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Can You Use Foam Board in the Attic? Insulation Options


Foam board insulation installed in attic

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If you are thinking about insulating your attic, there are several options. The trendy one is spray-in foam, but that can be costly and is not a DIY project. Foam board is cheaper, but can it be safely used in an attic?

You can use foam board in the attic. It is an excellent installation option for the attic because it is lightweight, easy to work with, and offers high R-values. Often the R-value of fiberglass insulation is too low and requires the rafters to be retrofitted, making foam board a better choice.

When building my current home, I was faced with a dilemma on how to insulate a bonus room that would be upstairs. Since the walls would be adjacent to the attic space, I wanted to make sure that I could properly insulate and air-seal them to prevent energy loss.

Spray foam was ridiculously expensive and although there are clear benefits in terms of insulation value, I just couldn’t talk myself into it. But foam board offered some real benefits of it’s own, including the ability for me to install it myself.

According to the EPA, insulating the attic could save you an average of 15% of your costs for cooling and heating your house. Let’s have a look at what rigid foam is and the different types, some tips on installing it, and special considerations to keep in mind.

What Is Rigid Foam Board?

Rigid foam board, also known as foam board or insulation board, can be an excellent option for insulation. It is lightweight and easy to work with. A 4 by 8 board weighs next to nothing and can easily be cut to fit.

Typical thicknesses range from ½ inch to 2 inches, with estimated R-values of R-3 for the ½ inch to R-10 for the 2-inch board.

Often you will hear that rigid foam is a closed-cell product.   

In an open-cell insulation product, like fiberglass, the air is trapped inside the fiber. However, the fiberglass can be compressed, decreasing the size of the air pockets.

When installing fiberglass, people often compress conventional fiberglass batts which reduces the product’s effectiveness.  

That’s not a problem with foam board insulation.

How Does Rigid Foam Board Work?

A rigid foam board works by trapping air, making heat conduction more difficult. Heat can be transferred by three methods—radiation, convection, and conduction.

When the sun’s heat hits your roof, that heat is radiated across the roof. For that heat to make it into your attic, it must be conducted through the roofing materials.

Using rigid foam to create an air cavity makes it more difficult for the heat to conduct itself into your living space.

This short video from a foam board manufacturer demonstrates installing foam board to allow for these air cavities.

R-TECH Insulation in an Attic or Ceiling Application

Are There Different Types of Rigid Foam Board?

There are currently three types of rigid foam board.  

Molded Expanded Polystyrene Foam Board

MEPS is a rigid board formed by molding polystyrene into boards. Polystyrene can be fashioned into various items, including shipping materials, such as packing peanuts and coffee cups, as well as sheets of foam board insulation.  

  • MEPS, also called beadboard, is made by pouring loose polystyrene beads into a container. The beads are heated and expanded. Next, the beads are poured into a mold where additional heat and pressure form them into blocks.
  • Beadboard is manufactured in a variety of densities and widths. Denser MEPS is used for roofing so that it won’t be damaged as installers walk on it. Beadboard used for walls is typically thicker but not as dense.
  • Expanded Polystyrene boards can be used above or below grade and have good moisture resistance. These boards are usually white and have a silver facing. 
  • The R-Value ranges between 3.8 to 4.4 per inch

Extruded Expanded Polystyrene Foam Board

XEPS, or XPS, is also made with polystyrene beads.  

  • The pellets are melted, as in MEPS, but instead of being poured into a mold, the liquid is then extruded through a die that shapes the board.
  • XEPS has a more consistent density and is less susceptible to being pressed down, making it a better choice for roofs. Extruded polystyrene also resists moisture better than MEPS.
  • Extruded foam boards are more expensive than expanded, but the R-value is around R-5 per inch.  
  • Pink Owens Corning Foamular is an extruded board, as is Dow Styrofoam, which is blue.

Polyisocyanurate and Polyurethane Foam Board

Both boards are urethane-based. This insulating material allows thinner boards to provide equal insulation values. An inch of polyiso insulation has an R 6 to R 8 value.

  • Polyiso foam boards are ideal for roofs because they remain stable over a more extensive temperature range than other boards.  
  • An aluminum foil facing added to Polyiso boards creates a vapor barrier. However, polyiso is not recommended for below-grade applications because it absorbs liquids easily.

Which Should You Pick?

If you are working with foam board in the attic, you need to consider your budget. But space and function are also important considerations.

If you have lots of room to work and are only trying to insulate your house better, then the molded or beadboard could be an option. It will reduce how much heat escapes in the winter and keep heat out in the summer, allowing you to heat and cool your house for less.

However, if you plan to convert part or all your attic into a living space, you should probably choose extruded. First, it is sturdier and will hold up better. Secondly, it is thinner, which will allow for easier installation in tight spaces and give you more room.

What Kind of Problems Can Rigid Foam Board Create?

The following are the concerns most cited:

  • Moisture. Water can condense inside the wall cavities if the air can infiltrate them. Condensation can occur both in the winter—as warm air from the house leaks into the cavities, or in the winter when heat from the sun condenses in the cavities. In either case, if the water stays in the cavities, then moisture-related problems like mold and rot can occur.  
  • Fire. Once foam insulation catches on fire, it burns rapidly. Also, the smoke that comes from rigid foam is thick and full of toxic gases. For that reason, you will need to use a fire barrier. Half-inch thick gypsum board is commonly used for that purpose.
  • Insects. Although insects can tunnel through foam boards, if you read about this as a potential problem, it usually refers to foundation installations. Foam boards treated with a borate compound prevent this problem. 

How Do You Install Rigid Foam Board?

If you do not plan on finishing your attic, then the installation process will be straightforward.

  1. Measure the space between rafters and cut the board to fit.  
  2. Use a chalk line or T-square to mark where it needs to be cut.
  3. You can use a utility knife to cut using the same cut and snap process you use with drywall. A table saw will also cut the board, but it will raise more dust.
  4. Put the board between the rafters. You can use a foamboard adhesive to glue it in place. Do not nail the board into the roof.

If you plan on finishing your attic, then you cannot install it directly.

  1. Your board must leave at least ½ inch gap between the board and the roof.  
  2. Use furring strips nailed into the sides of the rafters so that you have something to fasten the board to.  
  3. To create an airtight seal, use foil tape.
  4. Install foam boards with the foil side facing you.

Another installation method is to attach the board to the rafters just like you would drywall. When you get ready to hang the drywall, your screws need to be at least one inch longer than the width of the drywall and foam board combined.

Since the sidewall vents will be covered up when you finish your renovation, you need soffit and roof ridge vents so that hot air can escape.  

This short video by Owens Corning will give you a general idea of the installation process:

Insulation Installation Guide: FOAMULAR® XPS Insulation for Attic Ceilings

Bottom Line

If you can get to your attic easily, then insulating your attic with foam board is an excellent way to either turn it into an additional living space or cut down on energy costs.

It is a larger project, so you might want to consider hiring a pro who can do the job in half the time. I hired a young man in our neighborhood to help me with the install but it took some work, mainly getting around in the tight corners of the attic.

Conduct a Free DIY Energy Audit on your home using our guide and worksheet.

Paul

As a homeowner, I am constantly experimenting with making the structure of my house more energy-efficient, eliminating pests, and taking on DIY home improvement projects. Over the past two decades, my family has rehabbed houses and contracted new home builds and I've learned a lot along the way. I share my hard-learned lessons so that you can save time and money by not repeating my mistakes.

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