Year of Construction 1954
1100 SQFT
Maryville, TN

I am getting qoutes on repacing the wood siding on my house with HardiPlank.  When the wood is removed, I hope to remove the existing "blackboard" and reinsulate my walls, then add insulation board over the studs before the Hardi goes up.

Currently, there is 1" rock wool batts in the walls.  Should I remove these and apply R13 fiberglass kraft faced batts in the cavity?  Should I leave the rock wool, which has a kraft face and apply unfaced R13 fiberglass over them?  Or is some other solution better?

I would like to add 1" foam, may be better to do 1/2" in two layers, over the studs with 1/2" plywood in the corners with 1/2" foam over that.  Should the foam be XPS, polyiso, or another?  What about facing on the board?

I've looked through Building Science articles, but I'm not sure I understand the details regarding drainage plains and especially moisture barriers.

Thanks for the advice.

With only 1" of existing insulation in the walls it should be pretty easy to snake in a dense-packing tube and use blown insulation (cellulose or fiberglass), and achieve a better result for (likely) less money & labor than ripping out the old batts and applying new batts.   Cellulose would give you more thermal mass and good humidity-buffering, fiberglass would give you a slightly higher R value, but in mixed climates like yours the thermal mass might trump the higher R in real-world performance. Blown insulation fills in around all voids, and can't bunch up/compress like batt installations inevitably do.  (If in 25 years it sags a vertical inch in the cavity it only means it's deteriorated to the real-world performance batts had on day-1.  But dense-packed cellulose tends to not sag if blown to over 3lbs/ft^3 density.)  If going with cellulose, specify "no sulfates" or "borate only".  Sulfated fire retardents can react with metal if it ever gets damp (it's particularly fond of copper wiring & plumbing).  Borates don't corrode metals, but the DO react with the bio-chemistry of ants/wasps/termites, making it a pest-inhibitor as well.  (The same chemicals are commonly used in soaps & cosmetics, with extremely low toxicity to us.)

Not knowing the condition, permeability or which side of the cavity the facing of the existing batts is, or the permeability of the interior wall coverings/finishes, in the mixed climates of TN it's probably best to avoid foil-faced insulation.   An inch of XPS works. Taped/caulked/mastic sealed it's fine in just a single-layer, but double-layered/taped seam may be less fussy to get perfect.

Regardless of what the Hardi instructions might specify as OK for your climate, installing it on vertical furring strips with a 1/2-3/4" rainscreen gap between it and the foam is better.  Cement siding can hold a lot of water, and although XPS is nominally water proof, it's semi-permeable to water vapor (and you need it to be, to keep the studs from rotting), so when the sun breaks out and beats on that siding after a summer rain or morning dew, the inward vapor-drives can be extreme, similar to that of stucco or masonry under similar conditions.  (Stucco invariably has a vent gap behind it, as do most modern masonry walls.)  With a ventilation/rainscreen gap it keeps that moisture out of the wall structure.

Thanks for the reply.

I'm assuming you're suggesting placing the cellulose from the inside? Or are you suggesting leaving the batts and then once the foam is placed, blowing in the cellulose form the outside?

Leave the homosote on and blow behind it. The batts should be against the interior wall. Don't blow the insulation from the inside it is really messy.

Also check how the sheetrock was installed by checking for type of nails in a closet. Older sheetrock was installed by using smooth blue nails. These can pop when installing blowin insulation.

What smartwall said... definitely assess the condition of the interior wall for it's ability to handle some pressure while blowing. If it's iffy, dense packing might be out of the question, but 2-hole method blown insulation at lower pressure & density will probably work in 99% of the cases (and still out-perform batts.)

When you've stripped the old siding down to the sheathing, drilling & blowing from the outside is dead-easy. Put the foam board up only after you're done.

Thank you all for these responses.

I had not thought of the cellulose and because I have had some ant problems this would seem to be a good solution.  I know the sheetrock was installed with nails, but I think it is in good shape and seems to be strong.

One last point of clarification.  If I leave the "blackboard" sheathing on and foam over it with 2 layers of 1/2" then do 1X furring strips, am I reaching the limits for the hardiplank installation?

I wouldn't think so- as long as the furring is long-screwed into something structural like a stud, you can have as much as 3" or more between the siding and the studs.  At 5" it get's to be a very long screw indeed (figure on at least an inch of penetration into the structural member), but you're talking 2.5" of furring, foam, & sheathing tops, (and likely a 3.5-4" screw for fastening the furring).  Don't try to hang it on nails though...

You should probably use screws for the furring that are compatible with cement siding, since at least some direct contact is likely.  (IIRC Hardi board will corrode away common nails/screws.)