I'm in the process of renovating an older cinderblock home (built in the '50s, slab on grade foundation) into a passive solar design and am researching vapor barriers.  I would like to use hard foam insulation panels on the outside walls and a stucco finish.  I'm planning to continue the insulation panels down along the footings.  A wood stove will be the primary heating source.

I'm having trouble finding information specifically concerning where the vapor barrier should be located for such a home.  I'm thinking 6 mil polyethylene could go between the outside block surface and the insulation, but am unsure.  I know moisture and mold can be problematic for older cinder block homes, and I'm hoping to do all I can to avoid it.

Also, I'm planning to install an in-floor radiant fluid system and pour 1.5" to 2" inches of concrete (stamped) over the top.  I believe that the current old slab does not have a vapor barrier and thought it might be wise to lay down some polyethylene before installing the tubes to act as a vapor barrier between the old slab and my new stamped concrete floor.  Any thoughts?

The house is located in Medford, Oregon (hot/dry summers, cool/moist winters).

Big thanks for any advice.

The system I like and have had good luck with is using a cement based concrete sealant, such as thoroseal, that actually plugs the pores in the concrete blocks. We use it for basement waterproofing and have not had any dampness issues, so it would be more than fine for above ground. You might even want to use it between the concrete levels before you put the poly down, since it acts as a radon barrier.

Vapor retarders would be required for walls & ceilings for Medford's relatively dry marine-tempered climate, where summertime dewpoints rarely exceed 60F, and wintertime average dewpoints are well below the average daily temps (and close to the average daily lows.) If it won't even condense outside in winter, and it doesn't stay below freezing for days on end, it won't condense in your walls unless you're keeping the inside at some ridiculously high relative humidity.

But if you feel you MUST put a vapor retarder in, it should be between the cinder block and exterior insulation. The R-value of the cinder block is low, and it's permeabiltiy high, so it's essentially already fully "inside" the thermal boundary (the insulation), so keeping inside the pressure & vapor -boundary where humidity is better controlled is the better option.

Still, it's probably best to use a vapor permeable or semi-permeable air barrier between the cinder block and foam. This will allow at least some outward drying capacity to the cinder block to minimize efflorescence on the interior side from wicked ground moisture, but will block air movement from the interior into the cooler outer layers as well as wind-driven infiltration. Do NOT apply vapor retardent wall coverings like foil wallpaper or vapor-retardent paint on the interior side. With the insulation on the exterior the primary drying direction for the cinderblock will be toward the interior (with or without exterior vapor retarders).

A 6mil+ poly vapor retarder on the floor to impede soil gas penetration into your newly-tightened house is the right move. Poly by itself is an adequate radon barrier:

http://www.radonguide.com/detailed-protective-measures.html

Thanks for the very insightful response.  I sounds like you feel an tight vapor seal in the walls is not needed?  In your first sentence, "Vapor retarders would be required", did you intend to say they are not required ?

I've read that XPS greater than 1 inch thick (I'm think of using a total of 4 inches) acts as a semi-impermeable vapor membrane.  With this in mind, would you still recommend a vapor permeable or semi-permeable air barrier between the block and foam?  I don't understand what this membrane would accomplish with the XPS in place.

A little background:
My concern about the possible need for vapor barriers became significant as I removed the old interior wall materials from the house I'm renovating and found a lot of mold.  Foiled covered insulating sheathing and hard foam insulation, all of which had mold underneath (between it and the block).  Also found mold under carpets in the corners of rooms.  Thought this might be from a lack of heating in the winter and no vapor barrier.  Now, as I read more, it sounds like a tight vapor barrier would be detrimental as it does not allow the walls to breathe.  Not sure what to think, just don't want to find mold inside my walls a couple years from now.

Thanks a bunch for any further guidance.

From what you've written it sounds like the house was built with concrete block walls, insulated on the inside, and probably not protected from the exterior at all, or maybe just a layer of paint. A lot of houses were built like that in the 50's, and I don't know WHAT they were thinking. As you already know, concrete block acts like a sponge. If it were just kept dry from exterior sources of moisture then most of the mold and mildew problems would be absent. Your decision to insulate from the outside however will go far in correcting the problem.

Uh, yeah insert "not" where appropriate... (y'd think I'd read it before hitting "submit", eh? :-) )

Walls don't need to "breathe" exactly, they just need to be able to dry reasonably. Letting air move through the walls leads to warm moist air depositing it's moisture on the cooler bits, creating a mold potential.  The key is to stop bulk water migration where you can, but be sure that the structure can dry itself in reasonable amounts of time after wetting events.  Placement of vapor retarders defines the direction in which the drying must occur.

XPS is still only a class-II vapor retarder (Class-II = 0.1-1.0 perms), even at 4" thickness, and to achieve even that across the entier surface would take glued/caulked seams at every joint.  (It'll be on the order of 0.2-0.3 perms.  Class-I i= 0.1 perms and lower).  It's easier to get near-perfect air-barrier with roll-goods, but lapped joints, multiple layers, & caulk can also be pretty good, if more labor intensive than sheet goods in many instances.  But it's the air-barrier aspect you're looking for here.  Poly sheeting is a class I vapor retarder (~0.05 perms), but using poly vs. something else won't make a whole lot of difference in the humidity levels in the cinder block for the following reasons:

Cinder block construction (and masonry in general) passes moisture readily.  When the insulation was on the interior of the masonry every rain wetting event on the exterior brought moisture in, where it gets both wicked & pulled to the floor, and wicked by the slab to the interior.  On the exterior the wintertime temperature is also below the interior temp, so the vapor drive for drying after these bulk wetting events isn't super-high.  There is also some ground moisture being wicked up, but that's usually the lesser issue.

XPS on the exterior won't wick water inward-  bulk water from rain splash is shed entirely, and with reasonable drainage measures around the perimeter, can't find it's way back to the cinder block.  The temperature & humidity of the cinder block is now very close to the indoor temperature & humidity- much warmer in winter than when it was outside the thermal boundary.  This increases the vapor pressure of any moisture within the masonry, and unless you use vapor retardent wall materials it will dry toward the interior.   Being inside the pressure boundary (the air barrier), no cool air will find it's way into the cinder block creating localized cool spots either.  As long as you keep the interior warm & dry, the cinder block will remain warm & dry==no mold potential.



The slab is another story.  Being earth-coupled it's temperature will be lower than the room temp, especially if you put rugs over it which insulates it (~R1-ish), making it substantially cooler that if non-insulating materials were used. Lower temperatures== higher relative humidity, so if it's a comforable 68F & 60% RH at countertop height, if the slab is at 60F the same air at the bottom of the rug will be about 85%RH. See:

http://www.engineeringtoolbox.com/docs/documents/816/psychrometric_chart_29inHg.pdf

(Find the intersection of the 70F the 60% RH curve- move horizontally left to 60F and see which %-curve that is.)

The mold goes ballistic at anything over 70% RH, and with a rug on a cool slab you simply WILL get mold on any untreated mold-food  that the rug may contain (cotton, jute, wood, sisal, paper, etc. etc. ).  Even if you have a perfectly vapor-sealed (but uninsulated) slab, this will be the case.  If the slab itself is a water vapor source (groundwater wicking), it only get's worse from there.

The solution is to seal the floor and use non-insulating floor coverings. If the slab's going to feel too cold and you want to put something else on it, even 3/4" XPS  (~R3.5) between pressure-treated sleepers for nailing the floor decking will make a world of difference, and you can put whatever you want above without the temperature of the decking getting cold enough to be at the 70%+RH in for the room air, as long as you mechanically control the room air to 60% max with a dehumidifier or HRV, etc.  Plan on losing 1.5" of headroom if you go with the slab insulation, but it'll slightly reduce your heating load (and ever so slightly add to the AC load, but your R20 of wall insulation should mean a huge load-reduction on both.)  If you're really tight for headroom, 3/8" XPS sheathing (R1.5) & no sleepers, with 1/2" OSB floor decking thru-fastened to the slab can work.

Those with dust-mite allergies are advised to keep indoor humidity under 50% RH, but the generally-advised range for health & comfort is 30-60%, winter & summer.  Mold potential starts creeping in at 65% (the ASHRAE recommended upper limit, IIRC), but doesn't go really nuts until 70%+.  I keep the dehumidifier in my ~65F basement set to 60%, which keeps the musty smell at bay quite well.  But even though I've sealed the slab, corrugated boxes directly on the ~55F floor will still get a tiny bit mold going on the bottom after several months, yet none at all with as little as 1/4" of air space between box & slab.

The key to mold control is keeping all of the mold-food materials warm & dry. The particulars of how to make that happen will vary, but getting all of the building materials inside the thermal & pressure boundary of the structure the way you're doing is a very good start.