Under House Ventilation

It's usually best to keep crawl-space vents closed

Here in the humid South it has become evident that warm-humid air getting under the house actually worsens the moisture problem. As air conditioning is the norm, more so in this decade than ever, the floor over the crawl space is cold (compared to the 98-degree, moisture-laden outside air) and the moisture suspended in the air condenses directly on the cold floor joists and sub-flooring. This keeps the wood damp and subject to dry rot fungus that can eventually weaken the wood until it is brittle.

The consensus among websites devoted to this issue is "leave the crawl space vents closed to keep out the warm humid air, cover the crawl-space soil with heavy '6-mil polyethylene vapor diffusion retarder' to keep ground moisture in the ground, insulate the stem-wall inside the crawl space with R-11 to R-19 batt-type fiberglass insulation or rigid foam sheets with joints taped or sealed and all covered with 6-mil poly."

The Forest Products Laboratory offers a treatise on "crawl space ventilation" tulsaworld.com/ACcrawlspace that says, "In the southern United States, where high water tables often preclude basement construction, building houses on posts or piers or over crawl spaces is a tradition. Decades ago, it was common in the South to build houses on posts. In these houses, the under-floor area was extremely well vented and about as warm in summer and as cold in winter as outside ambient conditions.

"As decades passed and crawl spaces became more common, it became evident that they tended to become objectionably damp. Outside-air exchange rates are much lower in crawl spaces than in under-floor spaces of houses on posts or piers. As a result, seasonal temperatures in crawl spaces are more effectively moderated by the ground than in the under-floor spaces of houses on posts or piers.

"Crawl spaces are not as cool in winter or as warm in summer. This can reduce heating and cooling costs, and it reduces the chance of plumbing lines freezing in cold weather. In short, crawl space construction was recognized as having thermal benefits ... but it was also recognized as being more prone to moisture accumulation."

"By the 1950s, crawl space ventilation became mandated by building codes ... because of the combined idea that (1) the primary source of crawl space moisture was evaporation from the crawl space floor and (2) crawl space ventilation would allow for dissipation of this evaporated moisture.

Home Ventilation System |

Fresh air and adequate ventilation serve more than just the physical comfort of occupants. While a healthy supply of fresh air is important proper airflow also provides condensation control, removal of airborne pollutants and ensures the safe use of potentially combustible appliances.

Insulation, heating systems and overall ventilation installations need to be considered in a group context for the most energy efficient results. Generally speaking the aim of a good ventilation installation is to maximise energy efficiency without reducing indoor air quality.

Visually the most significant disadvantage of bad ventilation is mould damage associated with high relative humidity. Kitchen and bathroom activities utilise a lot of water in particular and the supply industry has responded by producing wide ranges of domestic fans for bathroom and kitchen installations. 

Environmentally-conscious home owners will be aware of the new best practise building standards available for energy efficient dwellings. Insulation or ‘airtight’ architecture is often used – with success – to reduce the heating energy expenditure. This ‘airtightness’ also contributes to more rapid build-up of internal pollutant sources such as gases, cleaning agent fumes, Allergens such as house dust mites and chemical by-products from home ware and furniture. Unless great insulation is coupled with effective ventilation negative health effects will be felt over time.

Natural airflow can easily be augmented and controlled by installation simple ventilation mechanism in the home. There is a three-point action plan to reach great home air quality goals:

One: Extractor fans installed in key areas such as kitchens, laundries and bathrooms allow for controlled air expulsion in heavy activity rooms.

Two: Whole building ventilation and air condition allows automated regulation air quality throughout the building usually via roof or floor ducting installations. Apart from timers whole building systems can also be installed to utilise sensors to determine appropriate operation times. For example an occupancy or motion sensor will allow a sensor to tailor its usage to times when the house is actually occupied and reduce the overall energy costs of ventilating.

The Homeowner's Handbook to Energy Efficiency, A Guide to Big and Small Improvements

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