Superinsulation is not a material, it is a system consisting of thermal insulation plus other building components which, when designed and installed properly in concert with each other, produce superb thermal performance in a home or business. Proper installation and design are paramount.
In case you were sleeping during your science classes, let's begin with the basics. Heat is transferred in three ways --
1. Conduction is the movement of heat energy (from hot to cold) through materials. Different materials conduct heat at different rates. That's why a down filled jacket is warmer than a fleece lining. Hold the end of a knife over a flame and the heat will travel to the other end. Heat inside a house is being conducted through walls, ceilings, windows, doors, etc. to the cold outside.
2. Convection is the transportation of heat energy by a moving fluid - water or air. Natural convection is fluid movement resulting from temperature differences - principle behind radiators transporting heat energy throughout a room by convection currents or heat loss inside an uninsulated wall. Forced convection occurs when an external force acts to move the fluid - forced air furnace or wind drawing heat energy from the house.
3. Radiation is the transfer of electromagnetic energy - e.g. heat transfer from a wood stove to people or materials. Near windows warm body heat is radiated to the cool glass surface making one feel cold. If surfaces are warm, people will feel warm regardless of the air temperature. A room has a mean radiant temeperature (MRT) - the 'averaged' temperature of all exposed surfaces within the room.
Insulation reduces heat transfer through walls, ceilings, windows, doors, etc. by dividing the large interior space of a building component into thousands of tiny air pockets. It is not the isulation material itself that creates the thermal resistance, but the low conductivity of the still air created by these air pockets. All insulation materials - fiberglass, mineral wool, cellulose, straw bales, foams - work by the same principle. Each material is given a conductivity rating (R-value) that measures its thermal resistance to heat transfer. Convection heat transfer is virtually eliminated because the air is trapped and prevented from moving. Radiant heat transfer is greatly reduced because long range infrared radiation is absorbed and/or scattered.
Easy enough, pick an insulation material with the highest R-value per inch! Not so fast. Remember that superinsulation not a material, it is a system. Superinsulation also avoids thermal defects that can reduce the overall thermal resistance of the building components:
- Insulation Voids: The component spaces must be totally and completey filled. For example, a superinsulated wall or roof installation should have as many voids as roof leaks or plumbing leaks - none. If a wall had 5% overall insulation voids, its overall resistance could be reduced by up to 25%.
- Thermal Bridges: Thermal bridges are points or components with relatively low thermal resistance that intrude or 'bridge' through the insulation layer of the thermal component - studs, rafters, plates, window & door openings, corner framing, etc. Unlike insulation voids being addressed by thorough and careful installation practices, thermal bridges are addressed through proper design detailing.
- Air Intrusion: Even if air is not allowed to flow all the way through the insulation system, it can degrade overall thermal performance by merely penetrating the insulation from one side.
- Convective Loops: Wherever there are hollow spaces around insulation in a wall or ceiling, heat can be transimitted through the system by convection, even if the insulation is completely sealed agains leakage or air intrusion and no inside or outside air penetrates the system. Faced insulation batts stapled to the side of studs leave an air gap between interior finish and he insulation facing. Voids in hollow concrete block walls can develop convection loops cooling the wall. Air chases around a flue can transmit warm air to the attic.
- Moisture: Moisture comes from human activity - breathing, cooking, bathing, plants, etc. Moisture can degrade the thermal performance of insulation by convection - moisture moving into insulation through gaps - and diffusion - moisture moving through materials. All insulation systems have a dew point - the location within the system where water vapor will condense into water due to temperature.
Air-tightness is a crucial element of the superinsulation system and requires careful attention to detailing and workmanship. Heat loss by poor installation of air/vapor barriers can negate all the beneifits of well installed insulation and can even result in building damage. Air/vapor barriers in a building system will be shown in greater detail when we look at construction detailing in later blogs.
The most typically used air/vapor barrier in buildings today is 6 mil polyethylene sheets. The air/vapor barrier is always located on the warm side of the exterior wall systems where it is will be above the wall's dew point temperature to prevent condensation. Practice over time has shown that an air/vapor barrier can be installed 1/3 of the way into the warm side of the overall insulation and still work effectively. This will come in handy later when electical and plumbing needs to be run in the wall system.
The air/vapor barrier must be meticulously sealed at all seams and at all penetrations including windows, doors, outlets, plumbing, vent stacks, vemt fans, etc. Techniques will be discussed later when we look at constuction details. The air/vapor barrier must be protected throughout the building process and all damages must be repaired as soon as they occur. Before any interior finishes are installed, the entire house should be blower door tested to insure air-tightness of the thermal envelope.
An air barrier is typially installed on the cold side of the exterior sheathing. This is commonly referred to as 'house wrap' in the building trade, e.g. TYVEK - a spun-bonded polyethylene. Unlike the air/vapor barrier that blocks both wind and vapor penetration, air barriers protects the insulated walls from air intrusion and makes the thermal envelope tighter, but allows water vapor to pass through to the outside. Care should be taken to tightly seal all wall penetrations (windows, doors, vents, etc.) and to tape all overlaps in the air barrier.
These are the structural elements or structural frame of the building - what holds up the building. Support structures must also (1) provide adequate containment of the insulation materials while minimizing the effects of thermal bridging, and (2) provide adequate support of the air/vapor barrier and the air barrier.
For the purpose of my blog, wood frame construction and materials typical of the maritime region will be incorporated. While I will be using double stud wall construction techniques in most details, other walls systems have successfully been used in superinsulated houses, e.g. 2x6 stud wall with rigid insulation as exterior sheathing; TJI joists used as wall studs; strapped walls; larsen trusses; straw bales; etc.
For the purpose of my blog, roof framing will incorporate raised heel trusses and parallel chord trusses to allow for full thickness of insulation material over the entire roof area. More later.
In my next post, I will be discussing heat loss, heat gain, and how superinsulation techniques can dramatically reduce the overall yearly heating requirements of a building.
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.