How to Insulate an Attic

One of the best ways to cut down on heating and cooling bills is to insulate your attic. There are a variety of methods and materials to consider, so the best way will depend on the type of attic you have, as well as the climate you live in.

Any house will benefit from attic insulation, however, as it helps with heating and cooling the entire home. Many applications are DIY-friendly and cost-effective, too. This article will go over everything you need to know about how to insulate an attic.

The Importance of R-Value

By insulating, you are increasing the home’s “R-value,” which is a way to measure thermal resistance. The higher the R-value, the better the material will be at insulating the space.

The US Department of Energy advises that “the amount of insulation or R-value you'll need depends on your climate, type of heating and cooling system, and the part of the house you plan to insulate.”

The lowest R-values recommended are R-30 for hot climates, R-38 for areas with moderate temperatures, and R-49 for very cold climates.

Since various climates have different minimum building code requirements, contact your local building inspector to find out the minimum R-value recommended for your specific location. It’s a good idea to choose a product with a higher R-value than the minimum.

Types of Attics

Attics come in a variety of shapes and sizes. Some are easily accessible by stairs, whereas others have a latch. Some have enough headroom to walk around in, while others are merely crawlspace height. The rafters and trusses may run directly against the ceiling, or they may be suspended.

Attics may be full size, one and a half stories, with cathedral, flat, gabled, or hip roof designs. All of these aspects must be considered when choosing whether to insulate the floor or ceiling of your attic space, with budget and ease of installation the top two considerations.

Attic Without a Floor

Some attics don’t have an actual floor on top of the joists, so if you ever walk around up there, you have to watch where you step, as you could step right through the drywall of the ceiling below.

This is the easiest and cheapest way to insulate an attic, as you can use blown in cellulose to cover in between the exposed joists, or lay down batts in between them.

It’s your choice to finish the space with a floor or not afterward, just remember if you insulate the floor, the space above will remain un-conditioned, and will warm or cool according to outside temperatures.

The insulation could be left exposed as long as no HVAC vents are installed in the attic that circulate air through the home. Applying a vapor barrier and/or floor would help contain the material and provide another layer of insulation, also giving you a way to walk around easier.

It's generally cheaper to insulate the floor instead of the ceiling, since there’s usually less square footage than the attic ceiling, and the process is less labor intensive. You don’t have to insulate around windows, vents, or electrical boxes, although there may be some ceiling lights and wires from the room below that you need to consider.

Attic with a Floor

If your unfinished attic has a floor, but an exposed ceiling, you'll most likely want to insulate the upper area. If the floor is old, or just sheets of plywood, you may consider removing them in order to insulate underneath, putting them back, or upgrading after the job is done.

If you don’t want to do that, then remember you’ll have to insulate the attic walls and the ceiling in order to reap the benefits. Merely insulating the ceiling with un-insulated exterior walls won’t be all that effective.

Types of Insulation

Once you’ve decided what area of the attic you are going to insulate, you can decide on what insulation to use. Technically, any insulation can be used for the ceiling, walls, or floor, but there are characteristics of various materials that make them better fits for particular applications.

Blown In (Loose Fill)

Loose fill insulation comes in bags filled with either fiberglass, cellulose, or mineral wool. Choosing the material depends on your budget and the R-value you want to achieve. To install it, you’ll need to rent a machine to do the job.

You’ll also want to wear protective clothing, safety glasses, and a good quality mask, as the fibers will be airborne as you blow the insulation into the space.

Loose fill is great for attics that are hard to access, have very little headroom, or don't have floorboards, especially ones with joists that haven’t been spaced according to normal standards. It’s also good for attics that you want to insulate but won’t be using for any storage or liveable space afterward.

Cellulose is the most commonly used loose fill because it has a high R-value and doesn’t require a vapor barrier.

Blowing insulation will fill in any gaps easily—just make sure you aren’t covering up anything you don’t want to like ceiling lights and wires from below. Check local codes, but also the specific lighting devices as not all of them are rated for being installed around insulation, which could pose a fire threat.

Keep in mind that increasing the thickness of loose fill does not actually increase its R-value, as a certain density starts to compress it, which makes it useless.

Otherwise, using loose fill is a relatively simple job that any DIY-er can do to save money, while reaping the benefits of an insulated attic.

Batts or Blanket Insulation

The most common type of batt insulation is fibreglass or mineral wool. They come packaged together in rolls that you can lay out between joists or in between studs. They are meant to fit between 16-inch framing centers, which is standard framing used for walls, floors, and ceilings.

If the space you want to insulate is not on 16-inch centers, then batts may not be ideal for your projects. You could add framing to make them standard sizes, and then cut a few batts into smaller pieces to fill the odd spaces. Just remember that batts must fit snug. Any gaps or spaces means air can escape, and your hard work was for nothing.

Whether fitting between joists or studs, make sure the ends of the rolls butt up against one another and continue until the entire cavity is covered with material. Never compress the rolls to make them fit, but cut segments to size to fit around any obstructions.

Cut the batts using long-bladed utility knives with gloved hands, and always be careful when cutting. You’ll want to wear the same amount of protective clothing and eyewear, as well as a mask when installing. The fibers also become airborne when installing batts, though it won’t be as intense as loose-fill.

Fiberglass and mineral wool batts come in various R-values, and may also include soundproofing and fire-resistant qualities, which may or may not be desirable depending on your overall goals for the attic space.

If it’s being left unfinished then soundproofing won’t be necessary, and fire-resistance isn’t either unless you want to create liveable space, or it’s required by code.

Batts may also come in a variety of other materials like cotton, recycled denim, cellulose, plastic, or other natural fibers, some of which boast environmentally friendly qualities. Keep in mind that denim is incredibly hard to cut, and some of these alternatives are very expensive.

If you are building to code, not every kind of insulation is acceptable. Some may fail the inspection, meaning you would have to take it all down, eating the cost and the labor of installing it.

Foam Boards

Foam boards come in handy 4’ by 8’ sheets that can be laid out fully or cut accordingly, as well as other sizes to fit into standard framing spaces. They are made out of polystyrene, polyisocyanurate, polyurethane, or phenolic materials and are approved for use in walls, floors, and ceilings.

In walls and floors, they can be cut or fit nicely in between framing, but it may not be as easy to do in the ceiling because of gravity, especially if you are cutting around electrical boxes. They either need to be glued or used in conjunction with stapled paper over top to hold them in place.

One of the benefits of foam boards is they have a very high R-value in a thinner size, which can be helpful if space is limited or you want to add more R-value on top of other insulation.

The sheets can be applied across joists and studs to block all thermal transfers. Generally, fire codes require that you cover any foam board with half-inch drywall. Check your local building codes for specifics.

Spray Foam

Spray foam is the highest quality and functioning insulation product out there, but it’s also the most expensive. Modern spray foam materials are now made without toxic CFC’s and HCFC’s and come in closed or open-cell polyurethane form.

Open-cell is cheaper and lighter, whereas closed-cell is overall stronger and has a greater R-value. Either are fine for attic insulation, though keep in mind that open-cell tends to absorb water.

While both products are superior to other insulation, there are some things to consider when it comes to where it's installed. When used in an attic ceiling against the roof side, it seals up the space completely without the need for a vapor barrier or ventilation since it still allows moisture to escape.

This can be an easy and quick way to achieve the best results, especially if the space is free from obstructions. Same goes for applying over exposed floor joists and in wall cavities.

Spray foam is incredibly hard to cut or remove once it has been applied, however, so you don’t want to cover anything you may need to access later. While it can be safely sprayed around electrical boxes or behind wires, make sure these elements can be repaired, accessed, or removed if necessary.

Other Installation Requirements

While insulation materials are the main concern of how to insulate an attic properly, things like airflow and proper venting are also important.

Most heat loss escapes through air leaks around exterior vents, pipes, electrical boxes, and chimneys, so insulating should be used in conjunction with weather stripping, caulking, and venting the attic space. Small cans of condensed spray foam can help seal up small gaps and holes or around areas of heat loss that you can't get other insulation around.

When it comes to applying a vapor barrier, colder regions often have specific requirements according to code. You don’t want to apply it improperly or use it in the wrong spot, however, as they can do more damage than not by trapping in moisture.

Batt insulation is especially prone to housing mold when it cannot breathe properly.

Costs of Insulating

If you want to know how much it will cost to insulate your attic, a simple calculation can help you estimate the job yourself. The first step is to measure the square footage of your attic space. If you want to insulate the ceiling, measure the length versus the width and multiply to get the total square footage.

If you have a gable or hip roof, measure all sections individually and then add them up. If you want to insulate the floor, measure the length and width, and multiply the number. Then, choose what kind of insulation you are considering and look up the cost per square foot.

This can be found on the manufacturer's website, or on the package itself if you are in a hardware store. Call any companies you are thinking of hiring to ask them what they charge per square foot.

If you are installing yourself, simply multiply your attic’s square footage by the cost of the insulation per square foot. Add 15 percent to this number as there will always be waste. If you are hiring a professional, then you’ll need to ask them what their labor costs are, or see if they will come and do their own estimate for free.

Either way, get to know the costs of the different insulation types and what the square footage of your space is, so that you can tell if the estimate they give you is reasonable. Make a few calls if necessary and always hire a reputable company.

Before You Install

Do not cover up any wires, pipes, or vents that need repair. Get rid of pests like mice and squirrels beforehand. Using spray foam on the attic ceiling is a great way to keep pests out, but it will also trap them in.

Professional grade spray foam is not easy to cut or remove once it’s been installed, so you don’t want to cover anything up that you will eventually want to get at. Keep this in mind if you are ever thinking of adding a dormer, skylights, vents or windows as any renovation will be made much harder if contractors have to cut through and build around spray foam.

Have a roofing contractor check on the state of your roof – both exterior and interior before you insulate, as they may recommend venting or other repairs which would be better to do beforehand. Do any renovation or reparation work before putting any kind of insulation in, but especially spray foam.

In Summary

Knowing how to insulate your attic space properly will help you make one of the best decisions when it comes to heating and cooling your home. Insulating with higher R-values will cost more initially, but remember, it will save you money on heating and cooling bills in the long run.

Higher quality insulation will eventually pay for itself in the future, but you'll notice your home is more comfortable immediately.

Now that your attic is cozy, check out our coverage of how to finish an attic and make it a living space.