Rigid foam board insulation

I'm replacing my siding and find lots of gaps in the fiberglass inulation (in-wall). I am thinking of adding foam boards to the exterior walls to better seal them and add little insulation. Going by R-value, this doesn't add much (1/2" R4) but maybe this would protect better against winds etc. I am installing hardie plank siding. Will this home 'breath' ? ventilation? Anyone has experience with this type of insulation?
Luke
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If you are replacing siding then don't even think about breathing problems. I promise, you have lots of breating going on, even if you wrap it twice.
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You won`t get R4 per 1/2" out of any board I know of, R7.2" new and stabilised at R6+" is the best Polyisosyanurate gets. I covered my house in 2" polyiso for R 14.4. Can you replace siding on a soft surface and not over nail it making it uneven? I covered mine in Osb.
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m Ransley wrote:

1/2" Dow blue foam board is R3 and that is the best I could find when I searched for materials for my basement remodel. Perhaps one of the foil-faced types might do better but I don't recall any.
--
John McGaw
[Knoxville, TN, USA]
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I tried just about every brand a few years ago on a west wall I rebuilt, the blue board won as somewhat effective, the foil covered white crap is landfill. Only the blue board is only slightly more effective than an empty space even when fitted with surgical precision. A poorly fitted R-13 fiberglass did much better. I've ripped most of it out and replaced it with R-16 fiberglass. The blue has its uses, and I would use it again, it has the advantage of being softer and tighter so accurate cuts can be made with out the shattering and crumbling the white stuff does. Any place the bats will fit, isn't one of its uses.
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R value sounds a little high for 1/2", but reguardless these R values are under 'ideal' conditions. Meaning as you pound down the nails, the insulation becomes damage. It does seem to help ensure your building envelope is increased. If you really want to reduce windy effects, might want to take the time to use a properly installed house wrap.
I heard on one DIY show, drafts account for the majority of heat loss in a home, followed closely by poor insulation.
Do your self a favor and take this time to get some professional advice. The cost of fuel is ever increasing, and a few bucks to a professional energy advisor might help in the long run.
Also, follow up with what you did, atleast I'm curious the end result.
imho,
tom @ www.FreelancingProjects.com
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1/2" Blue Dow is stated on the board R2.5
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Actually, the R value for the board I wanted to use, is about 7.2 per inch, 3.6 per 1/2". I decided to use a felt paper instead of Tyvek. Installing it right is the key :) Thanks for all replies. Luke
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