We're going to add additional insulation to our attic, which already has blown in insulation, and I was wondering if any of the expanding products like icynene would provide more benefit than adding to the blown in cellulose already there?  Is it worth the cost for icynene over additional blown cellulose?  Are there downsides to adding spray foam to a blown cellulose insulated space, such as moisture retention?

Additional blown in would add to depth of the existing product.  Spray up insulation would need to go on the underside of the roof, and we would have to make sure ridge and soffit vents remain unobstructed.  If we used icynene, the attic would remain unconditioned, but I assume it would not experience as much heat build up. 

Thanks

Cost - No. If fact I don't really see much benefit at all for adding foam(and keeping the cellulose). If heat gain is what you are trying to reduce, you could try adding a radiant barrier instead, and adding to the cellulose. If you are trying minimize Heat Loss, I would just beef up the cellulose and skip the foam.

how much cellulose insulation is now in the attic

R-44, I think.

The two systems are not really compatible.  Cellulose allows moisture to migrate through to unconditioned attic space, but not much heat, and then the soffit vents, etc., get rid of the moisture before it can condense on the underside of the roof and cause problems.  Spray on foam is for enclosing conditioned space.  With foam, there are no soffit vents and the moisture is mechanically expelled through an HRV or ERV. 

You could spray foam, add a mechanical air exchange system, and close off the external venting.  Then, you would get the benefit of both the R-44 cellulose and the RX? of the foam.  This would clearly be a more expensive $ per R value than just adding to the existing insulation.

Mark

If memory serves me right, as you increase your r-value past R-30 the gains are so small,that the cost associated with this type upgrande would be unwise.

I believe that they called that The Law Of Diminishing Returns. And, when concerning Insulation, they always used stable energy costs in their justifications. However, as you know, our current energy costs are anything but stable, and in many cases have risen more than 100% in just one year. So, what use to be unwise is now becoming mainstream.

Economics aren't the only driver.  We're looking to make the second floor more comfortable without turning the first floor into a refrigerator.  We've been able to improve the comfort by balancing the air system, but the radiant warmth from the attic is still apparent, so I suspect either a radiant barrier and/or additional insulation can be beneficial.

I had that problem in another house. I added more insulation(fiberglass in that case) and a powered attic vent. Worked like a charm. Now, they have solar powered attic vents, so even less electricity costs.

I've addressed a similar situation in my own home. Our attic have 16-inches of cellulose over 8-inches of mineral wool. The estimate R-value was 60. We had ice dams during winter months and high energy bills. We removed the loose fill insulation and replaced it with 6-inches of soy-based, spray-applied foam insulation. Estimated R-value of the foam was 23+/-. We reduced actual propane use by 50% the first year. The next year we had the rim/band joists and foundation walls insulated with open-cell spray-applied foam and reduced the remaining energy use by another 30%. The reason this was so successful:

R-value is used to measure resistance to heat flow through a solid surface (conductive heat loss).
Most heat loss in cold-weather states is convective in nature (heat travel in a liquid or vapor - air).
Loose fill insulation, even when placed in an air sealed cavity, developes convective loops within the insulation which downgrades its performance.
Open cell foam locks the air in tiny bubbles so there is no convective looping. The only heat loss is conductive in nature so spray-applied foam at R-24 can outperform loose fill insulation at R-60. The fluffier the insulation material the poorer the performance.

You don't have to remove the existing cellulose but you should move it so the foam can be applied directly to the floor of the attic. Once the foam is in place you can spread the cellulose out over the foam. The foam will stop the air leaks (whcch reduces the drafts in the house) and lowers your energy requirements. For our home, using propane, the cost payback was under 3-years.

As far as diminishing returns, Icynene recommends 5-inches of foam as the tipping point.

Mike

In the same vein here. I am building a new sip home with a truss roof and have a quote for icynene in my second floor ceiling. It seems that everbody is talking about heat loss through air movement why not put plastic sheeting under the sheetrock and put fiberglass over that? Ive also read about 1" of spay in and layer it with fiberglass. I am sure it is a stupid question but r30 icyne is 5x the cost of sme sq footage of fiberglass and icyne is r30 fiberglass is r38. Not only that code reqires an r38 ceiling. Why is plastic bad and what to do about getting the proper rating for code compliance?