I read alot of people on here arguing the merrits of 2lb, half lb foam, fiberglass etc.

So whats the skinny on the best most effective, economical way to insulate a ICF home in the attic ?  My attic isnt a usable space and is vented.

Based on all the discussions I've read about attic insulation here on the forums, I would say 1 to 2" of spray foam on the ceiling and around the perimeter to create a good seal between the attic and living space, covered with as much cellulose as needed for an R value you want to achieve. Search the threads to get ideas for assuring ventilation air inlet at the eaves.

That's what we did 5 yrs ago and it worked great!!!

I agree that the way dmaceld did it is best, but I took a different approach from a process related standpoint. I sprayed 3" of polyurethane on the bottom of the attic floor (for usable attic storage space) between the ceiling joists to completely seal the ceiling and ceiling-wall intersection. All plumbing and electrical was installed below this layer of closed-cell foam. The ceiling insulation was augmented with fiberglass batts below the foam followed by gypsum board drywall. The thickness of the foam and fiberglass layers is critical to keep the dewpoint within the interior of the foam during the coldest winter months (I live just southwest of Chicago.)

The reason I took this approach was so I could insulate the house before winter set in. Building regulations forbid installing insulation and drywall until the framing, rough plumbing and rough electrical inspections are passed. Since I was building the house myself and couldn't get to that point before cold weather set in, the inspector allowed me to spray the foam in place before the plumbing and electrical wiring was installed as long as nothing was hidden from the inspector later. I was able to keep the house warm all winter while the mechanical, plumbing and electrical systems went in.

The further south you are, the better this approach works.

Minor clarification. That isn't the way I did mine since I have a conditioned attic. I spray foamed the underside of the roof. That is the way I would do it if I had a vented unconditioned attic.

maybe I'm misunderstanding something from your guys posts. You spray the underside of the decking for your attic AND you have vents? I don't think I'm understanding that right. I was under the impression that if you spray the underaide of the attic, you are created a conditioned space and therefor no ridge or soffit vents are needed. Where did I get lsot there? Thanks!

Yep, you got lost! Thick spray foam (10" in my case) is generally applied to the underside of the roof deck for a sealed conditioned attic. For a vented attic you want as good a seal as possible between the attic space and living space. One of the best ways to achieve this is to spray about 1" to 2" of foam on the ceiling, covered with wallet friendly cellulose to achieve the desired insulation R value. You combine the air tightness of the foam with the R value of the cellulose and still have money left over to take the wife out for a nice dinner!!

newbie,
Did you by chance put any radiant barrier on the roof deck, such as foil coated OSB? If not, you'll want to consider putting some reflective material on the underside of the roof deck. This will cut down significantly the radiant heat from the roof in the summer. It'll help in the winter too by reflecting heat back to the ceiling. Of course there won't be much heat escaping if your R value is high, but it's worth considering.

Great responses from everyone. Balancing economics with Energy Efficiency can sometimes be challenging when applying building science methods. Question - Is this new construction or existing? I did not see mention of ductwork location, therefore I am assuming it is in the attic? If so, here are options: New Construction 1. Depending on geographic location, you first need to determine if your HVAC contractor correctly sized the HVAC system. This could have a HUGH impact on your insulation approach. In Arkansas we find ICF homes with excellent windows coming in around 1,000 sq/ft per system ton. Example: 1,850sq/ft with sealed attic - 2 ton 2 stage heat pump that operates in 1st stage 75% of the time. 2. If HVAC is sized at typical 500 - 600sq/ft per ton you may have no choice but follow typical insulation methods, otherwise you will deal with short cycling, poor humidity control and higher than needed operating costs. IF this was done, you may have some of these issues regardless how you insulate! - Sorry 3. If HVAC is or can be "right sized" then sealing the attic to prevent heat gain/loss on duct system can be quite beneficial, and cost effective compared to purchasing "extra" tonnage. It also adds to the overall "Comfort" effect of ICF. 4. If duct and HVAC is already installed,and sized correctly, then from a cost standpoint have a cellulose contractor install (by inches, not bag count) the correct "R" value for your climate zone (any where from R-38 to R-60) and blow extra over the ductwork if possible to reduce heat gain/loss issues Be sure they utilize the soffit vent air shields so the insulation does not spill over onto soffit vents areas. There is so many more options or concerns related to your question. If you are not comfortable with the answers you are getting, feel free to contact me directly. Building science approach to construction and right sized HVAC is our passion. Dan@comfortmadesimple.com