How to insulate garage ceiling rafters

How to Insulate Garage Ceiling Rafters

Some people use the garage only to park their car there. Other folks, however, use this space for all kinds of activities – from working on music to carpentry. If you belong in the latter category, adding insulation to the garage ceiling rafters should definitely be your top priority. 

The reason why you should do this is pretty simple – nobody likes spending time in a freezing-cold garage. As the heat rises, the attic heat loss occurs, i.e. the heat leaves the garage through the roof. And to keep it inside, you will have to insulate your garage – its ceiling rafters, to be more precise. 

In this article, I will tell you everything you need to know in order to carry out a successful insulation installation on your garage’s open rafters. I will also provide some extra tips related to this task and help you complete it like a professional. 

What’s the Difference Between Trusses and Rafters? 


The main difference between rafters and trusses is that the former is a better option for specialty projects, while the latter stands as a good choice for most projects. 

The trusses should be used by homeowners who don’t prefer a finished attic space and whose homesite is easily accessed. The rafters, on the other hand, should be your first choice if you’d prefer to have a steeply vaulted ceiling and if you’d like to maximize living space inside of your home. 

One important thing to mention here is that the rafters are a better option for garage ceiling insulation, as they allow one to tightly insulate the spaces between the beams. 

That is not to say that those with truss garage ceilings cannot install insulation. It’s just that it’s a more challenging task – with no spaces between the trusses, you’ll have no other choice but to drill holes in them. 

Another way to go about insulation installation to a truss garage ceiling would be to use rigid foam. You could glue the rigid foam boards straight onto the trusses and then just cover them with drywall later. 

Different Types of Garage Ceiling Insulation 

Garage insulation
Photo by Jesus Rodriguez via Flickr (CC BY 2.0)

To get your garage ceiling insulated, you’ll have to use one of the four main types of garage roof insulation: 

Spray Foam Insulation 

Made out of polyurethane, spray foam insulation is the most effective (but also the most expensive) type of ceiling insulation. 

It typically comes in cans and spraying it between the ceiling joists is an effortless task. It has a high sound absorbance, it’s extremely effective when it comes to preventing heat loss, and it’s also very fire-resistant (the last thing you want in your garage is a fire hazard!). 

Rigid Foam Insulation 

Another kind of insulation you could use for your garage’s space and the roof is rigid foam insulation. As I mentioned above, it comes in boards – which turns it into a great choice for a truss garage ceiling – and it’s made out of polystyrene. 

This is not the most popular type of garage insulation, however, as it’s not as sound-absorbing and can be quite expensive. Also, it’s not that good for exposed application – it has to be covered with drywall. 

Cellulose Insulation 

Made out of recycled paper, the cellulose insulation is available in a loose-fill form. 

Unfortunately, this type of insulation is not as effective as spray foam or fiberglass insulation – which I’ll talk about below – when it comes to preventing heat loss. It is, however, the most environmentally-friendly type of insulation available. 

Fiberglass Insulation 

Manufactured from glass wool, this type of insulation is one of the most popular options. It’s a great choice for those looking to insulate the garage ceiling and garage walls. It is available in the form of fiberglass batts or rolls which are very easy to install. 

Fiberglass insulation is exceptionally effective at preventing heat loss and it’s also very fire-resistant. Installing a fiberglass batt is as easy as it gets and can be done by pretty much anyone. 

A word of caution, though – fiberglass fibers can be irritating and itchy to the skin. 

The Supplies & Tools You Will Need 

Caulking gun tool

To insulate your open-rafter garage, you will need the following tools and supplies: 

  • Insulation material 
  • Long sleeves, goggles, gloves, and mask 
  • Pre-cut pieces of wood (for measuring the insulation) 
  • Expanding foam (in low-expanding or high-expanding form) 
  • Air vents 
  • Step ladder 
  • Caulking gun 
  • Screw gun 
  • Staple gun 
  • Hacksaw
  • Utility knife 
  • Tape measure 
  • Garage door insulation kit (if you’d like to insulate your garage door as well – more on that later) 

A Step-by-Step Guide on Insulating Garage Ceiling Rafters 

Insulating the rafters is simple roof insulation work that can be done even by those who aren’t that DIY-savvy. 

However, the overall difficulty of this job will still depend on the type of insulation you decide to go with. As far as I’m concerned, the fiberglass one is the best insulation for a DIY garage roof insulation project. 

Step #1 – Measure the Space 

Model house with a garage

Before installing insulation, it is very important to do a bit of accurate measuring. 

Start by measuring the linear feet around the entire room. Once you get the number, you’ll be able to calculate the square footage by multiplying that same number by the room’s height. 

The calculated square footage will let you know how much insulation you’ll need and allow you to buy the correct amount of it. I would recommend purchasing an extra unit of insulation so that you don’t have to order again in case you use all of it and need more. 

Step #2 – Check for Holes and Cracks 

Garage wood

Checking for cracks and holes in the garage roof is the next important step. Adding insulation to the ceiling before fixing these cracks and holes is not advisable, as these can let the moisture get inside and, consequently, cause the growth of mold. 

To take care of them, I would recommend using an expanding foam sealant for insulation. Most products of this type can fill gaps that are up to 3” wide. Our post on types of ceiling cracks (with pictures) may be of help.

Step #3 – Make Room for the Air Vents 

The purpose of the air roof vents is the prevention of a mold-causing moisture build-up. What is more, these also prevent the buildup of heat (brought by warm air during hot summer days). 

  • Measure the width between two joists and, with the help of a hacksaw, cut the air vent to that same size. 
  • In the center of the vent, drill a hole whose size will be big enough for the screw to get through. 
  • Finally, use a screw gun to attach the air vent to the joist. 

Step #4 – Cut and Install Insulation  

Man cuts insulation

Are you using batt insulation? If so, use a utility knife to cut the batts to size. The batts will have to fit adequately between the joists. 

Loose-fill insulation, on the other hand, needs to be fluffed up before it’s put into the cavity. 

Once you’ve placed the insulation, secure it to the joists by using a staple gun. To make the insulation even more secure, ensure that the staples are spaced evenly. 

Step #5 – Install the Drywall 

Giving the garage ceiling a more refined look can be done by installing drywall. One very important thing to point out here is that the drywall needs to be flush with the insulation’s edges. 

If you want the drywall to match the appearance of the rest of the garage, prime it and paint it. In addition to having a clean look, painted drywall will also be more resistant to dirt and moisture. If you have a wooden garage door, you may have a similar question. This post deals with the question of whether you have to use a primer before painting wood.

How Do I Insulate Finished Ceiling Rafters? 

For finished garage ceilings, the best type of insulation is cellulose or blow-in insulation. Since it doesn’t leave any gaps, it is the most efficient solution for this kind of project. 

  • When it comes to insulating a finished garage ceiling, the first thing you’ll need to do is locate the studs. Ceiling studs are identical to the wall studs found in the walls of your home and can be found by taping on the ceiling with knuckles. 
  • After locating the studs, make a hole in each one of them with your drill. It goes without saying that these holes have to be sufficiently big for the insulation to fit through. 
  • To make blowing cellulose insulation into these holes easier, wet it down first. 
  • To blow the cellulose insulation into the holes, you will have to use a blower. Distribute the insulation evenly between the aforementioned studs. 

Frequently Asked Questions 

Does Insulating a Garage Require a Vapor Barrier? 

Using a poly vapor barrier is not always required while adding insulation to a garage space. 

However, to prevent the moisture from coming into contact with insulation in case your garage isn’t adequately sealed, installing a vapor barrier may be beneficial. 

Should I Heat My Insulated Garage? 

If you’re someone who often spends a lot of time in the garage during the winter months, you may want to consider getting a space heater – or any other kind of heating source – since the insulation might not be enough. 

How Much Does it Cost to Insulate Garage Ceiling Rafters? 

The amount of money you’ll have to spend on this kind of project will depend on the R-value of the installation and the size of your garage. 

Speaking generally, the cost to insulate this part of your home with fiberglass insulation averages at about $2 per square foot. 

What R-value Insulation is the Best for the Ceiling of My Garage? 

The R-value indicates how efficient certain insulation is when it comes to resisting the heat flow. The bigger this number is, the more resistant particular insulation will be to the heat flow. 

To find what R-value will be required to properly insulate your garage, you will first have to determine the amount of insulation the ceiling in your garage can hold. A vast majority of homes have insulations whose R-value ranges between R-13 and R-38. 

An important thing to mention here is that a lot of homeowners like to go with thicker insulation. If you’re one of them, keep in mind that thick insulation loses its R-value if you compress the batt. This is particularly true for those who decide to install baffles (foam baffles). 

Should I Insulate My Garage Door As Well? 

Garage door frame

Insulating the garage door is not necessary in most cases. However, many homeowners want to have complete control over the temperature in their garages – if you’re one of them, your best bet is to just get a pre-insulated garage door. 

If you think that adding insulation to the door by yourself is a better option, keep in mind that by doing so you will also be adding weight to it. In other words, you’ll want to make the necessary changes to the door’s springs before insulating it. 

The Takeaway 

There are many benefits to insulating garage ceiling rafters – better temperature control, reduced noise, protection against carbon monoxide, as well as fire prevention. Insulating the ceiling in this room will also increase your home’s overall value. 

In my opinion, this is a relatively easy project and one that’s ideal for the not-so-experienced DIY-ers. Just make sure you have all the necessary tools and supplies for the task beforehand!

Check out other eco-energy solutions for your home as well.

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