How To Insulate A Metal Roof [A Complete Guide]

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Metal roofs have become increasingly popular because of their durability and look. If you have a metal roof, you might be wondering how to best insulate it. In this post, we combine industry professional knowledge and up-to-date research to thoroughly answer your question.

As with all building surfaces, the goal of insulating a metal roof is to provide a layer of material that is resistant to heat transfer below the weather-resistant roof material. For this particular job, there are a few options that all have their own benefits, drawbacks, and appropriate applications. These options include:

  • Fiberglass batts
  • Foam board
  • Spray foam

Keep reading the rest of this post for details on each of the metal roof insulation techniques. We'll include tips for how best to install each type of insulation and some benefits and drawbacks of each type. To conclude, we'll answer a few questions related to the topic of this post. Without further ado, let's get into it.

Construction worker wearing safety harness and safety line working on a metal industry roof new warehouse, How To Insulate A Metal Roof [A Complete Guide]

How To Insulate A Metal Roof

One of the biggest challenges unique to metal roofing is the fact that metal is a 100% air and moisture barrier. Therefore, if you install insulation under the roof and moisture can get through that insulation, it will condense and collect on the underside of the roof.

Amazing metal roof

This moisture collection will damage the roof, the roof support structure, and the insulation. Thus, all insulation techniques must take steps to either deal with that moisture or prevent the moisture from hitting the roof in the first place.

Here we'll discuss fiberglass and spray foam insulation which are both appropriate for adding insulation to an existing roof. Also, we'll discuss rigid foam board insulation which is most appropriate for insulation during construction.

Of the three techniques discussed here, spray foam is likely the method with the highest R-value, lowest maintenance, and least likelihood of generating moisture control issues. Thus, spray foam comes as the most highly recommended insulation for metal roofs.

Fiberglass Batts

Hand holding a fiberglass batt.

For traditional wood construction, fiberglass batts are an easy and cheap way to insulate. In the same way, these are also the cheapest option for metal roofs. However, the need for moisture control is particularly important because the fiberglass will lose a lot of its R-value if allowed to get wet.

Also, fiberglass batts are generally only appropriate for ceilings if there are relatively closely spaced wood rafters along the roof. Without these supports, it is difficult to impossible to ensure that the batts stay in place. Thus, only consider fiberglass batts if you have an appropriate rafter system.

To install, follow these directions.

1. Measure Space for Materials

When installing fiberglass batts, it is important to maximize the possible R-value and to fit the framing. Thus, measure the distance between rafters, the depth of the rafters, and the total square footage. When shopping, you will want to match your purchase to all of these dimensions.

Click here for fiberglass batts from Amazon.

Also, you will need to choose between batts that have a vapor barrier facing or adding a separate 6-millimeter plastic sheeting as a vapor barrier under the batts. Both these techniques will require staples and a staple gun. Take note that not all fiberglass facing works as a vapor barrier.

If you opt for faced insulation, you will also need high-end vapor barrier tape to tape all seams and edges.

2. Install Batts

Now, place the batts up between the rafter bays. For obstructions and odd-sized bays, be sure to cut the batts to size. The goal is to have fluffy batts that fully fill all available space without being compressed at all.

Click here for a staple gun from Amazon.

If your batts are faced, staple the facing onto the wooden rafters as you go. If the batts are unfaced, you will rely on friction to hold the batts in place until you can get the vapor barrier installed.

Take note that it is important to wear eye and breathing protection whenever working with fiberglass because the material can be very harmful if absorbed into any bodily opening.

3. Install Vapor Barrier

If you used faced batts, simply tape all joints and edges of the facing material. If you used unfaced batts, staple up the vapor barrier onto the rafters to completely cover the ceiling. For the best results, be sure to overlap all seams and/or tape the seams and edges.

Click here for 6-mil plastic from Amazon.

Foam Board

Extruded polystyrene foam boards

Foam board is most appropriately installed during new construction before the metal roofing itself has been installed. To install, follow these directions.

1. Add Sheathing

First, add plywood sheathing to the top of the roof rafter's construction. These should run perpendicular to the rafter direction. Attach these as you would normally attach sheathing on a roof.

2. Add Home Wrap

Now, staple a layer of home wrap on top of the sheathing. This will help keep moisture out of the insulated space. Be sure that the wrap is properly oriented and that you overlap all edges by at least 6 inches.

Click here for home wrap from Amazon.

3. Install One or Two Layers of Foam Board

On top of this sheathing and home wrap, install your foam board. Either install one or two layers of this material depending on how much insulative power you are looking for. For this job, you will want to use very long screws with wide washers to hold the relatively soft foam board in place.

Click here for foam board insulation from Amazon.

4. Tape or Caulk Seams

Now, tape or caulk all of the seams in the foam board. This completes the vapor barrier nature of the board and also improves the insulative properties of the assembly.

Click here for seaming tape from Amazon.

5. Add Spacing

Before adding the roofing, it is often best practice to add some sort of spacer that separates the insulation from the metal roofing. This provides a small amount of space to allow any water that does get on the underside of the roof space to flow down to the roof edges.

Spray Foam

Worker spraying in foam insulation on the wall

For adding insulation to an existing metal roof with any type of support/rafter system, spray foam is always a good choice. For the best result, use closed-cell spray foam which also makes a very solid and reliable vapor and air barrier. Closed-cell foam also has a higher R-value per inch.

Before considering this job as a DIY project, it is smart to get a bid from a local spray foam installer. Unfortunately, this insulation technique is relatively expensive whether you install it yourself or pay a professional.

The high-end equipment professionals use makes this job and easier faster, and it provides a better final product. However, below we'll provide basic DIY instructions for doing this type of insulation yourself.

To install, follow these directions.

1. Prepare Area

First, tape and tarp off all areas that you fear foam dripping onto. This product is very hard to remove from porous surfaces such as cloth. However, it will scrape off hard surfaces like concrete. The more tarping you do beforehand, the less cleanup you will have later down the line.

2. Spray Foam

Spray foam comes in containers that generally cover 100 or 200 square feet. Following the directions on the box, first, test the spray in a trash can or disposable surface. This will give you a sense of how fast the foam comes out and how much it expands.

Click here for spray foam from Amazon.

Now, draw the nozzle along your metal roof 6-8 inches away. This allows you to spray an even layer of foam. As you do this, be sure that joints, where rafters and other components touch the roof, are well coated.

For electrical and other similar building elements that may be on the underside of your roof, you may need to hold them in place as the foam expands around them.

Moving a ladder across your roof for this job is difficult because of the heavy spray foam container and short hose. Alternatively, you may want to erect scaffolding or rent a high-end sprayer that has a longer hose.

Take note that it is important to wear a respirator during the application process because of the dangerous off gassing.

3. Allow to Cure

Importantly, give the foam about 8-24 hours to cure before using the space. During this time, the foam will still be off gassing. According to the EPA, this gas can be dangerous if inhaled.

Do you need a vapor barrier under a metal roof?

Not necessarily. If you do not install insulation and if condensation is not a problem, then a vapor barrier under a metal roof is unnecessary. However, if you do insulate your metal roof, a vapor barrier is an essential addition.

What is the best underlayment for a metal roof?

The most common underlayment for a metal roof is to use felt. This material is also known as tar paper. That being said, the best underlayment depends on the specifics of your situation including the type of insulation, the slope of the roof, and your local climate.

Can you spray foam insulation on a metal roof?

As mentioned above, yes you can. Spray foam insulation is actually the best insulation for a metal roof, especially when applied as a remodel.

How do you insulate corrugated iron?

Stacks of metal corrugated sheet

Insulate corrugated iron using any of the insulation techniques outlined above. As a remodel, it is easiest to either insulate with fiberglass and a vapor barrier or with closed-cell spray foam.

In Closing

In this post, we answered the question of how to insulate a metal roof. We included advice both for insulating this surface as a remodel and for new construction. Good luck!

To learn more about insulating metal roofs, take a look at these other posts:

How To Insulate An Aluminum Patio Roof [A Complete Guide]

How To Insulate A Roof From The Inside?

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