Most six-year-olds can draw a house. The typical child’s depiction of a house shows a rectangular building with a door and a few windows, topped by a gable roof.
This type of house is fairly common in most areas of the U.S. If the house was built 100 or more years ago, it usually had a cellar or basement underneath the first floor and an attic above the top floor.
In the middle of the 20th century, modern architects attempted to do away with basements and attics. They designed flat-roofed homes on slab foundations. While many Americans are happy to live in this type of home, others feel that basements and attics are useful spaces, and have no desire to see them go away.
As originally conceived, attics were supposed to be outside a home’s conditioned envelope. Because of their location, however, attics (along with basements and unheated mudrooms) actually represent a type of in-between area that isn’t quite outdoors and isn’t quite indoors.
Types of attics
There are several kinds of attics:
Traditional attics under steep-sloped roofs are easy for builders and insulation contractors to work in, while cramped attics with difficult access are not builder-friendly.
What are the advantages of a traditional attic?
A house with a traditional attic has several benefits compared to a house without an attic:
- Ceiling air leaks are easier to fix when a builder has access to an attic above the ceiling.
- It’s usually easier and cheaper to install deep insulation on an attic floor than it is to try to insulate a sloped roof assembly.
- Roof sheathing facing an attic is less likely to have moisture problems, and is far easier to inspect, than roof sheathing facing insulated rafter bays.
Are attics usually dry or damp?
A well-built attic shouldn’t have any moisture sources like dripping plumbing pipes or roof leaks. Since most roofs get good solar exposure, attics tend to be dry spaces.
If an attic has signs of moisture problems — for example, sheathing rot or mold on the underside of the roof sheathing — something is clearly wrong. It’s usually fairly easy to identify a roof or flashing leak; telltale signs are soaked sheathing, especially after rainy weather.
If you’re seeing mold on the underside of the roof sheathing, however, a roof leak probably isn’t responsible. Attic mold is almost always caused by air leaks from the house below. If the leaking air holds enough moisture to contribute to attic mold, the usual explanation is a direct air path from a damp basement or damp crawl space to the attic — for example, a poorly sealed chimney chase or plumbing chase. The solution is simple: find the air leaks and seal them.
What about venting?
While most building codes require attics to be vented, the usefulness of attic venting is overrated. In the old days, code officials recommended attic venting to handle all the moisture entering the attic though leaky ceilings. These days, however, building scientists note that it makes more sense to build an airtight ceiling than it does to try to remove the moisture after it’s escaped through ceiling holes.
For more information on this issue, see All About Attic Venting.
My attic is hot. Should I install a powered attic ventilator?
Powered attic ventilators use more electricity than they save and are potentially dangerous. To learn why, see Fans in the Attic: Do They Help or Do They Hurt?
Is there a way to take advantage of the fact that attics are hot?
If you have ever climbed into an attic on a summer afternoon, you know that attics can get hot.
This fact has led many tinkerers to experiment with ways to put the hot air in an attic to good use. For example, inventers have installed coils of tubing in their attic, hoping to invent an inexpensive solar water heater without any glazing. Others have installed ducts that pull hot attic air down to their mechanical room, hoping to use the heat for one purpose or another.
These experiments have all reached the same conclusion: the heat collected in this manner isn’t valuable enough to justify the cost of the hardware needed to gather it.
How should an attic floor be insulated?
Before installing insulation on an attic floor, it’s important to seal all of the air leaks in the ceiling. For more information on this work, see Air Sealing an Attic.
In many areas of the country, the most common type of insulation installed on attic floors is fiberglass batts. This is a poor choice. Fiberglass batts don’t conform well to oddly shaped spaces, and most installations of batts are sloppy. If you inspect your attic insulation, you’ll probably find voids and areas where batts are compressed or overlapping.
Fiberglass batts also do a very poor job of reducing air leakage.
By far the best type of insulation for attic floors is cellulose. If you live in an area where cellulose isn’t sold, the next-best choice is probably blown-in fiberglass. (However, blown-in fiberglass is subject to more problems with wind-washing and convective loops than cellulose.)
Requirements for attic insulation vary by climate zone. (You can determine your climate zone by consulting this map.) The 2009 International Residential Code (IRC) and the 2009 International Energy Conservation Code (IECC) require the following minimum levels of insulation for attic floors:
- R-30 in Climate Zones 1, 2, and 3;
- R-38 in Zones 4 and 5; and
- R-49 in Zones 6, 7, and 8.
If you are using cellulose insulation, that means that you need at least:
- 8.5 inches of insulation in Zones 1, 2, and 3;
- 10.5 inches of insulation in Zones 4 and 5; and
- 14 inches in Zones 6, 7, and 8.
Code requirements are regularly updated, and some jurisdictions in the U.S. have already adopted the new 2012 codes. In the 2012 codes, minimum requirements for Climate Zones 2 and 3 have been increased from R-30 to R-38, while minimum requirements for Climate Zones 4 and 5 have been increased from R-38 to R-49.
If your attic has fiberglass batts on the floor, don’t despair. Here’s the good news: it’s perfectly acceptable to blow cellulose insulation on top of a layer of poorly installed fiberglass batts. The added cellulose insulation will fill the nooks and crannies that are presently uninsulated; will reduce convective loops that degrade the performance of the fiberglass insulation; and will improve the R-value of the insulation layer.
If you hire a contractor to do this work, don’t skimp on insulation depth. Much of the cost of the work is the cost of getting the crew and equipment on site; once they are there, the difference in cost between 8 inches of cellulose and 16 inches of cellulose isn’t very much.
Can I use my attic for storage?
In general, it’s a bad idea to use an attic for storage. In many homes, the joists or roof trusses aren’t designed to handle the extra loads from heavy objects stored in attics.
Every time a homeowner visits an attic, there’s a good chance that insulation will be disturbed, or that the gasket on the access hatch will be damaged. That’s why builders usually advise homeowners to stay out of their attics.
If you insist on using your attic for storage, and if you think that the existing framing can handle the expected loads, you’ll need to add new floor framing on top of your existing joists (at 90° to the joists) so that the top of the floor framing is at least as deep as your insulation. Then install a sturdy plywood or OSB subfloor in the area of the attic that you intend to use for storage. (You may have to cut the plywood into small pieces to get it through your attic access hatch.)
What do I need to know about access hatches?
Attics are usually accessed through hatches. In general, a well-designed attic hatch is preferable to pull-down attic stairs. (Most pull-down attic stairs are poorly insulated and extremely leaky.)
While some energy-conscious builders argue against attic hatches — they prefer to install an exterior access door in a gable wall, to prevent homeowners from entering their attic without an extension ladder — most homeowners would rather have a convenient hatch than one that is deliberately hard to use.
Interior hatches should be large enough to allow a person to get into the attic without difficulty. It’s better to locate the hatch above a hallway than above a closet. (Closet hatches are notoriously hard to use.)
Unless the hatch is carefully built and installed, it can be responsible for significant air leakage and heat loss. A good attic hatch has the same R-value as the insulation on the attic floor; this is best achieved by gluing several layers of rigid foam to the attic side of the hatch. The hatch needs weatherstripping at its perimeter, and it needs latches that pull the hatch tightly to the weatherstripping when closed.
Note that the 2009 version of the International Residential Code as well as subsequent versions require attic access hatches to “be weatherstripped and insulated to a level equivalent to the insulation on the surrounding surfaces.” In other words, if you have R-49 insulation on your attic floor, you need an R-49 attic access hatch. This requirement can be found in section N1102.2.3 of the 2009 IRC.
For more information on attic hatches, see How to Insulate and Air Seal an Attic Hatch.
Should I install a radiant barrier in my attic?
The short answer is: probably not.
Here’s a more nuanced answer: if your attic includes ductwork and HVAC equipment, and you live in a hot climate, you may want to staple a radiant barrier to the underside of your rafters. However, the energy savings from this work may not justify the cost of the radiant barrier. This type of retrofit has a very long payback period, especially if you have to pay a contractor to do the work, and other retrofit measures usually make more sense.
For more information on this question, see Radiant Barriers: A Solution in Search of a Problem.
What should I do about the ducts in my attic?
If the builders of your house installed ducts in an unconditioned attic, they made a big mistake. I have no idea why our building codes allow builders to make this mistake.
Ducts in unconditioned attics waste huge amounts of energy. Correcting this building flaw is expensive.
The best way to solve the problem is to install insulation along the roofline of your house, converting the vented unconditioned attic into an unvented conditioned attic. While there are many ways to insulate sloped roofs, the most common method is to install spray foam insulation on the underside of the roof sheathing. (For more information on this option, see Creating a Conditioned Attic.)
If you don’t want to create a conditioned attic — either because the work is too expensive or because you think that spray foam is environmentally irresponsible and risky — you can try to improve the thermal performance of your ducts by carefully sealing duct seams and by adding additional duct insulation.
For more information on this topic, see Keeping Ducts Indoors.
I want to convert my attic into living space. Are there any problems with this idea?
In some cases, converting your attic into a spare bedroom or home office may make sense. However, bad (or even dangerous) attic conversions are more common than attic conversions that are graceful and energy-efficient.
Some points to bear in mind:
- Some building codes require third-floor bedrooms to include two modes of egress — a requirement that is often interpreted to mean two separate stairways.
- Older homes often have 2×6 or 2×8 rafters that don’t provide enough depth for adequate roof insulation. Skimping on insulation to gain an inch or two of headroom is a flawed strategy; better approaches include popping a dormer or adding rigid foam insulation above the existing roof sheathing.
Your unfinished attic may appear spacious. Once it becomes clear that the space can’t be finished unless an adequate thickness of insulation is installed, however, ceilings usually end up lower than homeowners expect. When a conflict arises between a homeowner’s desire for adequate headroom and an energy consultant’s advice on minimum R-values, the energy consultant usually loses. Unfortunately, the result is often a space that is both cramped and inefficient.
That said, it’s possible to convert an attic into usable space and still maintain the integrity of your home’s thermal envelope. Before you commit to an attic conversion, however, consult a reputable architect.
Martin Holladay’s previous blog: “All About Microwave Ovens.”