Rigid (Foam Board) Insulation R-Value: Chart & FAQ

Out of the dozens of options you have for insulation, one of them is foam board insulation, which is also known as rigid board insulation. Foam board comes in three different types with each type having its different qualities and R-value. As far as its popularity is concerned, rigid board is often preferred for its fair insulation and ease of installation. Foam board insulation has around 3.5-5.5 R-value for every inch. However, these numbers may vary according to the variation of the original product being used. Keep on reading if you plan on learning a thing or two about foam board and the R-values of its different forms.

Types of Foam Board Insulation & R Values

Foam board is commonly available in three variations, namely polystyrene, polyisocyanurate, and polyurethane. Let us have a brief look over their main characteristics, applications, and R-values.

Foam Board R-Value/inch
Expanded Polystyrene (EPS) 4.0
Extruded Polystyrene (XPS) 5.0
Polyisocyanurate 8.0
Polyurethane 6.5
Rigid foam insulation R value chart

Polystyrene (Expanded and Extruded)

The first one is polystyrene, which is further divided into expanded and extruded foam. Starting with the former, expanded polystyrene falls in the category of the cheapest insulation materials. It’s lightweight and ideal for protecting fragile items. As for its application, expanded polystyrene is not that widely used for insulation due to its short life. However, it resists bacteria and doesn’t cost a leg. Expanded polystyrene has an R-value of around 4.0 for every inch.

Coming to the latter, extruded polystyrene has more of a dense composition as compared to its sibling. Consisting of closed cells, extruded polystyrene is relatively stronger, durable, and more effective at preventing heat loss. Therefore, it’s often preferred for insulation purposes. The best part is that its superior performance does not make too much of a difference in cost. You’ll find this type of rigid foam in both blue and pink colors. Extruded polystyrene can put up an R-value of up to 5 for an inch.

Polyisocyanurate

Polyisocyanurate, also known as polyiso, is another form of foam board insulation that has closed cells. Due to its extraordinary capabilities of insulating heat energy, polyiso is a costly product. As a matter of fact, it is the most expensive and effective form of foam board insulation. An inch of polyiso can deliver an R-value of 7 to 8. Because of its dense structure, polyiso is used mainly for insulating attics where external temperature strikes the hardest.

Polyurethane

Lastly, we have polyurethane, a thermal insulation that is capable of barring soundwaves as well. Although it is also a closed-cell insulation, it does not boast as much stiffness as polyiso. However, both share a common trait, which is the presence of low-conductivity gas in their cells. Not only does polyurethane do a great job at thermal insulation, but it is also resistant to water. Polyurethane can be sprayed as well as installed in the form of rigid foam. It’s usually preferred in the form of spray foam because of the foam being able to fill in tight openings. You can expect polyurethane to put up an R-value of up to 6.5 per inch.

Recommended R-value for Foam Board

If you’ve decided to insulate a building with foam board insulation, you’re probably curious about which rating to opt for. Here’s the thing: the recommended R-value for every area is different. The US is subcategorized into climate zones with each zone having its own R-value requirements for homeowners to follow. Below is a table given that shows the required R-value for each climate zone:

Climate Zone For Attic For Floor For Wooden Wall
1 R-30 R-13 R-13
2 R-49 R-13 R-13
3 R-49 R-19 R-19
4 R-60 R-30 R-19
5 R-60 R-30 R-21
6 R-60 R-30 R-21
7 R-60 R-38 R-21

Comparing Foam Board to Other Insulation Materials

Foam board insulation has its own pros and cons, but how well does it put up in competition with other popular insulation options such as fiberglass, rockwool, and spray foam? 

Foam Board vs Fiberglass

In comparison with the mainstream insulation material fiberglass, foam board insulation takes the win in terms of R-value. Foam board in its weakest form EPS gives you an R-value of 4 per inch, whereas fiberglass only gives you barely 2.5. However, fiberglass costs almost half as foam board insulation. So, you’re basically paying more for a denser and more effective material. Fiberglass has the benefit of ease of installation, unlike foam board. As we mentioned previously, foam board requires tight fits since it cannot be molded into place. Fiberglass, on the other hand, is relatively easier to set up but requires extra safety measures.

Foam Board vs Rockwool

Rockwool is made from melted minerals and is commonly used for insulating exteriors of structures. 40% of rockwool’s ingredients are recycled, which means its manufacturing holds little to no harmful impact on the planet’s health. Compared to foam board insulation, rockwool brings a variety of benefits to the table. It comes with several praiseworthy qualities such as noise, pest, moisture, and fire resistance. Foam board, on the other hand, shares only a few common traits with rockwool. However, the former is way better for minimizing heat conduction. Rockwool offers an R-value of max 3.3 per inch while rigid foam can go up to 8 at its best.

Foam Board vs Spray Foam

Spray foam is usually derived from polyurethane, a type of foam board insulation that we described previously. As the name tells us, spray foam is applied in the form of wet thick foam that expands rapidly upon contact with the surface. It holds outstanding insulating abilities and is ideal for insulating areas where space is a problem. In regards to R-value and opposition to moisture, there’s not much of a difference since both spray foam and foam board fall somewhere in the same bucket. The real difference arises in the application of the material. Spray foam is better at filling gaps because of its texture as compared to rigid board, but it also costs way more.

Effect of Moisture on Foam Board R value

All products of foam board insulation come with special moisture-resisting abilities. For an insulation material, this can do wonders. Foam board insulation is almost completely impermeable. Due to this advantage, it is free from the need for a vapor barrier which is an additional cost. However, this can be said for all forms of foam board insulation except one, expanded polystyrene. EPS’ open-cell formation and spacious assembly of bubbles are not as good at fighting wetness as the rest of foam board’s types. Over time, if EPS faces dampness, it’s probable that it can impact the material’s efficiency and promote the growth of mold. For this reason, the R-value/heat resistance of foam board remains unchanged even if wet.

Does compression affect R-value?

Compression can become a major issue during the installing process for insulation. In general, you’re not supposed to compress the material because doing so can negatively impact the R-value you’re trying to aim for. Foam board insulation cannot and should not be compressed. Hence, its R-value remains unaffected during the installation process. Since the material has a rigid build, it’s almost impossible to try and push the foam into tight spaces. It is essential to cut the foam for a perfect fit. Otherwise, you’ll just be wasting both money and effort in putting up insulation that can barely preserve the inner heat energy of the house. 

Why Do R-values Differ in Foam Board Products?

You’re probably wondering how a single insulation material has different R-values. The answer to this question is quite simple. Since foam board comes in different variations with different compositions, each product has its own effectiveness in preventing heat energy loss. For instance, expanded polystyrene has a lower density and lighter mass. This means it has a lower R-value per inch as compared to other forms of foam board, such as polyurethane. Nonetheless, all of these types count as foam board and bring significant insulation properties to the table. 

Where to Use Foam Board Insulation

Deciding whether foam board insulation is right for you or not is an important decision. Foam board insulation is great at combatting moisture. Polyurethane, in particular, holds excellent moisture-resistant qualities that make it one of the best options for insulating areas where moisture would build up or pipe leaks might happen. Crawlspaces, kitchens, and bathrooms are just a few of the places where foam board insulation would fit best. You’re free to insulate other areas such as walls, attics, and floors if needed. Rigid foam board insulation can fit pretty much anywhere. You will have to tape the foam pieces together tightly to reduce heat conduction as much as possible.

How Long Does Foam Board Insulation Last?

What good is an insulation that requires replacement after every couple of years? Foam board insulation is one of the most solid solutions out there that provides a durable barrier against the weather. Its closed-cell forms have a long life due to their high-density composition. Not to mention foam board’s resistance towards moisture, which is one of the key reasons behind insulation going bad before its due time. You can expect the R-value to remain constant as well because rigid board does not deteriorate with time. Therefore, durability is the last thing you need to worry about if you’re opting for foam board insulation.

Conclusion

Make sure you do your research about which type of foam board product would best suit your needs before buying the material. There’s no denying that it’s one of the best insulation materials in the market, so the high price is somewhat justified. Lastly, you can refer to our table of recommended R-value ratings for different climate zones in the US.

Charles John

A novice DIYer who learns about home ventilation. I am a mechanical engineer and have a basic knowledge of HVAC systems but I learn continuously to make myself the best blogger in that space.

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