Roof ventilation cooling house

[Lacustral]

I've been told that getting my roof ventilated is probably the most important thing to do for passive cooling. The ventilation is pretty poor now.

The basic building code asks for 1 sq foot of NFA (net free area) of ventilation per 300 sq. feet of attic space. And 1/150 if you have insulation in there without a vapor barrier, which i do.

Does more ventilation than that cool down the attic and the rest of the house better? It's not really a lot of ventilation.

It seems like intake ventilation is easier to get than exhaust ventilation. People have suggested ventilation plans to me that give far more intake than exhaust ventilation. Do you get any extra cooling from adding intake ventilation over the amount of exhaust ventilation?

For exhaust ventilation, people have suggested ridge vents and louvers. I might need both to get enough ventilation, the ridge is very short.

Laura

[Joseph Meehan]

Yes, but maybe not all that much. If you have insulation the specified amount is OK. If you are given the opportunity to add more at a low cost then do it. It is hard to have too much.

Not all that a good design. It will work based on the lowest common denominator and in that design it would be the exhaust. What comes in (cool air) must go out. You don't want a lot more exhaust however as that may draw conditioned air from your living spaces.

Either or both is fine. I now have both as I took a new roofing job as a opportunity to upgrade.

[Edwin Pawlowski]

Heat rises. Heat is building in the attic space from the su n beating down on the roof. The heat is the house iwll not go up any more as the space above it is already very hot. In fact, some may come down.

I'm not exactly sure what you mean by intake and exhaust ventilation. No matter how it is done, the net effect of intake and exhaust will be equal. No matter how much air you try to blow in, you can only blow in what will move out. No matter how much you try to suck out, you can only draw the same amount that comes in any inlet spaces. I don't know your house, but as you already have seen, many variations exist, from ridge and soffit venting, roof turbines and powered fans.

[tbasc]

The value of ventilation depends on the local climate. I suggest a look at the Building Science Corporation web site. It contains * researched * information on a number of issues including ventilation. I research this site when designing homes. TB

[SQLit]

Passive vents are the best. No worries about them breaking. Ridge vents, roof turbines, and gable vents all help. Downside is that if you in a area that gets cold your attic will be cooler in the winter time. not good.

Fans and forced ventilators come next. These are set up usually by temperature and sometimes humidity. They make noise and always get one larger than you need. As they never really perform to the package claims. Example. If the package on the fan says that their 10" inch fan will do you home then get a 12" at least. These can be noisy. I had one over my garage that ran 12 hours a day. It was a drone when cooking dinner, and caused some issues with watching TV early in the evening. I set mine at 150 F.

What kinds of ventilation are the folks near your home doing? That is the best indicator of what you should be looking for.

[trader4]

"They make noise and always get one larger than you need. As they never really perform to the package claims. Example. If the package on the fan says that their 10" inch fan will do you home then get a 12" at least. "

Also keep in mind that with a fan it's important to have intake areas, eg soffit vents, sufficient for the size of the fan. Otherwise the fan will not only be less efficient, it can also suck some cool air out of the living space via electrical outlet gaps, recessed lights, etc

[Lacustral]

Intake ventilation being lower than exhaust ventilation - the intake are soffit vents, the exhaust ventilation is around the top of the roof. Air would usually flow from the intake to exhaust, not say from one intake vent to another. That's the concept, anyway - maybe the reality is different.

Laura

[Joseph Meehan]

...

In an area where it really gets cold (any area with snow) you want the attic area to be cold so the snow on the roof over the attic does not melt while there is still snow and ice on the edges of the room (the overhang) is colder and the snow and ice on the edges makes a dam causing leaks.

[Lacustral]

The FHA minimum for roof ventilation is 1 sq foot of net free area per 300 sq. feet of attic area -

does anyone know what's optimal for keeping the house cool in summer - not the minimum, but the optimum amount?

It seems like more than 1 in 300 is optimal from what I've heard from various contractors.

Laura

[HeyBub]

Rule of thumb: You can't have too many/much soffit vents.

Say the house is 60x50 = 3000 sq ft. You have a 220 ft perimeter. Assuming the space for a vent is 8", you COULD have as much as 165 sq ft of soffit vent. That's 1:18.

Sounds about right.

[Lacustral]

It's not intake ventilation I'm concerned about. It's easy to put in soffit vents. The exhaust ventilation is the hard part.

So how much exhaust ventilation does one need to get about the max cooling effect from ventilation? Using the 1/300 rule for intake+exhaust ventilation is apparently less than necessary. How much more than that?

Laura

HeyBub ( [e-mail]) wrote: