Deep Energy Retrofit -- Moisture Proofing Details


These are some questions and Gordon's answers about certain aspects of the moisture proofing of Gordon's deep energy retrofit of his residence.


For more on the full project:

- Residential Renovation of a Schoolhouse  -- A Deep Energy Retrofit...

- The Thousand Homes Challenge -- Sue and Gord Scale Case Study...



I read with interest your schoolhouse retrofit article on BuildItSolar. I live in a solid 50 year old brick, stone, and wood-frame building, which is very solid, but is not adequately insulated for our climate. I am researching ways to improve that. I found many good ideas in your report, although I have a few questions:

1) What exactly is the function of the rainscreen in the truss walls? Is this used to keep external moisture out of the insulation, while letting the house still breathe? The rainscreen doesn't sound like the same thing as a vapor barrier. It also doesn't appear that your house has, or needs, a vapor barrier. Is this correct? Is the foam insulation in the outer truss walls free of condensation risk because it is all outside the concrete or ICF walls and protected from exterior moisture by the rainscreen and the siding?

Good questions; the rain screen provides a two functions (should be three :). Most siding materials allow some of the driving rain (liquid moisture) to enter the wall cavity, so the rainscreen provides a drainage plane (or small air gap) to allow the water to flow down and out the bottom to the outside, and during more favorable weather, allow natural air convection to dry the back of the siding and outside of the wall assembly. . In addition, a good rainscreen is relatively permeable to water vapor, enabling vapor that has been trapped within the walls to escape to the exterior, whilst providing a relatively good air barrier to
resist wind pressure (infiltration). A really good rainscreen (not mine:( ) includes design details (particularly at the top and bottom) to ensure insect pests can't get into the wall assembly (bees, wasps, carpenter ants, termites - at least we have no termites in this area - yet ?). BTW carpenter ants love bead foam, even type IV XPS foam and especially low density 'Big Gap' canned foam - better to use denser, low expansion canned foam within 2 ft of ground surface.

In our case we used a very porous, non-woven polypropylene sheeting (something like a 3M pot cleaner without grit) for the liquid water gap directly under the siding. It comes in huge rolls and is about 3/8" thick, squishing down to about 1/8" at fastenings. Under that we stapled and taped Tyvec for the air barrier (because it is less wind permeable than most non-woven air barriers - but still permeable to water vapor)! Neither has any resistance to UV, so construction timing is important. I did not use a vapor barrier (poly sheeting) in the wall assembly. I wanted water vapor near the outside to exit to outside, similarly on the inside - one doesn't want to trap moisture and get mold. But that means I had to choose an impermeable insulation (2.2lb/cu ft urethane) and come up with a means (by multiple crisscross framing) to not have full wood depth framing conveying water vapor through the wall. I did a lot of calculations to determine permeability and dew point locations, and am satisfied that the potential occurrences are rare (a few hours less than 5 days/yr). Until I looked at various materials and assembly configurations & did a lot of arithmetic, I never realized just how difficult this is to achieve. BTW, both soy & low density urethane foam are too permeable. Wet, dense pack cellulose might work - but I doubt it, and you have to get it fully dry before closing in, and that might not be possible during extended humid weather. Forget regular cellulose. Forget glass fibre - use it for furnace air filters :). Note that we used cedar against the masonry and towards the outside of the foam for rot resistance.

2) Did you lay the floor pavers directly on the Type 4 foam, with the 20 mil poly under the foam, and then the original floor? I am guessing this sequence was followed to prevent any ground moisture from seeping out of the original slab into the insulation. Is there any concern about a thermal bridge between the original (uninsulated) floor and the concrete block walls? What about ground moisture creeping up into the walls from the original slab? Have you prevented this possibility by waterproofing and insulating the exterior foundation walls? In which order is the foundation waterproofing and insulation applied? Fig. 24 seems to show that the wall is water-proofed before the insulation is attached. Is this right? Is the wall water-proofed again, after the insulation is applied?

More good questions. Correct, old slab, poly, then Type IV (no sleepers) then either conc. pavers, two ply of 1/2" concrete panel then slate, or two ply 1/2" fir ply then hardwood. Ground moisture was a major concern, and it was not possible to place a poly break at bottom of walls. From web, learned about moisture testing of slab, found ours was relatively bad and compounded during humid August. Thus our major effort to divert moisture out of sub-soil and prevent moisture infiltration (4 ways plus surface grading/swales). We also ensured that hardwood was finished on all sides/edges! Frost wall were waterproofed with Blue Seal on the outside prior to type IV foam insulation. Type IV is pretty impermeable. Not water-proofed again - but all new foundation & separate gutter drains - all to daylight! By draining & insulating, the sub-slab soil temperature is warmer, considerably so near the perimeter. If you can't drain to daylight, one should seriously consider an effective outer water-proofer such as dimple panel and perhaps additionally sprayed clay. I don't have sump pump - sure don't want to drag water into house - much better to effectively drain it well away - but not always possible.

BTW, the foundation contractor for the garage brought in a massive swing bucket, & that was just the tool to dig round the old house - in about 5 hrs he'd ripped out a hole 6" wide by 7' deep by 270' perimeter - (I figure about 50 days with my little Kubota backhoe:) ).

3) In the new addition, how did you attach the drywall to the inside of the ICF walls?

We used Arxx ICF, which has plastic cleats imbedded in it to resist the liquid concrete - the cleats form exterior strips on the surface for screwing either drywall direct or steel lath (our case). Note that electrician imbedded the wiring in the ICF foam prior to drywall contractors - but the latter were able to damage/destroy every circuit in the room (had to install surface wiring:( Would never consider ICF again - very slow, expensive, fraught with problems, mediocre R value, requires wider columns and beams over windows - thus smaller gain windows, than stick frame. I now know that with and experienced and keen mason/block-layer that concrete block would be 3 times faster and about 1/3 cheaper!

Thank you,

Donald C.






Gary January 8, 2011