ASHRAE Journal - February 2022 - 28

COLUMN BUILDING SCIENCES
time. They also recommend cleaning
the humidifier every three days. Yup,
no issues there.( " Humidifiers: Ease
Skin, Breathing Symptoms " : https://
tinyurl.com/4v4dteex)
How about controlling interior relative
humidity based on the exterior
temperature and the construction of
the house? In Chicago, keep the relative
humidity in a typical house to
20% December and January-35% in
November and March-and higher the
rest of the year. Save the people for
eight months, and save the building
for four months (Figure 1).
With continuous exterior insulation
(Figure 2) that meets the 2018
International Residential Code,
the interior relative humidity can
be maintained up to 35% during
the coldest months, December and
January. That is what Table R702.1
(Table 1) in the 2018 IRC is based on
(see " Hybrid Attics and Hybrid Walls, "
ASHRAE Journal, October 2017). You
want to go higher with interior relative
humidities? To 50% during the
coldest months? Easy, increase the
ratios by 50% or have all the insulation on the exterior
(Figure 3) and no insulation in the wall cavity framing
(Figure 4) (see " The Perfect Wall, " ASHRAE Journal, May
2007). It is pretty straightforward to plot curves for all
the major cities and make recommendations about
controlling indoor humidity based on average monthly
exterior temperatures and typical wall assemblies.
That is just the walls. How about the windows? Ah,
the windows are going to be a problem. You are going
to need R-4 to R-6 windows to control condensation
on the glazing. That means doubling or tripling the
performance of typical new construction windows. My
idea would be just to heat the inner pane during cold
weather. We already do that in museums. Or install
an interior storm window with a heating element
between the interior storm and the window unit. Lots
of buildings with artwork work use that approach. Or
you use the same approach proposed for walls: control
Truss
Metal Roof
Roof Sheathing
Fully Adhered Roofing Membrane
Metal Roof Deck
Gypsum Board
Air-Control
Layer
Liquid-Applied
Vapor Control
Layer
Metal Cladding
Hat Channel
Thermal Control Layer
Water Control/Air Control/
Vapor Control Layer
Service Space
Interior Lining
the indoor relative humidity based on the exterior
temperature.
What about roofs? It's hard not to love a vented R-60
roof, which easily handles high interior relative humidity.
Unvented with all the insulation above the deck also
works. Can you say " perfect roof " ? (See again the " The
Perfect Wall, " ASHRAE Journal, May 2007 and " Hybrid
Attics and Hybrid Walls, " ASHRAE Journal, October 2017).
Here is where I remind everyone how difficult it is to
build indoor swimming pools-natatoriums-in cold climates
(Figure 5) (see " In the Deep End: Indoor Swimming
Pools and Water Parks, " ASHRAE Journal, September,
2011). Yes, I know it is not quite the same-interior conditions
are around 80°F (27°C) and 50% relative humidity
in natatoriums, a higher moisture load than 72°F
(22°C) and 50% relative humidity. But they have taught
us things we should remember. The key to windows in
natatoriums in cold climates is to blow warm air against
them to keep them warm and not condensing. Yup, heat
the windows.
28
ASHRAE JOURNAL ashrae.o rg
F E B R U A RY 2022
Dropped Ceiling
FIGURE 5 Cold Climate Indoor Swimming Pool. Continuous exterior insulation, no cavity insulation, highly insulated
vented attic without thermal bridging, all mechanical systems located to the interior of a continuous air
control layer.
Roof Vents
Vented Soffit
2x Continuous Wood
Thermal Break
2 × 4 Thermal Bridge Furring
Sealant
Sheet Metal Closure Strip
Ducts
Ducts
SurfaceMounted
Fixtures

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ASHRAE Journal - February 2022

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