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Bonus Rooms Are Hot!


foreverkeeps's Avatar
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NC

06-19-06, 07:32 AM   #1  
Bonus Rooms Are Hot!

Our neighborhood is 6 years old.

Some of the houses have finished bonus rooms above the garage.

All of them I have been in have been pretty hot. Hotter than downstairs, even though there are two vents in the ceiling.

Why?

Do they not put insulation in the walls up there?

Is it because the ceiling is like a dormer room, that follows the roof line?

 
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chandler's Avatar
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06-19-06, 07:22 PM   #2  
Bonus rooms usually use the normal attic space for occupancy, and therefore you don't have much dead air space for insulation. In newer construction the roof rafters are probably only 2x6 or 2x8, allowing for maybe R19 if proper baffles are installed. Heat transfer is a bear where the sun is hitting less than a foot from the interior finish. Of course proper air conditioning, ridge and baffle venting, as well as ceiling fans will make it a little more bearable.

 
resercon's Avatar
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06-22-06, 08:43 AM   #3  
This is quite a common problem, especially with Cape Cods and houses with finished attics. Proper ventilation or if you prefer, "Adequate Free Venting" may help aleviate the temperature difference between floors but rarely will it ever make it the comfort level you desire.

To answer your question "Why?" is because the dominant heat transfer mechanism during the summer is Radiant. To illustrate this is looking at a boiling pot of water on your stove top. When you place your hand near the pot of boiling water you sense heat. If you were to move your hand closer to the pot, the more heat that you will sense. Basically this is heat radiating from the pot. There are two factors to consider here. One is how hot the object is, in this case the pot which represents the roof above this room. The other is your proximity to the hot object, in this case is your hand, which represents the room directly below the roof. This by itself does not fully answer your question "Why?", but gives you a basic understanding of Radiant Heat Transfer.

There are three heat transfer mechanism, namely Radiant, Conduction and Convection. For this explanation Convective heat transfer is illustrated with adequate free attic ventilation. As heat radiates fromthe roof into the attic, air inside the attic absorbs the heat. Warmer air than it surrounding air becomes bouyant and will escape the attic through high vents in the attic like ridge or gable vents. Since the volume of air within a confined space like an attic remains constant, the same volume of bouyant warm air that leaves the attic through the gable or ridge vents must enter the attic space. This air comes into the attic through the lower vents like your eave or soffit vents. This lower temperature air from the outside absorbs more heat from the roof and becomes bouyant and this process repeats. This illustrates adequate free ventilation.

Inadequate free attic ventilation can be described as not having an equal amount of lower vents compared to higher vents. Insulation installed directly against the roof decking, which prohibits air flow. Or insulation blocking soffit or eave vents or not even having lower vents. The amount of air leaving the attic is equal to the amount of air that can into the attic space and vice-versa. So if very little air can enter the attic, very little air can get out and vice-versa.

Since inadequate ventilation extracts less heat from the roof than an adequate ventilated roof, the roof with inadequate ventilation will be higher than the one with adequate ventilation. an adequate ventilated attic will usually have average roof temperatures between 115 degrees to 140 degrees Fahrenheit. An inadequate ventilated attic will usually average roof temperatures between 125 degrees to 170 degrees Fahrenheit. Finshed attic and Cape Cods that are insulated will exceed 170 degrees Fahrenheit. Remember one of the factors with Radiant heat transfer is how hot the object is.

This brings us to the dilema concerning your question with bonus rooms. Insulation resists heat flow by Conduction. Retention of heat is a key factor in this resistence of heat flow because it follows the rule high to low. In other words an object of lower temperature cannot give heat to an object of higher temperature, only the reverse is true. Remember the other factor concerning Radiant heat transfer, the proximity of an object to the object that is radiating heat. The insulation being so close to the roof will attempt to retain enough heat to bring its temperature to be as high or even higher than the roof temperature.

Even with adequate free ventilation, the proximity of the insulation to the roof will usually make these bonus rooms a higher temperature inside than outside.

The way to alleviate this situation is to lower the roof temperature. Shading is the most effective. However having tall trees around a fairly new home is rare and impractical to plant. Light colored roof shingles is an effective way to reduce roof temperatures but in most cases not practical. The most practical way to reduce roof temperatures is induced attic ventilation. Attic ventilating fans increase convective heat transfer, thereby reducing roofs temperatures.

 
foreverkeeps's Avatar
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06-23-06, 03:44 AM   #4  
The roof is black and no ridge vents, from what I can tell. WHen we re roof we will do that. The ceiling fan does help.

thanks!

 
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