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Discussion Starter #1
I am installing new Hardie siding on a homes great room and can't decide weather to add 1/2" rigid foam to the job. The room does have 2x6 walls with R-19. Here in Atlanta, we are in zone 3 which seems to mean half the people say do it and the other half say don't. Even the DOE info goes both ways. I usually err on the side of over building and with energy efficiency being such a focus now, I'm leaning towards doing it. However, I don't want to create moisture problems in the walls; especially considering the high humidity we have here in the Summers.

Siding and insulation are not our routine jobs and I can't seem to find anyone who can give me an answer that gives me the warm fuzzies; if you know what I mean.

Any advice / point of view would be appreciated.
 

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I'm not gonna help much either. But, I'll give you my opinion.... I would never use Fiber Cement in the first place. Up here, we've found that the back side of the FC is a haven for mold and mildew growth. I can't tell you how many FC panels we have pulled and the back side of the FC panel is covered in black mold.

Throwing rigid foam behind the FC panel will only accentuate that problem IMHO. You see.... FC is really no different than any siding panel in that, moisture will tend to condense on the cool side of the panel. In your case, you have a cool humid evening that warms right up when the sun comes up. The back side of your FC will be covered in condensation shortly after sunrise.

If you don't have a way to shed that moisture PDQ, the FC will tend to absorb it and hold that moisture over long periods of time.

I would tend to think that your regular wall insulation and an excellent house wrap and underlayment designed to shed moisture like the DC-14 would be better than any rigid foam board. This is the system we use under our EVERLAST siding jobs, but would work for FC as well.

http://www.trustgreenguard.com/moisturemanagement.aspx#overview
 

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think he's more interested in controlling condensation than attachment,Sean Lintow from SLS tech is the guy to ask
That attachment shows the GreenGuard DC-14 which is designed to shed moisture. We use it in conjunction with the GreenGuard RainDrop moisture wicking house wrap.

I probably should have posted these two links individually to start with.... sorry.
 

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he's worried more about condensation in the stud bays caused by the ''outsilation''thickness or lack there of.if i'm not mistaken..
 

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I'm not gonna help much either. But, I'll give you my opinion.... I would never use Fiber Cement in the first place. Up here, we've found that the back side of the FC is a haven for mold and mildew growth. I can't tell you how many FC panels we have pulled and the back side of the FC panel is covered in black mold.

Throwing rigid foam behind the FC panel will only accentuate that problem IMHO. You see.... FC is really no different than any siding panel in that, moisture will tend to condense on the cool side of the panel. In your case, you have a cool humid evening that warms right up when the sun comes up. The back side of your FC will be covered in condensation shortly after sunrise.

If you don't have a way to shed that moisture PDQ, the FC will tend to absorb it and hold that moisture over long periods of time.

I would tend to think that your regular wall insulation and an excellent house wrap and underlayment designed to shed moisture like the DC-14 would be better than any rigid foam board. This is the system we use under our EVERLAST siding jobs, but would work for FC as well.

http://www.trustgreenguard.com/moisturemanagement.aspx#overview
That's odd. I live in what seems like the wettest place on earth and haven't ran into that problem. Recently worked on a house that was built in mid 90s. It had rigid foam over the shear with FC on the outside. No house wrap whatsoever. Only places with mold were due to leaks.

FC with felt is the most common siding around here, with cedar lap and shingle behind it. Vinyl is downright laughable IMO, and T-111 is usually only used when you're just doing a partial siding job and you have to match it. We live in some of the harshest weather conditions, so I think it's a great measuring stick or basing a sidings durability/quality.

Although I did use Certainteed FC to match rest of house on a repair, absolute garbage compared to Hardie
 

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As I recall you are still under the 09 codes - 12 codes make it easy, you would have to install R5 continuous unless you find some magical R20 Batts or use something similar

No issues with foam - you do have to control the air & moisture movement which will be from the outside in - get your details right & you will be fine

Besides the manuf. directions you might want to check this piece out
http://blog.sls-construction.com/2013/common-sense-weather-resistive-barrier

One other item to consider is using a rain screed detail if you are truly worried about moisture issues - oh and as for one layer, better to go with 2 seams offset
 

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Discussion Starter #9
Thanks for the feedback guys. As Tom suggested, I'm not worried about attachment or drainage behind the siding. We've got all that under control. It's that moisture build up in the wall cavity or on the sheathing behind the foam that has me concerned.

Since my original post, I found some info on this site that would suggest the humidity should not be a problem and it appears that it doesn't get cold enough here on a regular basis to produce condensation on the sheathing.

As for any mold issues on the siding, I have to agree with Fender. We did think of this potential and I researched back priming/sealing as a potential safe guard. No need and not recommended. At least not here. Another contractor I know that does a lot of siding says she's never seen it be an issue. She said she's pulled siding from the 90's that still looked great. She said most cement siding mold issues come from poor storage and handing prior to install or not getting it painted in a timely manner.

But I digress. My apologies. The real concern is the moisture in the wall cavity. Whatever the outcome, I've learned a ton and can probably add siding as a regular service if I want too.
 
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