The external envelope of a building should be as airtight as possible - this is true for conventional as well as for passive houses. It is the only means to avoid damage caused by condensation of moist, room warm air penetrating the construction (see the figure on the left hand side). Such damage not only occur in cold climates; in hot and humid climates the problem can occur from airflows from the outside to the inside. The cause is the same in both cases: a leaky building envelope.
Drafts in living spaces are not tolerated by occupants any more: Therefore a very airtight construction is essential to fulfill modern thermal comfort expectations. Most building codes, worldwide, require airtight building
- A well insulated construction is not necessarily airtight, too. Air can easily pass through insulation made from coconut, mineral or glass wool. These materials have excellent insulation properties, but are not airtight.
- On the other hand an airtight construction is not necessarily well insulated: e.g. a single aluminium foil can achieve excellent air tightness, but has no relevant insulation property.
- Air tightness is an important, but not the most important requirement for energy efficient buildings (contrary to the impression given by some popular publications). Further, achieving air tightness should not be mistaken with the function of a "vapor barrier". The latter is a diffusion tight layer: An oiled paper e.g. is airtight, but it allows moisture vapor to pass through. Conventional room plastering (gypsum or lime plaster, cement plaster or reinforced clay plaster) is sufficiently airtight, but allows vapour diffusion.
Infiltration can not guarantee good indoor air quality. Houses built in Germany after 1985, for example, are so airtight that infiltration alone is inadequate to assure acceptable indoor air quality. Yet, these houses are still at risk regarding moisture damage to the construction from moist room air exfiltration. A greater level of air tightness is needed and these houses must be considered as "untight". Their n50-air leakage varied between 4 and 10 h-1. The consequences are draft-discomfort and moisture damage to the construction. The construction was too leaky to avoid exfiltration caused damages - but too tight for sufficient infiltration to maintain room air quality.
The new 2001 German building code ("EnEV" Energy Saving Standard) for the first time addresses the air tightness of new constructions. Without a ventilation system the n50-airchange- values have to be less than 3 h-1, with ventilation systems 1.5 h-1. From the experience in low energy houses we recommend tighter construction (lower n50) leakages.
In passive houses far better n50 leakage rates are frequently achieved. The requirement is n50 not greater than 0.6 h-1. In practice values between 0.2 and 0.6 h-1 have been measured in passive houses.
If a construction is not sufficiently airtight, moist room air can penetrate into the construction, condense and cause damage. The problem can be solved by thorough, air tight design.
Air tightness is not a mere nicety of energy saving construction, it is essential to avoid construction damage. Gaps in the construction will lead to substantial humidity transport by convection.
Isabella Ecohome air tightness & insulation approach
The air tightness for the Isabella Ecohome was achieved by first using a closed cell polyurethane spray foam insulation for the difficult to access areas. The product used was UCSC Polar Pro 1.9 the new name for the product is PP 1.1 by Bayer. This is both an air barrier and an insulation product. Here is an excerpt from US Department of Energy’s EERE site regarding closed cell polyurethane spray foam insulation. (http://apps1.eere.energy.gov/consumer)
While this type of insulation has greatly improved the ozone depleting blowing agent problems they have had in the past, it is still a hydrocarbon product so we tried to limit the use of it wherever possible.
We filled the majority of the cavities of the double wall system and the attic spaces with a dense packed cellulous material. We used Weather Blanket supplied by Modern Insulation, (http://moderninsulationinc.com/), which is a high quality, all-borate, loose-fill cellulose insulation for use in blowing caps in attics and in dense-packing wall and ceiling cavities. It was installed by Dave Joice owner of a company called “The Carpentry Works”.
Weather Blanket is composed of:
- Over-issue newsprint (yesterday's unsold newspapers), which is the cleanest and highest quality paper available
- Boric Acid, a highly effective fire retardant
- A small amount of a light white mineral oil for dust controlCellulose insulation complies fully with CPSP standard HH-I-515E; 16CFR 1209; and ASTM C-739. It is made from cellulosic fibers derived primarily from recycled, over-issue newsprint. This, along with hand sorting, virtually eliminates plastic and trash from our insulation, making it cleaner, less dusty, and more installer friendly. None of these fibers are sourced directly from wood.
In all applications, bags of cellulose insulation are placed in an industrial-quality blowing machine. The product is then blown through several hundred feet of 2" to 3" hose, either into attics or dense-packed into wall cavities. In new construction applications, the wall cavity is formed by stretching and stapling synthetic webbing or poly sheeting across the open-faced studs. A slit is made in the webbing for the hose, the hose is inserted, and the cavity can be viewed as it is filled with dense-packed cellulose insulation. The Sheetrock® is then installed on the studs over the membrane.
The R value of our walls was calculated to be 31, 51 and 60 respectively for each of the wall types. The majority of the wall has an R value of 51. The roof has a R value of 100. With temperatures reaching record lows of 60 degrees below zero F, you can start to understand the importance of insulation and the challenges we faced for this home.
Results of blower door test prior to Sheetrock installation
As we traveled to the project site on November 11, 2008, we wagered on what the results was likely to be once the test was completed. The initial results prior to finding a open vent stack and a few volume calculation errors was 1.1 air changes per hour. Once Mike LeBeau and Dave Joice fired up their handy dandy
infra-red thermography cameras they were able to find where the air leaks were occurring. The blower door test puts the house under negative pressure using the blower door set-up shown below.
After we sealed up problem areas and set a strategy for what needed to occur next, two weeks later we were able to accomplish a .6 air change per hour result, well within the range of comfort that we could meet and even exceed the results required by Passive House design standards once the sheetrock and final finishes were complete. Two areas of concern that we are still investigating include the window air leakage problems and the fact that we used a .4 air change per hour as the baseline for the PHPP energy modeling data. We are hopeful there are easy answers to fixing the air leaks occurring in the windows and we are feeling confident that when the Sheetrock is complete we will be able to reach a .4 in lieu of .6 air changes per hour result.