Insulating Existing Roofs and Attics

Insulating Existing Roofs and Attics


e sometimes remodel existing residences and have found that a high percentage of them are under-insulated, sometimes lacking viable insulation at all. If this is your case, you can upgrade fairly easily in most situations, particularly in insulating the roof and attic spaces. Tony Rosetti, a project manager here at Hendricks Architecture, provided the following post.

When deciding to install, add to, or replace attic insulation, a little thought must be taken in advance such as choosing the type of insulation, and to what levels of heat retention you wish to achieve.

You will first need to decide if you would like a “Cold Roof”, or a “Warm Roof”.

Cold Roof vs Warm Roof

A “cold roof” has insulation at joist level, just above the ceiling, which helps prevent heat from escaping from the living spaces directly below. The attic can be used for storage if heat extremes are not an issue (i.e. very cold in winter, extremely hot in summer).

cold roof, insulation

A “warm roof” has insulation tacked into the roof rafter bays, thus allowing heat into the attic space, but not beyond the roofing above. This is generally warranted if you are converting the attic space into useable storage (and are concerned about temperature extremes of stored items), or other heat regulated uses.

Warm roof, insulation, roof, attic

Insulation Types

The most commonly used types of insulation are lay-in fiberglass insulation (batt or blanket); loose-filled insulation (bagged or blown-in) insulation; and sprayed-in (spray foam). Batt/Blanket insulation comes in easy to work with rolls, but requires protective clothing and gloves to handle as it consists of fiberglass. It works well in the traditional ceiling joist space installation, and especially if wanting to tack into the roof rafters for a “Warm Roof”.

Loose-fill blown in or bagged insulation is typically made from cellulose, which is shredded newspaper treated with fire retardant, or recycled paper, a green product. Both are non-itching, and easy to handle. Another product used is mineral fibers, material similar to batt insulation that works much better in the blown-in form than in batts. Since these products are loose, they are suitable for installation in the joist spaces just above the ceiling, achieving the “cold roof” scenario.

The third major type of insulation is urethane spray foam. The main advantage of spray-in foam is that you can insulate at the roof rafter level; thus insulating the attic space as well. This allows additional energy savings if you have hot water pipes, and an HVAC system with ducts running through your attic. They would therefore not need additional wraps or insulating to hold their heat.

Spray, foam, insulation, roof, wall
Spray foam insulation at roof and walls

If you don’t have HVAC and ducts in the attic, spray foam in the roof rafters isn’t really necessary. I’d blow insulation on the attic floor. The big disadvantage with spray foam is cost. It can be three to four times what you’ll pay for blown cellulose or fiberglass. It also gets very hard and is a burden to remove, should you ever need to.

Prepping for Install

When preparing to select and buy your insulation, measure your attic surface area accurately. Check local building codes, and order for at least the minimum depth required for your area. Increase it if you wish to achieve greater “R” values, or heat retainage.

To prep your attic space for insulation, first clear all joist spaces of debris. A shop vac is very handy for this. If the ceiling drywall has no vapor barrier, it is recommended to install one. To do this, cut lengths of vapor barrier sheets for each bay, wide enough to staple each side to the sides of adjoining joists. Cut openings in the barrier around electrical fixtures and other hardware.

Installing Fiberglass Batt Insulation

If using fiberglass batt insulation, roll out the insulation between the joists, but do not compress it. Make sure sections of batt are butted securely to each other. Cut holes in the blanket @ electrical components to prevent overheating. Lift cables & wires above the insulation. Make sure to maintain a 2” air gap at eaves for attic air flow. Plastic baffles are typically used for this.

If you require more depth or layers (by code or preference) to increase heat retention, you can add a second layer by laying it perpendicular (at right angles) over the first layer. You can also do this at problem areas, such as rooms that are susceptible to being colder, or above rooms with a heat source, such as wood stoves.

Installing Loose-Fill Insulation

If the attic is awkwardly shaped, has numerous joist blocking or inaccessible voids, or if you just want an easier alternative; consider loose fill insulation. If a DYI install, consider bagged insulation; it is easy to transport, and easy to handle. Blown-in insulation is recommended to be installed by a professional. Before installing loose fill, cut small 8”-12” wide sections of batt insulation, and use in each joist bay as an outer barrier, where the joists meet the roof rafters (at eaves). This will keep the loose fill out of the eave space. Remember to maintain a minimum 2” gap between the batt and the roof sheathing, to allow air flow from eaves into attic space.

Fill all bays with the loose insulation to a min. uniform depth as recommended by code, or to a higher depth if you wish to achieve better insulation values.

The photos below show an attic being insulated with blown-in cellulose. Notice when complete, you don’t see any of the ceiling framing. You also don’t see any gaps down to the ceiling drywall. Blown-in insulation is great at filling gaps, thus providing a good, complete layer of insulation.

Installing Spray-In Foam Insulation

Spray foam insulation is best left to insulation professionals; it is fairly demanding to apply and the over-spray can be harmful to the lungs. If you attempt to apply yourself you'll need to wear disposable coveralls with a hood, and gloves, a face mask, and eye protection. It takes some practice to spray foam evenly and, because it expands so dramatically, to control its depth; 2 inches is all you need to seal the joist cavities. You need a clear area so that you can work without interruption; any pause longer than 30 seconds will clog the nozzle and require putting on a new one. It's also critical that the air temperature stays between 75 and 85 degrees while spraying.

During application, spray a consistent, even amount throughout the attic space. This is important, as any lack of coverage or an uneven application will result in significant heat loss through these areas.

One simpler and less expensive approach for do-it-yourself applications: Cut some 2-inch-thick rigid-foam insulation and glue it to the subfloor between the joists or support it with nails driven partway into the joists. Then fill any gaps between the edges of the foam boards and the joists using the canned spray foam sold at home centers or hardware stores. Again, protect yourself from dripping foam and possible inhalation with a full face shield, gloves, and a hat.


There are various other materials and systems to insulate your attic, including:

If you have existing batt insulation, you can combine with blown in insulation for a better insulating value. Or, combine the two products if putting in all new insulation. In this case batt insulation would be laid into the joists, and then covered with the blown-in type of insulation.

Another system is the structural insulated panel (SIP); a sandwich of rigid foam insulation and plywood or OSB (oriented strand board). This system is typically used in new construction, to insulate throughout the house.

Finishing Up

If water and/or other pipes are exposed above the insulation, you should insulate them as well. Custom sized tubular pipe insulation is readily available, and is typically pre-split down the length to allow easy installation over the pipes. Secure in position using tape or clips. Butt sections tightly together and tape together if possible.

If the insulation is not higher than the joists, plywood can be put down over them to provide platforms for storage. If insulation is higher, deeper joists may need to be added, to increase the height needed to clear the insulation. Do not compress any insulation, as it will decrease its heat retention capabilities.

To insulate the attic access panel, cut to fit a batt section, gluing it to the topside of the panel. This will help complete the coverage.

Your attic insulation is now complete!

Tony Rossetti, Project Manager
Hendricks Architecture

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