cooling shortfalls was to increase the capacity of the equipment. When it comes to furnaces and air conditioners, many builders and homeowners still mistakenly assume that “bigger is better.” But in today’s homes, over-sizing HVAC equipment seldom results in improved comfort or performance. Not only is over-sized equipment a waste of money for the homeowner, it can also cause a variety of operational problems, including short cycling, inadequate moisture removal, noisy operation, and shortening of equipment life. Over-sized air-handlers can lead to over-pressurization of the home, and increased leakage rates. For proper operation then, the HVAC system must be sized properly in consideration of the entire house system. I sure hope some of this helps you to shed some light on your situation.