How do I know what size insulation I need?

Author: Kaya Padberg Jr.  |  Last update: Friday, October 10, 2025

Start by measuring the height and length of each of your room walls to determine the amount of insulation needed. Write down the spacing of the studs and multiply the wall's length by the wall's height so you can determine what kind of house insulation will be needed.

How do I know what size insulation to use?

Depending on where you live and the part of your home you're insulating (walls, crawlspace, attic, etc.), you'll need a different R-Value. Typical recommendations for exterior walls are R-13 to R-23, while R-30, R-38 and R-49 are common for ceilings and attic spaces.

Is R19 or R30 better for an attic?

Yes, you can use R19 insulation in an attic, but whether it's suitable depends on a few factors: Climate Zone: R19 insulation is typically appropriate for attics in moderate climates. In colder climates, higher R-values (like R30 or R38) may be recommended to effectively prevent heat loss.

How do you calculate insulation requirements?

To calculate the amount of insulation needed, multiply the length and width of the loft space and divide by the area covered by each roll of insulation. Therefore, for this example, around 7 rolls of loft insulation would be required to cover the loft space of a typical semi-detached house.

What is the formula for calculating insulation?

U-value = ƛ-value / thickness of the insulation

The quotient of these two figures (the U-value) is therefore expressed in W/m2K, which stands for the number of Watts (W) per square metre (/m2) at a temperature difference of 1 degree Kelvin (K).

How much Insulation Do I Need? Expert advice.

What thickness of insulation do I need?

In general, 50-130mm of insulation is commonly used for wall insulation.To achieve the required U-Value of 0.17 W/m2K, 125mm of PIR insulation board should be used. 150-200mm of insulation is often used for roofs to achieve the recommended U-Value of 0.16 W/m2K.

How do I figure how much insulation I need?

Start by measuring the height and length of each of your room walls to determine the amount of insulation needed. Write down the spacing of the studs and multiply the wall's length by the wall's height so you can determine what kind of house insulation will be needed.

Is it OK to use R19 insulation in attics?

You will want to bring up your attic up to R-38:

If your attic has 5-6″ of existing insulation, you'll want to install an R-19 to achieve a total of R-38.

Where is R30 insulation used?

R30 insulation is primarily used in spaces requiring high thermal resistance to maintain energy efficiency and regulate indoor temperatures. It is common in attics, floors, and ceilings, where heat transfer is more likely due to exposure to outdoor temperatures.

Should you use faced or unfaced insulation in a crawl space?

Faced insulation is ideal for spaces that have no existing insulation, such as exterior walls, attic ceilings, crawl spaces, and exterior basement walls. Please note that the proper way to install it is to have it installed facing outward or towards the person installing it.

How do I choose the right insulation?

Choosing the right kind of insulation depends on where you're adding it, the desired R-value, and your budget. In the simplest terms, a higher R-value indicates greater insulation properties. The R-value you need is determined by where you live and what part of the house you're insulating.

Can you over insulate a house?

It is possible to over-insulate your house so much that it can't breathe. The whole point of home insulation is to tightly seal your home's interior. But if it becomes too tightly sealed with too many layers of insulation, moisture can get trapped inside those layers. That's when mold starts to grow.

Is it better to insulate a ceiling or roof?

The answer to these issues may be adding insulation, but that still leaves the question of where to apply additional thermal protection to see the most benefit. While both ceiling and roof insulation will increase your home's resistance to outside temperatures, insulating the ceiling should be your priority.

Is adding insulation to an attic worth it?

Should you insulate your attic roof? Insulation is a wise choice to increase the energy efficiency of your home. Quality insulation will make it more cost-effective to heat and cool your home, The Department of Energy's estimates range from a 10% to 50% savings, depending on several factors.

Can insulation R-value be too high?

The optimal R-value for most homes is 30, but you can go as high as 38 if you live in a tropical climate. Anything higher than 38 indicates an over-insulated space. While you might think that too much insulation can't be bad, there are some significant consequences to consider.

Do R30 and R38 make a difference?

R-value measures insulation's heat resistance, with higher values indicating better performance. R38 insulation has a higher R-value than R30, providing greater resistance to heat flow. This increased resistance translates to improved energy efficiency and better temperature regulation in homes.

How do I choose insulation thickness?

Choosing the right thickness for your insulation depends on various factors such as climate, location within your home, and type of material used. It's crucial to do thorough research, consult with professionals, and ensure proper installation for maximum efficiency.

Is 2 inches of insulation enough?

Two inches of closed cell spray foam might be enough insulation for some applications. However, for other applications, only two inches will not be able to deliver the results you need.

What is the labor cost to install insulation?

Most installers charge between $40 and $80 per hour to install insulation, so the more complex the project, the more it will cost. A few complexities that may increase the cost of labor include: Your installer needs to install insulation in hard-to-reach areas.

What is the best thickness of insulation for preventing heat loss?

The recommended thickness for EWI typically ranges between 60mm and 100mm, depending on the type of insulation material used, the construction of your existing walls, and the age of your home.

What thickness blows insulation?

How thick should blow-in insulation be in an attic? For attics, you should aim for an R-value between 38 and 49, depending on your specific needs and climate zone. To achieve an R-value of 38 to 49 in your attic, you should aim to install 11 to 14 inches of blow-in cellulose insulation.

What is the optimum thickness of insulation?

Optimum insulation thickness is 3.2 cm for cooling while it is 2.5 cm for heating. One should select 3.2 cm, the optimum insulation thickness based on cooling degree hours, for the design insulation thickness since it provides a higher energy savings for building.

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