Foam insulation: Is it worth it?

Although it can get cold out in winter, it doesn’t have to be that cold inside. The key to keeping your house warm and safe is to use the right insulation. I wrote about the Pitfalls and Uses of Old House Insulation. I have written about many types of insulation, including mineral wool and spray foam. Today, however, I will be focusing on foam insulation.

Foam insulation comes in many forms. The most common is spray foam. But foam board insulation is often called rigid foam insulation. This post will give you an overview of each type and how they perform best. All these are excellent types of foam insulation. However, they don’t all work in all situations. Some of these may pose problems if they are not properly matched for your particular application.

Spray foam insulation

Spray foam insulation has enjoyed a huge rise in popularity over the past 20 years. Spray foam insulation has a much higher R-value per inch than other insulations such as fiberglass, mineral wool, or blown-in.

Contractors who specialize in new construction can benefit from this high R-value and excellent air sealing. The cost of spray foam insulation can vary, here we have some examples of different types of insulation and prices per square foot

Foam for open-cell spray

  • R-Value: 3.5 per inch
  • Prices from $1.25 to $1.25 per SF

Open-cell spray foam is the most economical option. It can be used for roofs, walls and ceilings. Open-cell foam insulation expands rapidly upon installation and feels very soft, unlike closed-cell foam.

Open-cell foam can also be vapor permeable, which means that it doesn’t count as a barrier to vapor and must have one. Open-cell spray foam’s vapor permeability means it can absorb water and keep it. This can pose a danger. Open-cell foam can leak and will hold water against the sheathing and framing elements, facilitating mold growth and rot.

Bottom line: This is a cost-effective and great insulation option when there is no risk of water intrusion. However, you have to be careful if it gets wet.

Closed-Cell Spray Foam

  • R-value: 6.5 per inch
  • Prices from $2-$3 per SF

Closed-cell spray foam insulation is the best in terms of cost and R-value per square inch. There is nothing else that comes close. Closed-cell spray foam, unlike its open-cell counterpart, is not vapor permeable. It will not retain water. It is therefore a great option for water intrusion.

It can also include binders and glues that are used in many applications to help glue together a structure. Its strength can prevent roof deck uplift and give structures shear strength. The downside to closed-cell foam is hydrofluorocarbons which can be harmful to mother nature.

Bottom Line: This is a costly investment, but it will get you a well-insulated and structurally sound building.

Spray-foam insulations are my preference for new construction. They can be a great option (if used correctly). Find out more. For historic buildings and remodels, I don’t recommend this insulation because it can cause performance issues and was not intended for older homes.

Spray foam cannot be reversed, which can pose a problem in historic buildings and cause irreparable damage to historic fabric.

Rigid Foam Insulation

Spray foam insulation is not all that good. Rigid foam insulation can be another option, and unlike spray foam, it can be reversible. This is a big plus for older structures. There are many types of rigid foam insulation, and each manufacturer makes a different type.

These rigid foam insulation options come in 4×8 sheets ranging from 1/2” up to 2’’ thick. This allows you to choose the right option for your home.

Expanded polystyrene

  • R-value: 3.8 per inch
  • Costs: From $.26 to $0.32 per SF

Styrofoam (or expanded polystyrene) has been a popular product for many years. It is just as effective in insulating coffee cups as it is in older houses.

It may not have the highest R-value among rigid foam options but it has the lowest cost. Newer High-Density products made of EPS have seen an increase in R-values and have excelled in exterior applications.

Extruded polystyrene

  • R-value: 5 per inch
  • Costs: $.38 – $.45 per SF

Extruded polystyrene rigid foam (XPS), is often blue or pink and is less rigid than other foam insulations. This gives it more flexibility, in my view. Unlike polyiso (which is another plus column for XPS), XPS does not have problems absorbing water. It is not usually covered with foil, like the rigid foams mentioned above.


  • R-value: 6.8 per inch
  • Costs: $.65 to $0.75 per SF

The highest R-value for rigid insulation is polyisocyanurate or simply called Polyiso. The R-value of polyisocyanurate is susceptible to degradation with time. This foam is usually sold with a radiant barrier of foil along one side. It also has the advantage of stopping radiant heat. Learn more about radiant heat transfer. Polyiso is the board form for the closed-cell spray that we discussed in the first section.

Rigid foam insulation should not be stuffed into stud holes like mineral wool or fiberglass. Rigid foam insulation should be installed on the outside of the framing, then the siding can be installed over it. Retrofits may find this difficult or expensive.

Roof decks are often part of a re-roofing project and offer the best return on investment. You can increase the roof’s height by installing rigid foam decking. However, this will require some changes to the cornice or fascia. This won’t affect the overall look of the house.


It is up to you to decide whether foam insulation is right and, if so which type and where. Its always good to upgrade your insulation if you own any rentals, it will definitely make you more money in the long run. Yes, there are many questions, but not as many answers. I hope you will be able to make an informed decision about insuring your home. Enjoy your insulating journey and be comfortable!