Old Brick scenarios

[jiano]

I am having trouble interpretting results from a number of comparative scenarios. All the wall assemblies begin with 12 inches of "old brick" and then add various insulation and vapor barrier materials to the interior side of the wall as follows:

1. Solid, 12-inch, “old brick” wall
2. Same as 1., with inside finished with 1" extruded polystyrene insulation, polyethylene vapor barrier, and gypsum wallboard
3. Same as 2., but omit the vapor barrier
4. Same as 3., but with 1.25" expanded (rather than 1" extruded) polystyrene insulation
5. Same as 3., but with 1.25" fiberglass (rather than polystyrene) insulation
6. Same as 3., but with ½" expanded polystyrene insulation
7. Same as 3., but with 3" expanded polystyrene

All scenarios, other than 1., have significantly higher and very nearly identical levels of moisture accumulation in the brick portion of the wall. I was not surprised that insulating the wall reduced its drying ability, but I don't understand why there seems to be almost no difference in the results of scenarios 2 through 6, despite the variations in insulation level and permeability of the added materials.

Can you offer any insites? Thanks.

Details:
Location: Seattle, cold year
Orientation/Inclination: Short building, South/900
No surface coatings
Calculation Period: 2/1/2006 - 2/1/2008
Indoor Climate: Indoor condition, medium moisture load
Initial Conditions: Constant across component: 80% RH, 680 F
Software: WUFI Ver 4 ORNL (Lite version)

[Hartwig M. Kuenzel]

I have done similar simulations about 10 years ago. The results were published in Materials & Structures 1998 vol. 31 (WUFI-Publication E7 on the WUFI website; this paper will be downloadable next week).

The results correspond with what you describe. The thermal resistance of old brick is so poor that a thin layer of insulation makes a huge difference to the brick temperature. This could explain why you get pretty much the same results for cases 2-7. Of course there may be some condensation on the interior side of the brick wall when you add fiber glass insulation without vapor retarder but this is a small amount compared to the water absorbed during driving rain events.

[jiano]

Yes, I suppose that makes sense. (I believe the scenario with only 1/2" of foam did show a slight, but not significant, reduction in moisture levels.)

So as a followup question, to what extent can these results be applied to real-world conditions? Are these results a generally useful guideline for insulating historic brick masonry walls in this particular climate, or are they really only valid for the specific "old brick" as modeled in the software?

Thanks.

[Hartwig M. Kuenzel]

I think this result is rather independent of the type of brick as long as the water absorption and the thermal conductivity are not too small (which is hardly the case with old brick masonry). We have also done field tests where we recorded the water content of brick masonry elements exposed to natural weather over a period of several years by discontinous weighing. The elements with 5 cm of interior insulation (polystyrene foam) had a considerably higher (more than 50%) water content. We have published these results in a German Journal (WTA-Journal 2/03, S. 149-166; article in German can be downloaded from www.building-physics.com)