Restoration Guide: Foundations and Ventilation Practices

Roger Diez

Editor's Note: This is article 7 of 9 in Chapter 1: Foundations section of the Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.


Section 1--Overview

Ventilation in basement spaces, crawlspaces, and for soil gasses is important for a number of reasons. Direct ventilation via windows, vents, or fans is preferable to indirect ventilation via air infiltration.

Section 2--Ventilating Basement Spaces

If a basement space is not well ventilated, concentrations of soil gases, including radon, can occur. In addition, mold and mildew thrive in cool, damp, poorly ventilated spaces. Chemicals, fuels, and solvents stored in a basement can outgas volatile compounds, as well. Ventilation is especially important if the basement is used as a living space. If systems like heating furnaces and water heaters are placed in the basement, poor ventilation of these devices can cause depressurization as they draw air for combustion. This depressurization can draw soil gases into the basement atmosphere. If possible, a separate source of outside air such as a window or vent should be provided. Ideally, direct-ventilation (sealed combustion) units that draw outside air directly into the combustion chamber should be used.

2.1: Adequate Basement Ventilation Techniques

Here are six methods for providing adequate ventilation to basement spaces and combustion air for furnaces, boilers, and water heaters.

  1. Direct-ventilation for boilers or furnaces: A duct leading directly into the combustion chamber of this type of furnace or boiler provides the needed air for combustion. This prevents incomplete combustion and flue gas backflow.
  2. Fan with a motorized damper: Using this method, the damper to provide outside air is opened only when the furnace or boiler is firing. This method is second only to a direct-ventilation device.

  3. Screened, open vent: This allows air to enter the furnace or boiler area from the outside at all times. To prevent heat loss, the furnace or boiler should be in a separate, enclosed room. This is a lower cost solution, but requires that the heating equipment is situated next to an outside wall with an opening above grade. A vent should be used rather than a window, which could be closed inadvertently.

  4. Basement air separation from house: This technique can be used if the basement is not used as living space. The basement is closed off from the rest of the house with doors and a continuous floor. The basement must be ventilated with windows or fans, and heated with electric baseboard heaters or a separate heating system, and fan housings and ducts that lead to the house must be sealed. If fuels, chemicals, or solvents are stored in the basement, they must be kept in a separate, sealed space with continuous outside ventilation.

  5. Basement treated as a conditioned space open to the house: If the basement is finished as living space, it can be ventilated by the same air system that serves the house. To prevent low pressure that could lead to suction of soil gases, outside air should be supplied at the upper levels and exhausted from the basement, maintaining positive pressure in the basement. Adjustable dampers on heating ducts should be used to prevent too much warm air rising to upper levels.

  6. Ventilated "room within a room": A technology called the ECHO system creates rooms or areas in the basement that have continuous air exhaust under the floor and between the walls of the room and the basement walls. This is used when basement walls are continuously damp or when radon cannot be controlled by other means. Heating devices used with this system should use direct ventilation to avoid creating negative pressure.

Section 3--Ventilating Crawlspaces

In order to minimize moisture, mold, mildew, wood rot, and odor in crawl spaces, adequate ventilation is required. Building codes typically require one square foot of ventilation opening per 150 square feet of crawlspace area. If foundation vents are not used, codes usually require a mechanical ventilation system.

3.1: Adequate Crawl Space Ventilation Techniques

  1. Natural ventilation through foundation wall: If this is an old house renovation, openings must be made in the concrete or block foundation wall. Cover them with screening to provide natural ventilation, but you need to insulate the floor above the crawlspace and all pipes and ducting. Crawlspace dirt floors should be covered with polyethylene sheeting.
  2. Mechanical ventilation of unconditioned crawlspace: Create the same openings as natural ventilation, but install a mechanical ventilation system. Dampers on the openings that close when the system is not activated keep humid air out.
  3. Mechanical ventilation of semi-conditioned crawlspace: This method treats the crawlspace as a basement, with insulation, mechanical ventilation, and heating. It is more costly to install, but saves fuel costs in the long run.

Section 4--Ventilation for Soil Gases

Soil gases, particularly radon, are health hazards. The EPA estimates that 14,000 cancer deaths per year are caused by radon, and that one in fifteen houses has elevated levels of this radioactive gas.

4.1: Adequate Ventilation for Soil Gases

The EPA recommends these techniques for abatement of radon gas.

  1. Sub-slab ventilation of soil gases in basements: A vent pipe with a continually operating exhaust fan penetrates the slab and runs upward through the house to vent out the roof. All cracks and openings in the slab should be sealed. If a new slab is poured, it should rest on a four inch layer of sand or gravel with a gas-retarder membrane on top.

  2. Foundation wall depressurization: Continuously operating exhaust fans depressurize the foundation wall through weeping tiles. Seal all cracks and penetrations in the wall.

  3. Basement air separation from house, and pressurization to keep out soil gases: Maintain a positive air pressure in the basement by providing a separate air system for the basement with an outdoor air intake. A direct-ventilation furnace or boiler for the house, and a separate heating system (such as electric baseboard heating) should be used.

  4. Ventilation of soil gases from crawlspaces: If the crawlspace has a slab, this technique is identical to sub-slab ventilation in a basement. In the case of dirt crawlspace floor, a perforated pipe is laid parallel to the house's long axis at least six feet from the foundation walls. A T-fitting attaches the pipe to an exhaust stack that vents through the roof. The pipe and crawlspace floor are covered and sealed with a gas-retardant membrane.
  5. Ventilated "room within a room": The ECHO system is effective in reducing radon penetration.

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