Enough Ventilation??

[dfogleman]

What I understand you saying reqards the flow from the living space too the attic...the ventilation of the attic is the flow of air into and out of the attic space from the out side which carries away heat and moisture from the attic space.

From what you are saying if a house with less attic insulation allows more air to travel from the living space to the attic then would it not stand to reason that a greater air flow would carry more moisture and heat, thus in this situation a greater rate of attic ventilation would be necessary rather than a lesser amount.

[David P. Valley]

Originally Posted by dfogleman

What I understand you saying reqards the flow from the living space too the attic...the ventilation of the attic is the flow of air into and out of the attic space from the out side which carries away heat and moisture from the attic space.

From what you are saying if a house with less attic insulation allows more air to travel from the living space to the attic then would it not stand to reason that a greater air flow would carry more moisture and heat, thus in this situation a greater rate of attic ventilation would be necessary rather than a lesser amount.

True.

I never stated that the attic would need less ventilation in any situation.

[Harold E. Miller]

Originally Posted by dfogleman

I must take issue with the idea that 'older homes didn't need as much ventilation'...please explain your thoughts to me, I would like to better understand your rationale for this statement.

The function of ventilating the attic serves 2 purposes, removal of moisture that has escaped from the building envelope into the attic and cooling of the roof deck and the attic space from heat that has built up from the relentless rays of the sun.

The insulation in the attic has no bearing on the attic ventilation per se, it does effect ice damming (by keeping the heat inside the living space), it does effect the transfer of heat from the attic to the living space. Improperly installed attic insulation can block the flow of air in, through and out of the attic.

Ventilation of the attic for removal of heated air in the space and cooling of the underside of the decking is an effect produced by the movement of air. The life span of a shingle roof can be greatly reduced by overheating brought on by improper attic ventilation and this can happen on a roof that is installed on a building that does not have any insulation at all, for example a roof installed over a garage.

As I state that I take issue with the statement I will stay open to the reasoning that may be stated to the contrary.

Don
Consider that many homes built before insulation was installed in attics (and walls) also did not require ventilation, or at best it was limited.

The heat from the coal, wood, or oil heat sources also heated the attic space. This is a crude warm roof system, but effectively it increased the temperature of the roof system above the dewpoint, and you did not need all the soffit, gable and roof vents we have today.

On the other side of the coin is todays home with R-38 and more in certain areas. This high amount of thermal resistance keeps the roof system cold, and with out air circulation, condensation occurs.

The original home for this thread probably only had R-11 in the ceilings, or a close approximate. Therefore more heat is transferred to the attic requiring less ventilation, to perform the same job of a modern home with R-38. How warm the underside of the roof sheathing is maintained is a key element to condensation accumulation in the attic.

Not saying less ventilation is acceptable. Just trying to explain that there has been a distinct correlation between the required addition of more ventilation, when there has been an increase in required ceiling insulation.

Attic evolution.

[Harold E. Miller]

http://www.jlconline.com/cgi-local/v...177f0000010565

BTW-little different topic than above posts...but informational..
Here is a nice article on unvented attic systems from Journal of Light Construction.