Hello Listmates,

I am a little over half way through my SIPs project in MN.  The ceiling is flat (no SIPs panels for the ceilings) and the attic has 2 inches of closed cell foam and cellulose was supposed to go on top of that for an R 50 total.  I guess the insulation company is not used to doing cellulose and just automatically put in their usual blown in fiberglass.  Little did I know that blowing cellulose was such an unusual process.  They have agreed to vacuum out the fiberglass (they will save it and use it for another house) and install the cellulose.  I thought for sure that cellulose for attic spaces was supposed to be installed wet but they say do not have the capability to do this.  They suggested blowing in 20% more cellulose (for additional cost) so that when it settles it will be the same as if it were blown in wet.  This does not seem right to me because if the house is speced for R50 in the ceiling there may be a greater volume of dry cellulose than wet, but the R value should be the same.  Can anyone chime in with suggestions/comments?  Can you suggest the best way to install this to get the maximum performance of the cellulose? I am stuck with this insulation company because they are half way through the project.

Thanks,

Ellen

Cellulose in the attic is blown in dry, at least from my experience. The closed cell spray foam of 2" should get you around R12 or so... so blowing cellulose on top of the foam, maybe 12" or so should get you around the R50.  As an option, you can tell them to leave the blown in fiberglass there, and have them blow in a couple inches of cellulose over.  They may rather do that than take the time to vacuum out all of the blown in fiberglass.  Might actually be to your benefit and get a higher R value... if isn't going to cost YOU anymore.

attic is almost always dry blown

Looks like they are trying to get a few extra bucks of you.

Thanks Jere,

I had thought about that, but was not sure that would be the best way to go.  Does anyone else have any thoughts on whether the insulation would be better if I add cellulose to the top of the blown in fiberglass or if I make them change it out to the cellulose.  Thr oof is a 4/12 pitch so my concern is the eaves, since I think there will not be any more room to blow more there because the baffles should be full; there, in spite of the fact that there are energy heel trusses.  Any thoughts?

Ellen

Posted By Ellen Johnson on 03 Aug 2010 11:12 AM
Thanks Jere,

I had thought about that, but was not sure that would be the best way to go.  Does anyone else have any thoughts on whether the insulation would be better if I add cellulose to the top of the blown in fiberglass or if I make them change it out to the cellulose.  Thr oof is a 4/12 pitch so my concern is the eaves, since I think there will not be any more room to blow more there because the baffles should be full; there, in spite of the fact that there are energy heel trusses.  Any thoughts?

Ellen

You need at least 3" of cellulose over the top of the fiberglass to block it's convective losses, and the weight of the (denser) cellulose will likely cause the fiberglass to sag a bit over time, but probably nothing like 3".  If you can get a consistent 3"+ overblow (which may be difficult, or not, depending on the particulars), you'll have  a higher initial-R, and comparable "aged" R.

Whether it's more labor/cost to vacuum it all out and do a full replacement with cellulose, or do an overblow of sufficient depth to guarantee performance depends on how easy it is to do the vacuum job.  If they just vaccuumed out around the eaves for a full-depth re-blow of cellulose there, and did a 5" overblow on the rest (to guarantee 3" min in the thin spots), it might be a reasonable compromise.

Thanks Dana, that makes a lot of sense.

Ellen