"what are the cons of building such a level without an attic?"

At best, possibly sagging the ceiling,at worst , caveing it in. The place wasn't constructed to support a secound story.

Building a second storey is more than just stacking on a second set of Lincoln Logs. There's foundation work and framing work to make sure that it doesn't colapse as Klem says. Once that's engineered for, how the new second floor roof is done is mostly a design and budget consideration. However, getting no attic trusses up to the second floor level rather than the first floor level will probably need a crane for a day. It will be easier to stick build, with attic space.

You need to describe the existing structure and explain what you mean by no attic. It seems to me that only a flat roof or a sloping roof with a cathedral ceiling would have no attic.

Unbearably hot and poor energy efficiency. Everyone is talking about the risk you won't do it right. I'm going to assume your builder knows what she is doing and at least makes it structurally sound.

Heat rises. Human bodies, ovens, and showers produce heat and humidity. If you don't have an attic, that heat tends to build up in the top floor. Also, black shingles heat up in the summer sun. This makes the top floor miserable in the summer...unless you spend a fortune on centrail air.

Is your roof going to be sloped? If not, it won't shed snow., and will tend to leak (or cave in) in a heavy snowstorm unless someone climbs on the roof to shovel it.

If yes, parts of the ceiling will be low and it will be hard to apply insulation to the ceiling. (If you have an attic, insulation is usually applied to the floor, which is flat.)

<sitting in a miserably hot top story apartment right now>

SIPs roof?

An attic will not necessarily affect the energy efficiency of the design. An attic is just a space you don't want to be a part of the top floor space. It can still be included in the conditioned space of the house or it can be unconditioned. I can think of no disadvantage of not having an attic except that the finishes and insulation might be more expensive but I don't really know what you are proposing.

There's more to cathedral ceiling desighn than meets the eye of casual observers. That can be seen in low cost Double wides. In a few years the ceiling starts to show rot from ongoing condensation.

It seems pointless to speculate about a design that has not yet been described. But I find it difficult to imagine a roof configuration that couldn't be properly designed.

Almost any thing *CAN* be properly designed if you throw enough money at it. I've lived in a number of apartments on the top floor of building without attics, and they've all been miserable in the summer.

ed I've found most any level around apartments to be miserable,not intirly due to the heat. (: Seriously,you are correct. Heat travels up from lower space,radiation effects top floor much more than lower ones and heat has no place to go. Despite all the hype over r-vaules of various material including insulation,air is one of the best and accounts for much of heat blocking ability of commercialy produced insulation which contains millions of tiny air pockets. An atic full of air is in effect an attic full of insulation of sorts.

Thank you everyone for commenting. I am talking about a home in the Mid-Atlantic Region currently with an attic and sloped roof. I have seen an example of an upper level with a sloped roof without an attic built on such a house. I would assume a SIPs roof is involved. Designs cost a lot, so I am trying to collect some data before deciding to pay for a design. Thanks again.

The air in an attic can do very little to slow the transfer of heat from the roof to the attic floor.

The heat from the roof warms the attic air which transfers the heat to the attic floor by convection in a circular air pattern. Heat is also transferred through the air to the attic floor by infra-red radiation without heating the air.

Insulation placed between the rafters reduces the heat transfer that might otherwise be carried to the floor by convection. An aluminum foil radiant barrier under or on top of the rafters, or on the attic floor, greatly reduces radiant heat transfer.

I assume you are thinking of remodeling an existing attic space or tearing a roof off and rebuilding. I can't think of an advantage in using SIPs. 2x12 rafters with dense-pac cellulose or spray foam insulation should work well. A foil backing on the roof sheathing is also helpful.

You really need to describe the design condition better to get useful advice. You might be better off hiring a professional designer now rather than sifting through misinformation.

We were simultaneously considering three projects. Tearing off the roof and rebuilding was far too expensive, so we are left with two options. Either (1) leaving our attic unconditioned, sealing the attic "floor", and spraying cellulose insulation (the newspaper kind), or (2) conditioning the existing attic.

We have conflicting information from different energy efficiency and attic remodeling companies. For example, let's take Option 1. One company recommends ridge vents. The more airflow in the attic, the better, this company says. Another company says our existing gable vents are more than enough, and in fact the ridge vent would disturb my airflow. This company says an attic's primary purpose is to alleviate moisture in the air so you don't have condensation. I don't understand how the moisture will get in the attic if everything is sealed. I also don't understand how, if I go with Option 2, the moisture will escape my conditioned attic space.

Very few people really completely seal their attic. I'm sure it's technically possible, but it is almost impossible. Lighting fixtures cut into the air seal, there are gaps where the wall meets the roof, etc. Every time you use the shower or washing machine, you add moisture to the air in your house. Moisture is bad for insulation and can cause mold or rot. This is assuming your roof doesn't actually leak.

The peaked roof sheds snow in the winter. An attic avoids having black roof shingles that heat up to 100F within a foot of your head. It's possible to insulate a peaked roof, but it's easier to add a thick layer of insulation to a flat attic floor.

There wasn't necessarily a problem so to speak. We are remodeling a home and want to make sound decisions, and balance those with what we can afford. After speaking with different contractors, my goal is to understand the concepts before proceeding. Every contractor has been in business 20+ years, but there are still many differing viewpoints. I want to make decisions I'm comfortable with. Right now I'm not comfortable with any of the decisions. I would prefer to make the decisions before we move into the house.

At a minimum, I would like to make the attic energy efficient. This involves sealing all the gaps edlincoln talked about and spraying cellulose insulation in the attic (right now we have "rock wool".)

An alternative is to insulate with a very thick foam layer and make the attic a conditioned space. Before even considering this option, I want to understand if an unconditioned attic really does help get rid of moisture. And, if so, how does the moisture get into the attic in the first place?

Unless the roof or pipes leak or ducts discharge into the attic, during hot humid weather moisture can get into an attic through oversized wall vents that allow moisture laden air to enter the attic and/or from a bathroom directly below the attic floor.

Moisture from a bathroom can be controlled with an exhaust fan and a vapor impermeable ceiling paint.

Moisture from vents can be reduced by substituting smaller eave and ridge vents

In a sealed conditioned attic with a moisture barrier at the ceiling of bathrooms, moisture is not expected to enter the attic but if it does the conditioned air should dry it. Even HVAC ducts in the attic will leak enough dry air to keep the space dry.

These issues can be solved regardless of whether the attic is vented or sealed so you should decide what you want, then design the attic. Since it is expensive, the primary reason people seal an attic is to allow HVAC equipment and ducts to be located there but they might also want to achieve a higher R value without changing the structure or the create a better environment for storage.

If you are planning to replace the roofing I recommend applying Grace Ice & Water Shield over the entire roof surface and using coated aluminum or copper step flashing at side walls.