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attics and vents


zeb
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I have a cape cod house coming up for inspection. My views of the house at realtor.com shows rooms upstairs and it appears to not have any ridge vents. It appears that room trusses were used to create living space upstairs. My question is this. Does there need to be attics vents the same as you would see in a typical gable trussed house. also I can see that there is no attic entrance mainly because there really is no attic.

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I am in the process of building a house that utilizes what would have been the attic for living space.

I am installing an in line exhaust fan to the outside with venting from each room and hallway. The fan will operate with a thermostat.

My reason for this exhaust fan is to remove the warmer air that will rise into the space from the lower levels. This is for cooling efficiency. (summer)

Prior to insulating between the rafters, I installed raftermates between the insulation and the roof sheathing. I also installed ridge and eave vents in conjunction with the raftermates.

My reason for the raftermates and vents is for air exchange the same as you would desire in an attic to allow the roof to breath. I felt since the space was warm on one side and cold on the other, moisture would definately be a problem and this would be an important feature.

I am also considering installing an additional fan or some type of a switching box to the first fan to "recycle" the heated air that has risen back down to the first floor. (winter)

I have used this technique in warehouses with high ceilings by suspending tubes with exhast fans on them to bring the warmer air from up high to down low where the men were working.

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Steven, you did not ask for input on this, but it shoulds like you are mixing your systems and headed for trouble. I would recommend going to http://www.buildingscience.com/default.htm and look at various construction methods and attic ventilation. In general one should never make a penetration in the air barrier (sheet rock) so that interior air mixes with attic air if you are cooling or heating the interior space. Warehouses with no seperate attic space are different. Also, think of what is being drawn into the house when placing the attic and house under negative pressure from an exhaust fan, radon, unconditioned air, insecticides from crawl spaces, etc. only need a slight pressure differential to move to the interior living space. A general understanding of the physics of air and moisture movement in the structure is needed for any building design, especially when departing from standard systems. I am not meaning to bust your chops on this, just hoping to help.

Jim

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Hi Jim,

I didn't think that I was offering input that was not on subject, since the venting issues regarding an area similar to attic space (cold) was being used as living space (warm) was being questioned. These are issues that came to mind when I encountered a similar situation.

As far as mixing my venting systems, I'm not. The ridge vent/raftermate/eave vent system, which is for the purpose of air circulation within the sealed area of the roof is totally independant and separate from any living space area and any exhausting. I feel that this is just as necessary and for the same reasons as venting a traditional cold attic.

Since the interrior space below the rafters/sheetrock ceiling is heated and the exterrior above the rafters/roof is cold, don't you think that condensation within that space is an issue and has to be addressed?

You may be right regarding the possibility of creating a negative pressure situation by exhausting the warmer air entrapped in the upper levels of the living space during warm seasons using an exhaust fan (for cooling efficiency of that level). But if so, wouldn't an exhaust fan in a kitchen create the same phenomena? Also bear in mind that I have make up air venting in the mechanical room (which is not air tight) and when the upper exhaust fan would opperate (summer) the heating system for the house would not.

Regardless, I appreciate bringing the issue to mind, I will consider it and perhaps I will eliminate the fan and simply vent the area by natural convection or allow for additional make up air.

The third item I mentioned, regarding recycling the heated air from the upper level to the lower level would not be a problem either since all I would be doing is circulating the air.

Finally, at no time did I think that you were busting my chops in your reply. I appreciate any constructive critisism that is offered in good faith and respectably. Especially when it causes me to think and consider new possible angles or flaws. We are all friends here and I have no problem with my ego and appreciate all of the knowledge that flows on this site.

"A smart man learns from his mistakes, a wise man learns from other peoples mistakes"

Sincerely,

Steve

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Steven, it seems we are more or less on the same page. I may have misinterpreted your original post. I read it as using the "in-line exhaust fan" as somehow venting the attic and bedrooms. My bad.

If you are using traditional A/C though I would still advise against ventilating the hot air to the outdoors since the makeup air would have to be drawn from somewhere which is usually going to be hotter and more humid than the interior air which is exhausted.

I am not familiar with your climate and cooling needs, but in our (Texas) cooling climate it is best not to try to mix ventilation and cooling to accomplish cooling. Now ventilation for fresh air, etc. is needed but at much lower rates than what would be needed to accomplish cooling of the upstairs as you mentioned.

If you are using just ventilation instead of A/C for cooling, that is what we in the south used years ago (before A/C was common) and referred to as attic fans or whole house fans. It is very effective at times, but has severe limitations in extremley hot humid climates.

On your plan to recycle the hot air to the lower level, might I suggest designing your heating/cooling ductwork with appropriately placed vents and returns and using one of the new high effeciency variable speed blowers. I have one which runs at about 10% normal speed and uses only 70 watts. I run it continuously to keep the house at uniform temperatures summer and winter.

Sounds like your roof vent system is right on track.

I hope this helps,

Jim

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