Monday, February 1, 2010

HEAT LOSS, HEAT GAIN & SUPERINSULATION

This will be my last post dedicated to background technical information, but is necessary to understand why a superinsulated house is superior to an insulated house. Simply put, a home gains heat and loses heat. If the home gains more heat than it loses, then we need to open the windows and/or air-condition. If the home loses more heat than it gains, then we need to shut the windows and turn on the furnace or other source of heat. The amount of heat and the length of year heat is required is referred to as the heating season. Superinsulation design will reduce the amount of heat required each day as well as shorten the heating season.

HEAT LOSS

Every house loses heat when the outside temeprature is lower than the inside temperature, always moving from warm to cold. Home heat is lost by three basic mechanisms:

1. Conduction - heat loss through materials and assemblies.

Most homes built in the past, as well as today, only incorporate the level of insulation and construction detailing that is "commonly practiced" or required by building codes. Superinsulation techniques reduce the amount of conductive heat loss by the intentional application of high levels of insulation (R40 or greater in walls / R60 in ceilings), reduction or elimination of thermal bridging in the structural framing, and installation of energy efficient doors,windows and storm doors, etc.

2. Infiltration - heat loss through air gaps.

Air-infiltration can often be the largest component of overall heat loss. Most houses built in the past, and unfortunately most today, did not fully address the need to reduce the amount of air infiltration for a multitude of reasons - "a house needs to breathe" or "don't worry, we'll put in a larger furnace" or "wood is cheap". Superinsulation techniques reduce the amount of infiltration heat loss by the design and installation of a complete, air-tight vapor barrier around the home's thermal envelope reducing air-infiltration to a minimum.

3. Ventilation - heat loss through exhausting warm air out of the house and cold intake replacement air into the house through bathroom vents, rangehoods, dryers, etc.

Superinsulation techniques reduce the potentially significant amount of ventilation heat loss by replacing traditional ventilation systems with heat recovery ventilators (HRV) that uses the warm exhaust air to preheat the incoming fresh air. This is critically important to provide a controlled source of fresh air in the air-tight environment of the thermal envelope.
In milder climates, exhaust-only ventilation system can use air-to-water heatpumps to transfer warmed exhaust air to heat domestic water and hydronic space heating.


Total Heat Loss Coefficent (HLC) is the heat loss rate for each degree of temperature difference between the inside air of the thermal envelope and the outside air measured in Btu per hour per degree Fahrenheit (Btu/h-°F). It the total sum of heat loss through all building components (walls, windows, doors, ceilings, basement walls, floor,etc.) due to conduction, infiltaration and ventilation. When multiplied by the indoor/outdoor temperature differential, it will provide the total home heat loss, but only for a point in time.


HEAT GAIN

Superinsulation design does not by itself increase or decrease heat gain mechanisms, but by intentional construction and design techniuqes takes full advantage of heat gain mechanisms to increase the home's energy efficiency and to reduce the home's heating load. While a home in a cold climate is losing heat through the heat loss mechanisms mentioned above, it is also gaining heat through three basic mechanisms:

1. Intrinsic heat gain is the embodied heat source produced from processes and activities occurring within the home - lighting, cooking, bathing, hair drying, human metabolism, appliances, refrigeration, etc. In a non-superinsulated home, most of the intrinsic heat is insignificant in comparison to or lost though high levels of conductive, infiltration and ventilation heat loss. In a superinsulated home, intrinsic heat sources are available for the heating of the home. The level of intrinsic heat gain available will vary depending on the activities occurring and when they occur, e.g. a television generates intrinsic heat, but only when it's in use. In a typical home, intrinsic heat can amount to between 2000-3000 Btu per hour.

2. Solar heat gain though either passive design or active collection systems can add significant heat gain depending upon the latitude of the home, the time of year, the amount of sunshine and cloud cover, the orientation of windows and the amount of shading. Maximum solar heat gain is typically received at midday between 10 am and 2 pm. While advantageous to receive this 'free' solar heat gain, it is not an imperative to successful superinsulated design.

3. Auxiliary heat gain is simply the the additional heat required through some controllable heat source to maintain comfort, i.e. the difference bewteen the home heat loss and the sum of the intrinsic heat gain and solar heat gain. The amount of auxiliary heat required will vary throughout the heating season and during each day. In superinsulated homes, the auxiliary heat gain requirement is so reduced, it is often a problem to design a heating system that is small enough to provide the required auxiliary heat.


BALANCE-POINT TEMPERATURE

The balance-point temperature (B-PT) is the outdoor temperature at which the total heat loss (HLC) equals the intrinsic heat plus the solar heat gains. When the outdoor temperature is at or above the B-PT, no auxiliary heat is required.

BP = Ti - [intrinsic heat input + solar heat input] / HLC

The balance-point temperature will vary throughout the day as the intrinsic and solar heat gains vary. Since superinsulated homes have a lower heat loss coefficient (HLC), they tend to have a much lower balance-point temperature (B-PT) even without solar heat gain. Factor in solar heat gain and the B-PT falls even further.

Typical heat loss / heat gain calculations used for mechanical equipment sizing assumes an average 65°F balance-point temperature as most homes have lower levels of insulation, are prone to air-leak infiltration and have exhaust-only ventilation systems.

In a superinsulated home with higher levels of insulation, air-tight construction and controlled heat recovery ventilation, the average balance-point temperature could be 42°F or even lower. This reduced B-PT can dramatically reduce the number of degree days (measurement of the difference between the average daily outdoor temperature and a specified base-point temperature) when auxiliary heat is required and could reduce the heating season from 8 months to 4 months. In essence, the lower balance-point temperature of a superinsulated house is the overall reason behind why superinsulation design techniques can so dramatically reduce the daily/seasonal heating load and cost for a building.


Now that the principles of superinsulation have been outlined, my next postings will provide several home designs and detailing that incorporate these principles. They will represent a series of solutions, but are by no means the only solutions. Any home can be superinsulated and alternative construction details exist and are possible if the basic principles of superinsualtion are understood and utilized as a total design system.



Many of the principles and concepts presented in my posts are gleaned from "The Superinsulated Home Book" by J.D. Ned Nisson & Gautam Dutt published 1985 by John Wiley & Sons. Most of the principles they presented in 1985 still hold true today and are still in practice. Unfortunately the book is no longer in print, but can be found in some public libraries and on Amazon for a price (I was lucky and got my copy for $35). If you are serious about building a superinsulated house, I would highly recommend investing in this book.

2 comments:

Ventilation System said...

Nice explanation, they are all facts,
it is the so called "cycle", everything will become complicated with out it.

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