How and Why to Insulate Your Attic

man insulating attic wearing protective mask
  • Intermediate

The mere fact that you’re actually considering the question if you should better insulate certain areas of your house or not is your realization that your energy bill is a serious part of your budget that you could probably gain some control over.

Heating your house and the cost of doing so becomes a common subject of conversation when the fall season comes around. As the outside temperature goes lower and lower and you notice that the heaters or the furnace tend to run more often and for longer periods, it becomes a concern.

Put in a different context, you suddenly realize that the heat that was generated in a room of your home to keep you comfortable seems to dissipate more rapidly as the outside air gets colder.

Heat and Conductivity

Heat is a form of energy closely related to thermal energy, which is the movement of tiny particles that all solid, liquid, and gas elements are made of. Those particles are atoms and molecules in constant motion in all matter.

As those particles move, the kinetic energy they contain produces thermal energy that tends to increase as the particles’ movements speed up.

The elements’ thermal energy is always moving from one element to another, always flowing from something warmer to something cooler. It can therefore transfer from any one element to any of the other elements in a solid, liquid, or gas state.

Three forces in nature are constantly at work transferring heat energy from one element to another, or in a more specific context, from inside the house to the outside —conductivity, convection, and radiation.

Some materials are better than others at conducting heat and are known as “conductors”—silver, copper, aluminum, etc. How well a material conducts or transfers heat is its measure of thermal conductivity.

an insulated attic with skylight

Conduction is one of three heat transfer processes that occur whenever one material or object is warmer than another one and in direct contact with it.

This can be from the warmth of a hot cup of coffee to your cooler hands, from the heat of a hot pot on a stove to the pot’s handles and its cover, or from the warm air inside a room to the room’s walls, floor, ceiling, and beyond.

Materials with low conductivity, such as Fiberglass, Cellulose, Polystyrene, Polyurethane, etc., are known as insulators, and are used to keep homes from losing too much thermal energy to the surrounding environment.

The degree of thermal resistance an insulator will provide to prevent the heat transfer from one object to the next is relative to the type of material, and the amount of it (or thickness) used to resist the thermal transfer. When doing homebuilding, this thermal resistance would be referred to as the R-value.

A second heat transfer process that occurs in gases and liquids is “convection.” The low density of gases and liquids leaves more room for their molecules to move around than in solids.

So instead of a warmer particle warming the particles next to it by conduction, warm molecules are becoming lighter than cold particles, where the forces of physics make it that the lighter molecules move up to the top while the colder ones are pulled down to replace them through the convection process.

Convection is the movement in a gas or liquid caused by heat differences where warmer air naturally rises towards the ceiling as it is carried away from the walls while the cooler air sinks towards the floor.

Since fluids such as air typically become less dense (or lighter) as it gets warmer, it causes convection currents to be pushed sideways and upwards as the heavier, more dense fluids are pulled down by gravity.

The sun warming the earth is the simplest example of how “thermal radiation” works. Unlike conduction or convection, radiation doesn’t require matter to transfer thermal energy but relies instead on electromagnetic energy, which is radiant energy propagating in waves through space at the speed of light.

It should also be mentioned that radiation can transfer energy through windows, making the installation of energy-efficient windows an asset in your home.

To describe the three processes at work, you can imagine the sun heating the roof of your house through radiation, then that same heat being transferred inside the attic through the conduction process, to the attic floor or the ceiling of the room below through the convection process and into the living spaces below through conduction.

In warmer climates, it causes the house to get super hot, while in colder climates, it causes ice dams and leaks for poorly ventilated attics.

attic window with insulation around it.

Insulating the Attic

If your attic was a space left unheated and possibly only used for storage, the effects stored up in there will present all sorts of obstacles to deal with, especially if you intend to build up a blanket of insulation over the joists.

As you go up into the attic to check for insulation thickness, if there is any insulation at all, pay attention to insulation that is compressed, water stained, or moldy making sure to remove it as you go.

In order to insulate such an attic, you’ll first have to determine the total R-value that you need.

You can then measure the thickness of the insulation already there and try to evaluate its R-value, and after estimating how much insulation you’re missing for the desired thermal resistance, you can decide on what type and R-value insulation are needed.

At that point, you’ll have to decide which will be best suited for you, the loose fill or the batt insulation.

Most insulation is made from fiberglass, cellulose, mineral wool, or cotton. The format that it comes in can be loose fill or batt insulation. Both types can be used to blanket an attic floor, although inch for inch, the batt insulation will generally provide better R-value than the loose fill insulation.

Batt comes in rolls or pre-cut 4-ft lengths to fit studs and joists spacing at 14-1/2 or 22-1/2 inches wide. Loose fill comes in bags and can be applied with a blower available for rental.

blown in insulation between wood slats

Caution: Working with insulation will generate a noticeable amount of particle dust that you should avoid breathing in. Make sure that you wear a proper breathing mask or apparatus when working with such materials.

Batt is normally easier to install and just needs to be cut to fit specific spaces using a sharp utility knife. It can then be laid in place as you fluff it in, but you should avoid compressing or squeezing it in place as this will reduce its thermal resistance.

Some types of batt insulation can also come with a layer of paper or foil vapor barrier. Installed in an attic, batt insulation can get awkward or difficult to install in low headroom clearance areas.

On the other hand, though, loose-fill insulation provides a much easier solution to insulating those small nooks and restrictive areas as the loose insulation is simply dumped into a machine that blows it through a hose to specific areas of your attic.

When applying loose insulation, nails or screws can be used as guides placed into the joists at intervals to provide you with some kind of gauge to maintain a more even layer of the material throughout.

A handyman installing batts of insulation in an attic.

A Great Ally for Your Attic’s Efficiency

Whether or not you’re heating a section of the attic, plenty of insulation is necessary to keep the heat inside your living quarters, but insulation is only part of the solution.

Most insulation materials slow down the “conductive” heat, but they’re not as efficient in slowing or blocking the “convective” heat flow. There is, however, a system that is designed to reduce heat transfer in a more efficient way.

The reflective insulation or/and the radiant barrier are very effective methods of controlling the heat gain, providing that the reflective surface is facing an air space. It should also be installed in a manner as to reduce dust accumulation on the reflective surface, as this could impair its reflective capability.

Ideally, the unheated attic space should always be at the same temperature as the outside air. But the situation here is always anything but ideal, with the attic air always warmer than the outside air—so no matter what, the attic must be vented or perhaps ventilated.

As mentioned earlier, different forces are constantly at play, trying to transfer the heat to cooler objects and areas. Radiant barriers are designed with one or two reflective surfaces to make sure that most of the heat is repelled back in as soon as it reaches the foil.

With the shiny side of the foil up against the rafters, the barrier blanket will repel the heat radiating from the sun right back outside, thus greatly reducing the amount of heat warming the interior of your attic.

Furthermore, a barrier with a layer of foil on each surface will also keep the heat already inside the attic from warming the roofing material, which would melt the snow in colder regions, forming damaging ice dams during winter seasons.

The same goes for a radiant barrier laid flat on the floor or against the exterior side of a partitioned living area with the shiny side facing towards the roof or towards the un-heated side of a lived-in attic room to keep the warmth from infiltrating the living quarters, during the summer months.

So for the colder regions in the north, the double-sided barrier is a double win!

A finished attic with bookshelves and a couch.

Venting the Attic

Even with the attic well insulated and with proper radiant barriers everywhere inside, there is still always a certain amount of warm and stagnant air that needs to be circulated and expelled from the premises.

The best way to get rid of that unwanted heat is to create an airflow throughout the attic where fresh air will be pulled in from the soffit vents to force the warmer molecules to move up towards the roof and gable vents.

Through this process of convection, the warmer air is then expelled from the attic through the upper vents creating, at the same time, the airflow that will also keep moisture from forming.

But if a section of your attic has been partitioned into forming some cubical room, there’s a very good chance that part of the ceiling is formed from the roof rafters.

You’ll have to ensure that an air space is still maintained under the roof decking from the outside section of the attic at floor level through to the section at the peak of the roof to permit sufficient air circulation.

There are several ways to accomplish that, one of the simpler methods being the use of rafter vents. It should also be noted that the radiant barrier should completely wrap the whole complete cubicle before sealing it in.

What Exactly Are Radiant Barriers?

There is a lot written here about radiant barriers. In building construction, you have two different types of barriers to wrap over and around walls, ceilings, and floors to block moisture, heat, or both to transfer from one area of the building to another or to the outside.

The vapor barriers come in rolls and can be stapled directly to studs and joists.

Vapor barriers can be of paper-backed aluminum, asphalt-coated Kraft paper, polyethylene, or other materials that can keep moisture from going in through the walls, floors, or ceilings to form condensation.

But unlike radiant barriers, vapor barriers can only serve to block moisture.

Radiant barriers, however, can serve as a vapor barrier while performing their very specialized duty to block any heat from transferring to other objects, mediums, or spaces.

Such materials used in construction can greatly reduce the amount of heat gain in the summer and heat loss in the winter.

Radiant barriers are made up of a number of substrate materials that hinder air infiltration, laid into a blanket which is then covered over on either one or both sides with a highly reflective material such as foil.

The hindering core material can be kraft paper, plastic film, cardboard, oriented strand board, and other insulating materials.

The reflective radiant barriers will effectively repel back most of the radiant heat, which is the most intrusive type of heat transfer, preventing the heat from getting through.

Even if it doesn’t have any insulation value, it does block most of the radiant heat trying to come in or leave the structure of a building.

You can find more on the subject of improving the efficiency of attic space by linking to some of our sites, such as “Tips for Preventing Attic Ventilation Problems,” “Making Your Own Attic Stairs Insulation Cover,” and “Will Insulating Attic Flooring Increase How Much Weight It Can Support?”