CTSNic
If you absolutely cannot afford exterior foam*, work on making the house as airtight as possible. Use ZIP sheathing, tape all the seams, caulk the framing around the bottom, top & side edges before installing. Use some type of rain screen - plaster lath is very cheap, 3/8" thick and will work; it never hurts to keep the wood siding dry on both sides regardless of whether the manufacturer requires you to do so. Since you are already planning to strap the inside and add insulation, strap it horizontally - that will give you some thermal break. *Or for a much better result, forget the inside strapping and add extra foam which will help far more than another little bit of insulation in the wall. The exterior foam helps with air sealing and in general keeps the framing, and the house, warmer.

Since you are concerned about your budget, remember that the better you do with the envelope the less your heating and AC will be. I built a 1500 SF ranch with 2x6 walls & 2" foam (less than the current recommended thickness) in zone 5/6 which costs the owners $300 to heat annually. It does face south and they do like it relatively cool, but still... The house has R-60 roof insulation (cellulose is cheap) and the air tightness is .9ACH50. And the basement is fully insulated - under the slab and on the walls. Basements are critical!!!

Thick, non-air-barrier insulation + OSB (or Zip) + no foam + very little rain screen gap is not a good design from a ex-filtration/condensation moisture standpoint.

With 2x6 studs, you should be able to use 24" spacing. Also consider cellulose instead of mineral wool.

Consider replacing the OSB with any thickness of rigid foam + diagonal metal strapping.

Posted By CTSNic on 05 Aug 2015 11:43 PM
Dana & Others,

Assuming Zone 5B - Cold Winters, some wet...some dry + Hot Hot summers, some wet...some dry. So every temperature with every humidity possibility here is what we are talking about.

If I want a energy efficient house without going over my budget (*important because if I go over budget, I'll flop on the mortgage payment as time presses on, and makes this all pointless*) --- I will plan on one of two sidings, Either a thick vinyl siding or the Diamond coat LP Smart Siding. After siding, I will have installed: Greenguard Raindrop 3D rippled House Wrap **Taped on every single inch of seams, unless told not to..** > then 7/16" OSB > 2x6 Stud walls 16" O.C. with 1.5" Furring strips to allow extra insulation which will be > 7" thick Thermafiber Mineral Wool Insulation R-30 > and then MemBrain > 1/2" or 5/8" Drywall > and lastly PAINT.

How can this be improved upon without too much extra cost?

Questions are: Should I tape the OSB Seams, at all?
Should I be diligent about taping every inch of seam with the house wrap?
Should I have any concern with MemBrain in my climate conditions with my choice of insulation, paint, and siding?

Bonus: Why does LP Smart Side not suggest / require the siding be installed over a rain screen? They have warranty on the product but the local supplier said it was non-issue. So I just want to inquire more into it.

Thanks!

The answers are:

Hell yes you want to tape the OSB seams, and you want to caulk the framing to the OSB at every stud bay too (which goes quicker than you might think with a powered caulk gun.). You want to caulk under the bottom plate and between doubled top plates too.

Yes you need to be diligent about taping the housewrap, taking extra care that it's lapped properly so that even if the tape fails it directs the water outward, rather than behind the sheet below the horizontal seam & or flashing.  The housewrap is your primary bulk-water management layer. While it's possible to detail it as a primary air barrier, it's pretty hard to make it air tight even for the short term, let alone for the long haul.  A few dings and holes here and there aren't much of a bulk-water problem, but it ruins the air tightness. That's why detailing the sheathing as the primary air barrier makes more sense- it's air-tightness is far more durable.

MemBrain is close to perfect for all of your climate conditions, and offers a huge resilience margin in your stackup. With a rainscreen gap it's not essential, but it is still cheap insurance, and FAR better than super-tight Class-I vapor barriers like polyethylene sheeting.

LP Smart Side is not affected by the presence or absence of a rainscreen gap, which is why it doesn't affect the warranty of the siding. They really couldn't care less about how you manage the moisture content of your sheathing- that's between you and your architect (and code inspectors.) They're only a siding company, and it's a moisture-tolerant siding. The rainscreen is to ensure the long term integrity of your structural sheathing by giving it a much faster drying path, not to protect the siding. 

But a rainscreen does protect other types of siding. With old-school wood siding the rainscreen helps preserve the siding AND paint, by keeping the peak moisture content of the siding bounded (since it can dry to both sides- the back side of which is usually dry) and by making the drying rate on the back side about the same as it is on the front side. Without the rainscreen the siding dries primarily to the exterior side which can induce cupping/warping of the siding, and paint blistering or peeling.

Makes a little more sense now. Interesting ideas.

On the MemBrain, should the seams be taped and lapped like house wrap? Should I worry about staples for fastening it to the studs? And the other thought is should I also use it on the ceiling before drywall...? I am hoping the walls are the areas that need the air seal most. If it's my air seal, I plan on taping the seams and being careful with the method I attach it to the wall. I was hoping if I was diligent with MemBrain that I would not have to tape the exterior OSB seams.

Posted By CTSNic on 06 Aug 2015 11:36 PM
Makes a little more sense now. Interesting ideas.

On the MemBrain, should the seams be taped and lapped like house wrap? Should I worry about staples for fastening it to the studs? And the other thought is should I also use it on the ceiling before drywall...? I am hoping the walls are the areas that need the air seal most. If it's my air seal, I plan on taping the seams and being careful with the method I attach it to the wall. I was hoping if I was diligent with MemBrain that I would not have to tape the exterior OSB seams.

The instructions for installation come with it.

Yes, seams need to be lapped & taped, but it's fussier than houswrap.

Staple it to the framing and tape over the staples.  Make sure that all lapped seams coincide with framing elements, so that the lapped seam get's stapled to a stable  supporting object, and won't come undone over time due to flexing.

The MemBrain cannot be your primary air barrier since it's flexible and on the interior, far easier to breach and far likely to be breached over time. (Ever have to re-hang a picture?)  Even with a bunch of nicks cuts and nail-holes it's vapor retardency function is still there (tape any damage that you find anyway), but that would destroy it's air tightness.

The OSB sheathing is the right layer for the primary air barrier.  If you caulk the sheathing to the framing in every stud bay, and under the bottom plates of the studs, and between doubled top plates & doubled-up window & door framing (should be standard practice, but isn't), then you'd only have to tape the horizontal seams in the OSB that don't coincide with framing.  You may be pleasantly surprised just how quickly the caulking goes when you use a powered caulking gun- it's not a big deal, and caulk (even acoustic sealants) is quite a bit cheaper than tapes capable of sticking to OSB forever.

I'd use tape and gaskets, but if you are going to caulk, review the points here. ASTM movement rating, bond breakers or gaps > 1/4", adhesion strength, etc. A seal that will last involves much more than buying any cheap caulk and squirting it into a crack.

For decades Tremco acoustic sealant (which never becomes brittle) was something of the gold standard for these applications, but there are now purpose-made caulks for sealing framing to sheathing prior to insulating, eg:

http://www.ocenergycomplete.com/

http://www.knaufinsulation.us/e...us-sealant

It would be interesting to see discussion of just rigid foam on the outside and OSB (+gypsum) on the inside. Both detailed as air barriers.

Tape seams and staple studs and tape staples in MemBrain - Got it.

Tape OSB Seams if at all possible - Got it.

Worry not about having the OSB Taped, and sealed from the inside with caulking even with an air barrier (or best attempt) with MemBrain on the interior - Got it. (Mostly)

Now new questions arise - best performance sealant that is readily available at either a Menards or local Lumber Yard? Nothing special order, and nothing that costs an arm and a leg since quantity is an issue (using a lot!) - and it would negate the initial point that caulking/sealant is cheaper than sticky tape for OSB sheathing.

Note: house is already sheathed with 7/16" OSB and studs are installed (2x6x8' 16" o.c.)

Consider Tremco Dymonic 100 (PU) or some brand of >= +/- 50% movement silicon. The former adheres well enough to act as a glue.

Maybe Dow Liquidarmor™ too.