Attic insulation

[Rixter58]

Going to blow fiberglass insulation in the attic of my shop next weekend. 33x48 pole type building with 16'sidewalls. Walls are insulated with 1.5" pink styrofoam (R7.5) between the purlins and R19 Batt insulation. Heat source is 150,000 BTU tube type radiant. 35 miles NW of Minneapolis. I have purchased enough insulation to achieve R38. So here is the question......at what point does the law of diminishing returns come into play with insulation? Am I shorting myself?.......or would more be a waste? I've had advice both ways. What are your thoughts and why? Thanks

 

[jimp]

R 30 is real close to the point of diminishing returns. See the attached ASHRAE graph. After that it better to spend the money on other items, such as air infiltration, new hvac equipment etc. (Not real world)

Real world, your R 38 is likely best for the attic. The fiberglass will settle over time and moisture in the insulation will degrade the R value. (I actually have R 60 in my roof and R 30 in my walls. The R 60 was selected because of a poor job by the installer of the first R 30 layer.)

 

[cabman]

This might be the best way to find out:
Insulation Upgrade Cost Saving Calculator
http://www.builditsolar.com/References/Calculators/InsulUpgrd/InsulUpgrade.htm

 

[northern skink]

Make sure you agree with the installer how much product will be used, in order to avoid the contractor 'fluffing' of the fiberglass product, which is just what it sounds like; loose fiberglass gets fluffed up and looks like a full depth job, and you get less than what you paid for. Only problem, if they fluff it and you don't watch them use all the product that's required, you'll only figure it out when the blown fiberglass settles, and the contractor is far down the road. Blown cellulose, on the other hand, can't be fluffed.

 

[jklingel]

loose fiberglass is not a great product. dense packed in walls (Spider) it does well. loose, no. cellulose loose is better. a icf cat made a funny statement yesterday: the guys who make fg for insulation also make fg for air filters. ironic. fiberglass lets air move through it, if not dense packed. cellulose in a ceiling.