How the Sun Heats a Home Interior

[Kenton H. Shepard, CMI]

How the Sun Heats a Home Interior

Conduction
Conduction is the transfer of heat energy through solid matter from particle to particle. Heat is conducted through a solid material much the same way electricity is. Conduction is slowed by insulation.
Heat always moves from warm areas to cooler areas according to a physical property called the Thermal Gradient.

Convection
Convection is the transfer of heat energy through a gas or liquid by movement of currents. The heat moves with the fluid. When warm air comes out of a register, it rises. This action is called thermal buoyancy. As it cools, it falls. This movement of air is called convection current. Convection can be interrupted by a physical barrier.
Fluids carrying heat always move from areas of high pressure toward areas of low pressure according to a physical property called the Pressure Gradient.

Radiation
Radiation is electromagnetic waves which directly transport heat energy through space. Sunlight is radiated through space to our planet without the aid of fluids or solids. Through radiation, the sun transfers heat through 93 million miles of empty space!

How Heat Moves from the Sun to the Home Interior

Depending on the roof color, a large part of the sunlight radiated to earth is absorbed by a roof as heat, which moves downward through the roof covering material into the sheathing and rafters through conduction, since these are solid materials. As the solid parts of the roof become warmer, they warm the air next to their surfaces, which is then carried to other parts of the attic by convection currents.

As they warm, the roof covering and framing materials also begin to radiate heat to the ceiling framing members and insulation between the ceiling joists. Ceiling joists absorb this heat and conduction helps spread it throughout the length and width of each joist.

As radiation hits the insulation and is absorbed by the fibers on the upward-facing surface, the fibers begin to warm. As they warm, they pass the heat they contain to the microscopic air pockets nearest to them. Through conduction and convection, heat moves slowly downward through the insulation.

As heat from the roof increases, this process happens more quickly until the ceiling has warmed enough to begin to radiate heat into the home interior where it’s absorbed by objects, including people. Heat is distributed through these solid objects by conduction, and they in turn warm the air around their surfaces. This warm air is spread through the home by convection currents, natural air movement, fans and blowers.

Feedback?

[Brian E. Kelly]

Here is my comment Kenton.

It's friggin hot here.

Phoenix reached a shoe-melting, spirit-crushing milestone Wednesday: 29 days of temperatures 110 degrees or higher in a single year.

[Brian A. MacNeish]

Originally Posted by kshepard

How the Sun Heats a Home Interior

How Heat Moves from the Sun to the Home Interior


(1) Depending on the roof color, a large part of the sunlight radiated to earth is absorbed by a roof as heat, which moves downward through the roof covering material into the sheathing and rafters through conduction, since these are solid materials. As the solid parts of the roof become warmer, they warm the air next to their surfaces, (2) which is then carried to another part of the attic by convection currents.

As they warm, the roof covering and framing materials also begin to (3) radiate heat to the ceiling framing members and insulation between the ceiling joists. Ceiling joists absorb this heat and conduction helps spread it throughout the length and width of each joist.

As radiation hits the insulation and is absorbed by the fibers on the upward-facing surface, the fibers begin to warm. As they warm, they pass the heat they contain to the microscopic air pockets nearest to them. Through conduction and (2) convection, heat moves slowly downward through the insulation.

As heat from the roof increases, this process happens more quickly until the ceiling has warmed enough to begin to radiate heat into the home interior where it’s absorbed by objects, including people. Heat is distributed through these solid objects by conduction, and they in turn warm the air around their surfaces. This warm air is spread through the home by (4) convection currents, natural air movement, fans and blowers.

Feedback?

YES!!

(1) Poorly written! If it's a dark roof, a large part of the incident solar radiation may be absorbed. If it's a light colour, a large portion may be reflected. Some darker roof colours now being researched are being designed with granular materials that have higher reflectivities than those in use.

Solar radiation is not absorbed as heat; it's absorbed as short wave radiation which then turns to heat. If it was heat radiation, the upper layers of the atmosphere would be warmed first.....but the radiation passes through these upper layers to hit the earth thus turning to sensible heat.(or be partially reflected by clouds, etc)

(2) In a flat roof situation, convection will not be a factor since the hottest air will be already at the highest point, just underneath the roof sheathing! This also is the case with air pockets in the insulation- the warmest air molecules will be at the top of the cavity since the heat is moving downward by conduction. Radiation and probably mostly conduction will be occuring in the small air pockets.

(3) This must be an older piece of writing. To have this low level of insulation today with ceiling joists/bottom chord exposed is criminal. Here's some recommendations from DOE. The article is based on heating but when A/C comes into play, IMO, the R22 listed for gas heat in warm/hot climates will jump to the R35+. And attic insulation is usually the cheapest to install.....no extra framing needed, no expensive foam boards, can be re-cycled cellulose (the best product for loose fill- higher R/inch; it restricts air leakage better than loose fiberglass)
http://www1.eere.energy.gov/consumer...nsulation.html

(4) Aren't these the same or is the writer talking about "wind" as natural air movement.

(5) With a well insulated attic, the greatest heat gain for a house will be through the east, south, west windows, or lower R value insulated/ uninsulated walls, or infiltration of heated exterior air.

[Brian A. MacNeish]

Originally Posted by bkelly2

Here is my comment Kenton.

It's friggin hot here.

And mine:

The winter ice and snow, ocean, temperate climate with no real need for A/C, low crime rates and no poisonous reptile/insects are starting to look quite good in Nova Scotia!!!

[Kenton H. Shepard, CMI]

Great feedback Brian, Thank you. Exactly what I was looking for.

Writer's me, working on this Green Building course.

1. a. Right, I wasn't sure whether to mention it. I think absorbtion rate is white-as low as 25%, black- as high as 95%.

b. Right here too. Skipped a process to show a result.

2. Even in a flat roof, that heated air would begin to cool and sink. Wouldn't that develop convection currents? We're talking about a roof with an attic here, and some kind of ventilation system which would also act to develop air currents, either convection or natural air movent.

I guess it depends on the type ventilation system.
With soffit and ridge vents, thermal boyancy would move a stream of air fromthe eaves to the ridge vent. with any kind of breeze, especially if the ridge line is perpendicular to the prevailing winds, I think low air pressure on the downwind side of a gable and high pressure against the soffit on the upwind side is responsible for pulling more air out the ridge vent than thermal boyancy. Course it depends on how hot the roof is and how hard the wind is blowing.
With gable vents, natural air movement would have more effect, especially if they were located on the upwind and downwind ends of the home.

Man, you start trying to nail this stuff down and realize it's really homesite, home design and weather-condition specific.

3. True too. If ceiling joists are covered with insulation, then it's mostly a question of how heat moves down through the insulation. It seems to me that the upper fibers would be warmed by radiation and convection from warm air. The fibers would transfer heat downward via conduction, since they're solid, and heat would be passed from air pocket to air pocket by convection, since air is a fluid. Except that if air doesn't actually circulate within those microscopic air pockets but heat spreads in them by warm air being in contact with cooler air and exciting those molecules, then it's actually conduction which is taking place.

4. I was thinking of open windows, yes... wind.

5. Right, but that's getting covered in another part of the course.

[Kenton H. Shepard, CMI]

Originally Posted by bkelly2

Here is my comment Kenton.

It's friggin hot here.

If it's 115 outside, what is it in the attic?

[rdawes]

Not sure who your intended audience is. Obviously there is no need to go to an engineering dissertation for a typical homeowner audience. Building science professionals or those in training to be such need the more technical explainations. Overall, your initial cut and the comments provided are both good.

[Kenton H. Shepard, CMI]

Originally Posted by rdawes

Not sure who your intended audience is...

Home Inspectors, Ronald. I'm trying to figure out how far to go into Building Science.
It's actually a green building course, so how heat, moisture and air move through a home are all part of indoor air quality and energy-efficiency.

[rdawes]

Kenton,

I found these pretty good:

http://www.buildingmedia.com/texas/
This can also be ordered as a CD for free and you don't have to live in TX to get it.
https://www3.cpa.state.tx.us/registe...estCD?OpenForm

http://www.bestofbuildingscience.com/
http://www.energycodes.gov/moodle/course/

[Kenton H. Shepard, CMI]

Good links, thanks.

[phinsperger]

Originally Posted by bkelly2

Here is my comment Kenton.

It's friggin hot here.

I know what you mean Brian. The other day I was almost tempted to take the second comforter off the bed.

[Keith Swift, PhD.]

Kenton, I was impressed. I always end up feeling dumb, just as I do when someone tries to teach me something about electricity, but thanks anyway.

[ldapkus]

Conduction
Conduction is the transfer of heat energy through solid matter from particle to particle. Heat is conducted through a solid material much the same way electricity is. Conduction is slowed by insulation.
Heat always moves from warm areas to cooler areas according to a physical property called the Thermal Gradient.

Convection
Convection is the transfer of heat energy through a gas or liquid by movement of currents. The heat moves with the fluid. When warm air comes out of a register, it rises. This action is called thermal buoyancy. As it cools, it falls. This movement of air is called convection current. Convection can be interrupted by a physical barrier.
Fluids carrying heat always move from areas of high pressure toward areas of low pressure according to a physical property called the Pressure Gradient.

Radiation
Radiation is electromagnetic waves which directly transport heat energy through space. Sunlight is radiated through space to our planet without the aid of fluids or solids. Through radiation, the sun transfers heat through 93 million miles of empty space!

All covered in IR Building Science class byThe Building Science Institute
www.buildingscienceinstitute.com