I am in VA in a mixed-humid climate (23223).  I am going to reside the longest wall of the house this summer.  I am required by code to add 5/8" gypsum sheathing.   Currently the wall has no sheathing, only wood siding.  New siding will be hardie plank.

 I am also considering 1/2" foam board sheathing  to provide a thermal break. 

The windows are original, and will remain so.  I plan to tighten them up and add appropriate storm windows later.  The difficult detail of furring the window trim out around my additional  sheathing and insulation thickness is why I plan to use 1/2" foam board instead of something thicker.

I am wondering if the foam is worth the trouble at that thickness, and if I am putting myself in danger of condensation issues. I have done a little research, but have not found answers for this situation. 

Another option is a thin fan folded product nominally r1 that has drain channels for behind the sidiing.  I like the drain screen and that this product is thinnner for my trim issue.  I wonder if it really adds insulation value at all in the real world.


The wall is currently insulated with r11 or13 fiberglass batts kraft faced, one layer of interior 5/8 rock.

Any input would be appreciated

If I understand the situation, the wall cavity will be exposed once the old siding is removed? What a perfect time to upgrade the glass batts with something that will increase the R-value and address condensation issues. May make the 1/2" foam board needless. Lots of experts on this board that can probably give you some good ideas. Also, why 5/8 rock exterior sheathing, why not OSB/Tyvek etc., or whatever is the local custom for a framed wall ?

With either half-inch or fan-fold XPS, you won't create any condensation issues, and it'll make a good drain-plane as well as providing a modest amount of thermal break on the studs. Cement fiber siding is a moisture reservoir, and XPS underlayments provide pretty good protection from the high vapor drives that occur when the sun hits rain or dew wetted siding.

While you have the off is a good time to consider replacing or enhancing the cavity insulation. Low density batts pass a lot of air unless there are good air-barriers on both sides of the cavity, and will convect considerably within the layer during temperature extremes, underperforming it's rated value. Ripping it out and using wet-sprayed cellulose before putting up the sheathing, or dry-blown with the sheathing installed would make it much tighter, and perform better over time. Dense-packing superfine high density fiberglass such as Certainteed Optima or JM Spider to 1.8lbs density would have a similar air-retarder effect, and yield a slightly higher R (but for more money.) If you're flush with cash, 3" of closed cell foam would boost the R-value by more than 50% and seal it up pretty well (at a cost of $3/square foot or more- way more than any fiber solution.) Middle of the road would be to use 3"of closed cell Icynene(tm) rather than standard polyurethane foams, or 3.5" of open cell Demilec which would seal better than dense-packed fiber and yield similar or slightly hgher R values than than high density fiberglass. (Cellulose is usually the better value per unit R though.) Low density blown fiberglass is slightly better than batts in that it doesn't leave voids and will usually hit close to it's rated R except at the temperature extremes.

If you caulk/glue the sheathing to the studs to form a good air barrier you'll maximize the performance of whatever is in there, but blown or sprayed goods is really the right way to go when you can.

Thanks for the replies.

I agree with you both that it could be a good time to redo the glass batts.  But I later plan interior work that will remove sheetrock from the interior.  House has been "redone" in the past and most of it is poor quality work.

So, since I am doing this myself, I think I will save reinsulating for later.  I am sure I will feel pressed enough getting old siding off, adding blocking, correcting any framing issues and getting it covered again before the next storm!

At that time I am going to investigate options, and figured at the least I would do a more detailed installation than the previous installers and use the dense fiberglass batts at a minimum, though a sprayed in product will also be a consideration.

Dana I like the idea of gluing the sheathing to the studs.  Also probably adds a modest amount of structutral strength.

3cityblue, I have to use gypsum since I am less than 5' from the property line, fire code issue. 

Thanks again for the input