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Cellulose Insulation Manufacturers Association

Determine Current Attic Insulation R-Value

Here Is How To Determine Current Home Attic R-Value

One of the most common questions homeowners ask is how to determine the R-Value of their current attic insulation. This typically results when a homeowner begins to think about improving their home energy efficiency by adding or upgrading the insulation in their attic. And it’s a good place to start. Understanding the current amount of insulation and the R-Value is critical to determining how much is needed to bring the attic to the optimum energy efficiency. Fortunately, for those with blown in cellulose insulation the calculation process is fairly simple and here is the process.

The first step for assessing the current attic insulation is a simple visual check. As a general rule, looking across the attic floor, insulation should be above the top of the floor joists. So if the joists are visible it’s time to consider adding or replacing the insulation.

To understand the current R-Value, an easy way to estimate is by measuring the average depth of the insulation. This chart provides general values for estimating purposes of the most common types of insulation products. Fortunately, for those with blown in cellulose insulation or fiberglass batts the calculation process is fairly simple. Measuring blown in fiberglass is more complicated and really takes specialized equipment and knowledge so best to have an inspection by a qualified insulation contractor.

Chart to Estimate R-Value of Existing Attic Insulation CIMA

Once the current R-Value is estimated, check the recommended R-Value based on the region of the country.

So, for example, a home in Kansas is recommended to have a minimum R38 for attic insulation. With blown-in cellulose insulation, that would require insulation to a depth of 10 inches. If there is already insulation, measure the current depth and subtract to determine the amount needed to bring the attic to the recommended level for the desired R-Value. If this house has 4 inches already, another 6 inches of blown-in cellulose insulation would be needed. Use an insulation savings calculator to get an idea of how much the home would save on energy costs by adding more insulation.

Remember, R-Value is only one of four factors for determining the insulating quality. Learn more about What R-Value really means.

Keep in mind, a home energy audit is recommended when considering energy saving home improvements. Also, a professional insulation contractor is always the best installation option, regardless of the product used, to obtain maximum performance and energy savings. Have more questions, contact CIMA for answers anytime.

2019-11-25 04:38:57
Paul Commons says:


I am a contractor working with cellulose to put into ceilings for builders in Australia. We have currently been questioned on the issue of the weight limitations for dry cellulose in ceilings (putting excessive pressure on plasterboard ceiling sheets) as compared to fibreglass batts. We would typically install R4.0 - R6.0 into new build projects which we would estimate somewhere between 4kg and 6kg per m2.

While the same in fibreglass batts is estimated at only 2kg/m2.

Would you have any information or reference material that would assist us in determining appropriate weight limitations for cellulose installed in ceilings? I realise this may vary depending on how the ceiling has been constructed - however at this point we would be grateful for any information that may be able to assist us to use as a reference for supporting the use of cellulose insulation into ceilings.



2020-01-15 12:21:40
CIMA Admin says:

Great question and one we have not received before. This is really a non-issue. The 6 kg/m2 is about 1.23 psf. For ½-inch US Gypsum Sheetrock installed 24” On Center, it will support 2.2 psf, per USG data sheets. I don’t imagine the products available in Australia are much different. Even with currently-published data you could install RSI R10 (US R-60) and still be well below the published weight limit. As chance would have it over the last several days we have had an e-mail dialog with a senior researcher at the USG Corporate Innovation Center. He is studying insulation loading on ceiling Sheetrock as it pertains to ICC Evaluation Services Acceptance Criteria. They have looked at their Sheetrock product with up to 4 psf of insulation on it and found no problem.

2020-12-22 19:59:22
Sam Duncan says:

Are calculations of thickness for the depth just after installation or after it has settled for 3-4 years as mine has. Original depth was 16 inches, now it is about 12 " so what is the true R value?

2020-12-29 11:20:58
CIMA Admin says:

Thanks for your question. Like all loose fill insulation coverage charts, cellulose insulation coverage charts have both settled thickness and installed thickness columns. Installed thickness is determined by ASTM C1374. Settled thickness is determined by laboratories using methodology consistent with the Federal Trade Commission R-Value Rule (16 CFR Part 460). R-values in cellulose insulation are based on settled thickness and bag count, not installed thickness. There are no such things as a standard coverage chart or R-factor for any insulation material, but for cellulose it would be reasonable to assume an R-factor of 3.8. For the installation level of 12" mentioned that would amount to about R45 – R46.