It gets cold outside in the winter but that doesn’t mean it has to be cold inside your house. Using the right kind of insulation in the right way is the key to staying warm and safe indoors. I recently talked about the Pitfalls of Old House Insulation and I’ve written about a variety of different types of insulation including blown-in, spray foam, and mineral wool, but today I want to focus on foam insulation.
Foam insulation can come in a few different forms, the most popular of which is spray foam, but there is also foam board insulation (often called rigid foam insulation). In this post, I’ll give you a break down of all three types and when and where they work best. These are all fine types of foam insulation, but don’t be fooled that they all work in every situation. When poorly matched to your application, some of these can pose major problems.
Spray Foam Insulation
In the last 20 years, spray foam insulation’s popularity has grown immensely and for good reason. It has a very high R-value per inch when compared to other insulations like fiberglass, mineral wool, and blown-in insulations. This high R-value, coupled with an ability to provide excellent air sealing helps a lot of contractors who specialize in new construction meets the more stringent air sealing requirements of the building codes today.
Open-Cell Spray Foam
- R-Value: 3.5 per inch
- Cost $1 to $1.25 per SF
The most affordable of the spray foam options, open-cell spray foam, is used in many applications like roof, walls, and ceilings. Open-cell foam insulation expands greatly upon installation and is fairly soft to the touch, unlike closed-cell foam. Open-cell foam is also vapor permeable which means it does not count as a vapor barrier and needs to have one applied over it. The vapor permeability of open-cell spray foam means that it can take in water and hold it, which can be a real danger. In the case of leaks, open-cell foam will absorb water and hold it against the framing and sheathing elements facilitating mold and rot.
Bottom Line: It’s a great and affordable insulation option when there is no chance of water intrusion, but you’re rolling the dice if you think it might get wet.
Closed-Cell Spray Foam
- R-value: 6.5 per inch
- Cost $2 to $3 per SF
Closed-cell spray foam is the king of the insulations when it comes to both R-value per inch and cost. Nothing else comes close, really. Unlike its open-cell cousin, closed-cell spray foam is not vapor permeable and will not hold water. This makes it an excellent option in case of water intrusion. It also includes binders and glues in many applications that help it to literally glue a structure together. Its strength can help prevent uplift when applied on roof decks and provide shear strength to structures. One downside is that often the blowing agents for closed-cell foam are hydrofluorocarbons, which are not so good for mother nature.
Bottom Line: It’s expensive, but wow, will this get you a structurally sound and well insulated building.
My personal preference on spray-foam insulations is that for new construction, they are an excellent option (if applied correctly, since user error during installation can cause catastrophic results. Learn more here). On historic buildings and remodels, I’m not a fan because the design of older homes especially historic in nature was never meant for this type of insulation and can cause performance problems.
Spray foam is NOT reversible, which is a major problem in historic buildings, which may cause irrevocable damage to historic fabric.
Rigid Foam Insulation
Not all foam insulation is spray foam. Rigid foam insulation is another option for insulation that should be considered, especially since unlike spray foam, it is easily reversible, a major plus for historic structures. Rigid foam insulation come in several varieties and of course, there are different makers of each type.
All of these rigid foam insulation options are usually sold in 4×8 sheets of varying thickness from 1/2″ to 2″ so you can pick and choose the best option for your house.
Expanded Polystyrene (EPS)
- R-value: 3.8 per inch
- Cost: $.26 to $.32 per SF
Expanded polystyrene (EPS), often known as Styrofoam, has been popular for years. It does just as well at insulating coffee cups as it does old houses. While it may have the lowest R-value of the rigid foam options, it does also follow with the lowest cost of any we’ve mentioned so far. There have been newer High-Density EPS products that have managed to bump up the R-value and excel in exterior applications.
Extruded Polystyrene (XPS)
- R-value: 5 per inch
- Cost: $.38 to $.45 per SF
Extruded polystyrene (XPS) rigid foam is usually blue or pink in color and is not quite as rigid as the other foam insulations, which gives it a little more versatility, in my opinion. XPS has no problems with absorbing water, unlike polyiso, which is another mark on the plus column for XPS. It typically does not come faced with foil like the other two types of rigid foam mentioned here.
- R-value: 6.8 per inch
- Cost: $.65 to $.75 per SF
Polyisocyanurate (polyiso for short) has the highest R-value and (no surprise) correspondingly the highest cost for any rigid insulation. Its R-value does have a tendency to degrade over time with exposure. Typically sold with a radiant barrier of foil on both sides, you also get the benefit of stopping radiant heat with this foam. Read about radiant heat transfer here! Polyiso is essentially the board form of the closed-cell spray we talked about in the first section.
Rigid foam insulation is not something you cut and stuff into the stud bays like fiberglass or mineral wool. Rigid foam insulation is best installed on the exterior of the framing and then the siding is installed over top of it. This may be particularly difficult and expensive for retrofit installations.
Installation on roof decks as a part of a re-roof project often makes the most sense and provides the biggest payback. If you’re going to add 2″ to the height of your roof by decking with rigid foam prior to installing new shingles, you’ll need to make some modifications to the cornice and fascia, but this can be done in ways that don’t impact the overall appearance of the house too much.
It’s really for you to make up your mind as to if foam insulation is right for you, and if so, which type and where. I know, I know, lots of question and not as many answers. My hope is that armed with the information here, you can make a better decision when it does come time to insulate your house. Happy insulating and stay comfy!
Founder & Senior Editor
I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.