Hip roof ventalation questions

[ivwshane]

I was in my attic pulling some cable this weekend and it seemed like it was 200 degrees up there!

My house is two stories and the top level has always been hot in the summer (northern California Sacramento area) and I have thought about how to make it cooler. I originally planned on getting a whole house fan but without proper attic ventilation it would be pointless, so I need to address the problem first.

My roof is an asphalt shingle hip roof (roughly 45'x31') and currently only has soffit vents (two per side).

I figured the best way to vent this would be to install dormer vents like this:


I've looked into other options (ridge vents, powered fans, turbines) and this seems to be the best option (in terms of cost, efficiency, and ease of installation).

My question is this, how do I figure out how many I need (from what I've read I need three) and how do I figure out what sides to put them on (I figured I'd put one on the back and two sides but I wasn't sure if I had to go east and west or north and south, etc).

Any comments or suggestions welcome.

[Uncle Cracker]

The only problem with passive attic ventilation is that you've got to have some breeze to really get the air moving. True, you'll get some movement through convection, but it's nothing like you'll get with a thermostatically-controlled power vent. Worth reconsidering, I think...

[ivwshane]

True but how would I mount one on the roof?

My roof sort of looks like this (2nd story):

[LCHIEN]

looks like you could ad more soffit vents simply by cutting some holes under the eves and covering them with a grille.

Then add one or two powered thermostatically controlled power vents near the top (rear so they won't show from the street) to get some positive flow thru when its hot and not move any air when its cold!

they would fit same as the dormer vents you showed, have to lift some shingles and cut a hole. And for power vents run some power.


Here's a good link with pictures:
http://www.roofroofer.com/roofinfo.html

picture of powered roof vent is about 1/4 down the page.

[Uncle Cracker]

^^^ Yup... What Loring said... ^^^

[woodturner]

Then add one or two powered thermostatically controlled power vents near the top (rear so they won't show from the street) to get some positive flow thru when its hot and not move any air when its cold!

I'd want to do a good bit of research prior to installing powered vents. The studies I have seen recommended against them and concluded the electricity cost outweighed any possible ventilation benefit. However, I don't know the specific roof configuration, etc. they tested - so I would want to make sure that research was applicable to my situation.

[Uncle Cracker]

I'd want to do a good bit of research prior to installing powered vents. The studies I have seen recommended against them and concluded the electricity cost outweighed any possible ventilation benefit.

Power vents made a big difference in one of my previous houses, although mine was a louver-type installed in a gable end. My A/C didn't run nearly as much as it had before, and my power bill was cut by 20% or so. It paid for itself in a single summer.

[parnelli]

I'd want to do a good bit of research prior to installing powered vents. The studies I have seen recommended against them and concluded the electricity cost outweighed any possible ventilation benefit. However, I don't know the specific roof configuration, etc. they tested - so I would want to make sure that research was applicable to my situation.

The biggest thing I've heard- outside of electricity usage- is the not having a air tight attic sealed from the conditioned part of the house.

Translation: The powered fan sucks conditioned air from ceiling electrical boxes, chimney gap, plumbing stack, electrical runs etc.

I would think that if you were to make sure to seal all that well, as well as having adequate soffit ventilation you'd be good.

As a bonus- from what I researched over the winter, sealing those air leaks in the attic stops a chimney effect of air from the basement flowing up to the attic and is a very good (and cheap) way to reduce heating and cooling costs.

[Mr__Bill]

A large area of soffit vents and either passive venting near the peek or something like this from Costco should help.

Note though this one is solar and will not be much help in the night when cooler air is available.

For the soffit they make continuous AL and vinyl vents that you just cut a slot in the soffit and install. They provide screening and lot of area for venting. The continuous run often looks better than a lot of vent plugs or screened openings. You may wan to put the upper vents on the side of the roof that is less visible.

Remember hot air at the top cooler air at the bottom so the soffit vents lets in the cooler air and pushes the hot air out the top. If the venting is only at the bottom there is very little air movement.

Installing a whole house fan that discharges into a Hip Roof Attic is really not a good idea. What happens is all the dirt from the attic is forced back into the house through any and all openings from the house to the attic. What happens is you pressurize the attic because the fan blowing into the attic needs a large opening to then vent to the outside. If you decide that a fan really is what is needed, adding a dormer with the fan in what would be the window and then louvered openings to the attic from the second floor ceiling will make for good ventilation for the whole house for those times when the outside temp is lower than the inside temp.

Personally I would install the soffit vents and passive vents near the top of the roof and then lots of insulation on the attic floor. This has the advantage of having nothing to break, wearout or make noise.

Bill
on the Sunny but cool Oregon Coast

[woodturner]

The biggest thing I've heard- outside of electricity usage- is the not having a air tight attic sealed from the conditioned part of the house.

Translation: The powered fan sucks conditioned air from ceiling electrical boxes, chimney gap, plumbing stack, electrical runs etc.

I think that's a lot of it. My suspicion is that it depends a lot on the particular application and installation.

[woodturner]

Power vents made a big difference in one of my previous houses, although mine was a louver-type installed in a gable end. My A/C didn't run nearly as much as it had before, and my power bill was cut by 20% or so.

One of the challenges in verifying energy savings is removing all the other variables. Many variables, such as the weather, cannot be controlled. Thus, it is very difficult to determine whether a change is saving energy or not.

I installed a thermostatically controlled gable fan in a prior house. I carefully monitored electricity usage, ambient temperature, and weather for several years prior and after installation of the fan. Bottom line, the fan provided no measurable benefit and increase (a little) electricity consumption when weather changes were factored in. Without this careful monitoring and testing, though, I would have thought that it was saving me money.

I think the specifics of the installation make a huge difference. If one has an under-ventilated attic, the fans probably help more. If one has a well ventilated attic where convection keeps the air moving in the attic, the fans probably provide little benefit.

[chopnhack]

Most roof's are installed without any regard to soffit open area. Unless of course they are industry trained roofers....and honestly there are not a whole lot of those doing residential that I know of. So they slap shingle on run down the ladder to collect pay and in a few years the shingles are curling from heat damage. There are some calculators online that tell you how much sq ft of open soffit you need for your attic. Check out GAF's website, I believe I last saw it there. If you have inadequate open soffit area, you will draw air from the conditioned spaces by adding a powered vent fan. If you have enough soffit area then sometimes you get lucky and have great passive convection occur.

[HarmsWay]

Our house and outbuildings all have hip roofs. I think most in the neighborhood do. Our building code specifies how much ventilation is required. I don't recall what the number is, but for a 12x14 building one roof vent was not enough. Typically here, the soffits are continuous vents (perforated aluminum now, previously slots in plywood) so the amount of ventilation is determined purely by the roof vents. I would think more is always going to be better in terms of the soffit vent area.

Bob

[pelligrini]

This post has an excerpt from the 2003 IRC (International Residential Code) about attic ventilation.

http://www.bt3central.com/showpost.p...49&postcount=9