There are homes designed without attics, and there are homeowners who wish to raise their ceilings, but there are energy efficiency concerns with having no attic.
How I would love not to have to crawl around my attic, adding insulation, repairing ducts, or dealing with the equipment up there. We know that running ducts in a conditioned space can be more energy efficient, as can having the AC unit in a conditioned space. Water heaters tend to be more efficient when placed in the attic in Houston. You really should not use the attic as a storage space, because this can be a fire hazard. (I have seen attics so filled with items on my home inspections that I could hardly move through the attic space). In a way, you could think of the attic as wasted space inside your building envelope, so why do we need one.
I have noticed energy efficiency problems with homes that do not have attics, which can be fixed. Here is the scenario: the framing for the roof is made with a 2×6, and it has a duct running between the rafters to deliver conditioned air to the space below. The roof faces east at a forty five degree angle. The shingles are black. Houston, we have a problem (alright, figure me that line; I am in Houston after all). The duct is about six inches itself, so there can be little insulation on it. The shingles are perfect for catching the heat of the sun. Plus, I failed to mention that there are lights in this space too. I should include that the roof sheathing has no radiant film on the underside as well. Sounds like I am making up the perfect scenario for failure doe it not. I am describing a town home that I inspected. What annoys me is the fact that this scenario is not out of the ordinary.
So how do we make a home without an attic? I know this may not be practical from all design styles, but we need to avoid running and placing any appliances or ducts in the little space that we have. We also need to consider making the most of the space under the roof. Depending on your climate, choose the best roof material to achieve efficiency. Hotter climates like Houston should have lighter colored roofs or roofs with a reflective coating element in them. If you are in Toronto, you will want a darker color roof. Sticking with Houston, you want the radiant barrier under the roof. The best option is to have the sheathing with the foil on the underside, because we need to make use of every inch of space. Under the sheathing, we will need to run baffles from the soffit to the ridge of the roof. The air flow will help with energy usage, but we need this air flow to deal with moisture. No matter how good your roof is now, you will have to deal with moisture penetration at some point. This means we need vents at the soffit and the ridge. Now we have five inches left (most baffles are about one inch thick and the rafter is six inches (2×6). Each inch of insulated space may give us an R3 insulating factor. (Since I discovered mentioning R factors was not commonly understood, I give this basic definition: R-factor is a measure of resistance to heat transfer). With five inches, we will be obtaining an R15. Not really great in our climate. Other homes have 2×8 rafters, which would give us an R21. We may also want to be concerned with thermal bridging (heat will transfer through the wood of the rafters past your insulation, but wood itself has an R-factor, which varies with wood species). The most common solution is to use spray foam insulation. You could add firring strips to increase the size of the rafters to create a deeper space for the insulation. It would be great to achieve at least an R30, which means having a space that is eleven inches deep. There are films that can go over the insulation to help stop thermal bridging. In Houston, this film should not be a moisture barrier due to our humidity problem. Finally, we can put on our ceiling covering (drywall).
If I am not suppose to run the wires and ducts in that space, where do they go? Create utility chases for the ducts under the ceiling covering or along the walls. These chases can be created for wiring too. Instead of recessed lights in the ceiling, consider track lighting or floor lamps. The wiring chases can be hidden in architectural elements in the ceiling. Here again, we have to consider the home decor and style. Many cathedral ceilings have not attic above them. On these ceilings, I typically see a decorative feature made to look like wood beams. I have not seen this done often (hardly at all), but I have seen designers create pattern with sheetrock to create an interesting ceiling scape.Other designers create a raised ceiling by making a lower ceiling along the edges of the room. These lower ceilings would make great chases for the ducts, wiring, recessed lights.
You have to plan carefully; however, eliminating the attic might be anenergy efficient solution. The real problem is determining how to meet other needs when we do not have use of that space. I would not run a metal conduit tube along a wall or ceiling to a fixture. I would not run the duct without a covering to hide it. I have seen both of these things done, and this detracts from the appearance of the home.