A good friend of mine is changing siding on his house and some insulation. The contractor recommended installation of “Alside” extruded polyesterene board underlayment over the fiberglass insulation instead of just Tyvek house wrap. Has anyone had any experience with it? Is it worth doing? Thanks for sharing your knowledge.
Is this what he is proposing?
http://www.alside.com/index.aspx?page=30
I don’t quite understand your statement that it would be installed over the fiberglass insulation.
Are you saying there is no sheathing on the exterior.?
Marcel 
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IMHO, No it is not woth it. The return on investment is not that great. Adds some insulating value but not that much. It is just one of those contractor “extras” that make more for profit forthe contractor, but in terms of making your home more energy efficent, it lacks. The best application for the money is just house wrap, it really cuts the air infiltration, which is a good value for the money. Now, if your friend went with the insulated backed siding that is another story. JMHO
Now, if your friend went with the insulated backed siding that is another story. JMHO
Not a thing I would do!! Not much R value in a piece of insulated siding and usually no airsealing.
If there is no sheathing under the siding, airseal at the inner wall surface with foam/caulk/etc. Fill the cavity (assuming 2x4) with R12 glass/rockwool batts, install full 1-1+1/2" rigid foam board and then put on a properly sealed weatherbarrier/exterior air barrier. Depending on the type of siding, it may be advisable to install a rainscreen system (wood based sidings). Install your siding usineg long nails through the foam into the studs (this will have better holding power than nails through a 3/4" sheathing board.)
Another way may to be spray the open wall cavities with Icynene or similar and then install siding.
Brian, are we having a bad day? The original post was talking about fan fold insulation, which has hardly any insulating value at all.
When I installed new siding at my house, I used fan fold not for its insulating value but for its ability to smooth the surface out. I was siding over old wood siding. I still installed a house wrap over that prior to new siding. It worked great.
Being in the siding business thirty years , I know Alside now has thier own rigid board insulation. It is dark grey in color , and shiplapped on the edges. What a pain to install. Might better use “DOW” board or Protech. They both come with toung&Groove. From the time house wrapp came on the market , we always put the house wrapp over the studding , then the insulation board. Use the nails in the insulation board as guids for hanging the siding. NOTE: pro’s and con’s about useing both together. Not that we ever had any problems. If I had to choose one or the other , I’d choose the house wrapp , as Robert R. Cramer had suggested. Much bigger payback!