We want to add insulation to our attic, and are considering cellulose. However the ceiling trusses are on 24" centers and the ceiling sheet rock is only nailed to the truss. (The house was built in 1965.). There is currently about 2.5 inches of cellulose in the attic and we would like to get the R-value to R-39. Is it likely that the added weight of the new cellulose would cause the ceiling to sag?

CONTINUED QUESTION: Thanks for the information below, but the specs for my application are a little different. The house was built around 1965 and the sheetrock seems to have been nailed rather than screwed & glued. I'm assuming it is 1/2 sheetrock. This means the weight load capacity is lower than the 2.2 lb per sq. foot listed below. Assuming 1/2 sheetrock nailed* on joists on 24 inch centers, What would the weight load capacity be?


Regarding your question about the weight of the cellulose on 24" trusses, the weight should not be a problem in this application. You stated that you would like to increase your R-value to R-38. Your current R-value with the 2-1/2" of cellulose is approximately R-8. Therefore, you would need to add an R-30. This would be adding approximately 8-1/4". The weight ratio per square foot of 8-1/2" of cellulose is usually .97 pounds per square foot. This should not cause a problem.

I would recommend adding an R-40 which would bring you to the maxiumum insulation and the point of diminishing returns on your investment. An R-40 would be 11.15" with a rate ratio of approximately 1.29 per square foot. (If your drywall has been properly installed, which is glued and screwed, it is capable of carrying a weight load of approximately 2 lbs).

Note: You also will want to add baffles for ventilation inside the attic and beware of walking on the trusses and putting all your weight on one truss. It is imperative that you distribute yours or the installers weightload on more than one truss at a time. This can be accomplished by sliding a few boards into the attic and walking on them. You will need to be careful not to approach the end of any board or it will flip and maybe cause you to fall through the ceiling.

CONTINUEDANSWER FROM THE INSULATION DOCTOR: In response to your question regarding the 1/2" sheet rock and the weight of the cellulose insulation. 1/2" sheet rock on a 24" on-center trussed ceiling is considered standard by today's building standards. The fact that the sheet rock is not screwed may not be relevant to any significant difference in the weight load. More than likely your sheet rock was installed with more nails AND GLUE. In that era, glue was always used. The advantage in today's method of screwing the sheet rock is that it requires fewer fasteners.

If you are still concerned with the weight load, you have the option to consider installing blown-in fiberglas insulation. For example, an R-30 fiberglas weightload per square foot is .531 or an R-40 the weightload would be under .786. This is considerably less weight.

Bruce Jones
The Insulation Doctor