[IamMark]

Hello,


I'm located in the SF Bay Area and have a two story home. Our home is about 22 years old with the original roof (asphalt shingles). We are looking at installing solar, so given the age and condition of the roof, we're going to replace the roof first.

This has opened a can of worms regarding attic ventilation. Our house does not have any eaves or soffit vents. The home builder installed one westward gable vent on the first floor and westward and eastward facing gable vents on the second floor. The only other ventilation are two large eyebrow vents facing south that I believe are acting as intake. We currently have a powered gable fan on the 2nd story east facing gable vent.

Our attic is stifling hot during the summer. I'm not sure if this is normal or if it is because of inadequate ventilation. At a minimum, we don't have problems with moisture or mold, so the ventilation seems to be doing something.

Several roofers have different ideas on how to improve the attic ventilation. A ridge vent is not possible due to our house using steel frame construction - all roofers advised not to do this. The suggestions so far are as follows:

1) Install a gable vent fan on the second story west facing gable vent. Remove the eyebrow vents and install 2 O'Hagin vents on the first floor

2) Install O'Hagin vents (13) both high and low spread out between the first and second floors. This was based on a design consultation with an O'Hagin rep (see design below). Remove the eyebrow vents and close off all gable vents.

.

3) Install DCI SmartVents along the gutter line and ridge line. Remove the eyebrow vents and close off all gable vents.

Based on my research, option 1 does not seem to be the best idea. Between the other two, I'm not really sure what to choose. I'm hoping that you can provide advise on what may be the best approach. I've attached a Google Earth picture of our house.

Thank you!

 

[Bud9051]

Hi Mark

"stifling hot during the summer" is pretty normal. Protection from that heat comes from the insulation. Ventilation will struggle to cool when the incoming air is hot as well.

Eyebrow vents and existing gable vents can probably stay. Defining vents as intake or exhaust is determined by their high or low position in relation to other vents.

First, select a shingle with a high reflectivity, assuming some will still be exposed. Attic heat comes from solar energy.

Complex roof so hard to design ventilation from long distance. Side view pictures might help with elevations. Are the lower attic and upper attic isolated from each other?

Bud

 

[Nealtw]

steel frame construction


Can you post a picture of that.

 

[IamMark]

Hi Bud,

Here's a picture of our house from the front. Its about the same perspective as the O'Hagin design picture.

I'm going with CertainTeed's Landmark Pro Solaris product, which is supposed to be solar reflective.

I'm hesitant to actually change the ventilation at all. I guess in my head I'm thinking that the home builders knew what they were doing, so the ventilation should be sufficient. At a minimum I would change out the eyebrow vents to O'Hagin low profile vents, so solar panels can be installed over them. At the same time, I'm wondering if a different ventilation design will cause a noticeable difference in attic temps in the summer, which will hopefully lead to a cooler second story and less A/C use.

Thanks.

 

[IamMark]

steel frame construction


Can you post a picture of that.

Hi,

Here you go:

 

[Nealtw]

Hi,

Here you go:


View attachment 602515 View attachment 602517

Thank you, I have heard tell but never seen one.

 

[Bud9051]

Still hard to get specific but I can outline the principles of passive attic ventilation.

There is a pressure difference created between inside and outside based upon the height difference between low and high vents and the difference between inside and outside temperature. Whatever the resulting total pressure it is self adjusting between high and low vents so intake will equal exhaust and the total air flow is than determined by the NFA (net free area) of all vents.

The wild card is any powered vent, always much stronger than passive venting, but needs a lot of vent area so it does not pull conditioned air from the house. Also some serious air sealing between house and attics is always beneficial.

It doesn't look like the high attic is common with the low attic so each needs to be calculated separately.

I don't see the eyebrow vents so not sure why they need to be removed.

Bud

 

[Windows on Wash]

Very cool framing.


Appears that you are lacking on soffits. I would look into one of the eave venting products to work in conjunction with your ridge vent. She will move a bunch of air at that point. I would still look into some air sealing and insulation too.

 

[Bud9051]

Can't tell from the picture, but if the ceiling joists are also metal like we see for rafters and bracing, then that could be a major source of heat. I can't judge how much ceiling insulation is up there but I have seen data on metal studs in walls and how it drastically compromises the insulation.

Even with a foot of insulation the braces would be conducting heat down to the ceiling joist.

How much insulation?
Are the ceiling joists metal and how deep?

Bud

 

[Nealtw]

Can't tell from the picture, but if the ceiling joists are also metal like we see for rafters and bracing, then that could be a major source of heat. I can't judge how much ceiling insulation is up there but I have seen data on metal studs in walls and how it drastically compromises the insulation.

Even with a foot of insulation the braces would be conducting heat down to the ceiling joist.

How much insulation?
Are the ceiling joists metal and how deep?

Bud

Unless they built in a heal cut you would not get much insulation over the wall, never mind venting. :surprise:

 

[Bud9051]

Did some digging and a 2x5 metal stud wall loses 63% of its r-value due to the metal and that involves studs covered with drywall and sheathing. Here those braces act as thermal antenna connecting the ceiling joists directly to the hot air above.

Would be interesting to see if they attempted any thermal isolation for that meta. My search found no examples.

Mark, is there a brand on that attic framing?

Bud

 

[IamMark]

Here's a picture of the eyebrow vents that are in the back of the house facing south.

Everything is metal in the house, including the ceiling joists and wall studs. I don't see a brand on the attic framing.

The design that I included in my original post was done by O'Hagin themselves and suggested blocking off the gable vents and removing the eyebrow vents. They said that leaving them there would short-circuit their design and ventilation. The roofer who suggested the SmartVents also said the gable vents and eyebrow vents should be removed for the same reason.

As a reminder, due to the metal framing, a ridge vent isn't possible.

Thanks.

 

[Bud9051]

The "short circuit theory" was introduced decades ago and because it sounds so logical no one questioned it, until I came along, pat on the back. But convincing the establishment that it was ill-founded and worse has led to incorrect attic ventilation assumptions has proven a serious challenge. To provide a full explanation will take a book (I'm working on that) but I can give you an example that is part of the full explanation.

The SCT says that air will follow the path of least resistance thus taking the closer vent and not the intended vent. Here they are using a common phrase but applying it wrong. Air will follow the path of least resistance IF the those paths have the same pressure across them and in attic ventilation that pressure varies with elevation. Understanding the air pressure in an attic is a fundamental part of determining ventilation. There are 4 parts to the SCT and all 4 are wrong.

Your particular arrangement is even more complicated as it is not the traditional soffit to ridge vent combination so their blanket statement is even more inappropriate. But you would struggle to find anyone explaining it correctly even though some have changed their thinking, but they are few.

I think Neal mentioned "under shingle vents" and since you are installing a new roof that would be an easy option, both high and low. Snow is not a concern where you are o edge vents might also be a problem.

If the eyebrow vents are in the way for the solar, yes they should be removed. Do the high to low vent area calculations to ensure you have enough NFA (net free area).

Bud

 

[IamMark]

Let's assume I changed out our current ventilation for something more efficient. My ONLY concern is heat and specifically reducing inside temps on hot days. Realistically, how much of a difference in inside temps can be expected by improving your attic ventilation? On 90-100 degree days, will having better attic ventilation really allow for lower attic temps? Even if airflow is improved, you're still taking in hot air.

 

[Bud9051]

IMO, the only way to make the ventilation effective would be with an exhaust fan up there and taking advantage of cooler evening air. 140° attics are common and it is the insulation and air sealing that protects the house. But attic fans without air sealing between house and attic can result in cooling your attic with conditioned air from your home. This gets worse with limited attic intake.

We ventilate attics here in the north to eliminate a real problem, condensation and ice dams. In warmer climates those are rarely issues thus the typical NFA of ventilation really doesn't apply. IMO, if you have some ventilation it is probably enough. Adding more would produce little discernible difference.

Bud

 

[carpdad]

"Adding more would produce little discernible difference."


I think this is the best explanation. Attic is ALWAYS going to be hot unless the roof itself is insulated and the attic made into a "conditioned" space which would be such as making the attic into a living space. Yours is just an attic so why not measure the temps and forget it feels too hot?
I'm in nj. My attic was not insulated enough by previous owners. Below, I could feel radiated heat through the drywall. I changed the old attic fan and that problem was solved. I think I set the thermometer on the fan to the lowest temp. Sure, it would run more and use more energy but i save on not using the ac as much.
I really like those eyebrow vents. If my choice, I'd have couple more.:smile: Make sure the screens on those vents are clear. Just blowing them off from outside with a leaf blower may be enough. But make sure, from inside, the screens are secure and won't easily detach.

 

[IamMark]

Thanks everyone who responded.

Bud - I've modified the design given to me by O'Hagin to show the existing ventilation:

Their design shows 13 of their low profile vents (it assumes no other vents are installed). I've added the location of the existing two eyebrow vents (red) as well as the gable vents (purple). The gable vent on the very far right is located on the downstairs dining/living room (Area 3 in the diagram). There is no other room above it. I guess it may provide airflow for in between the floors, but I really don't know. The other two gable vents are on the upstairs section of the house.

The section of the house that I outlined in blue has absolutely no ventilation whatsoever (intake or exhaust). If we keep the existing gable vents and eyebrow vents, does it make sense to add attic ventilation to the area in blue? Or will that not provide much benefit as we've discussed?

Thanks.

 

[IamMark]

Any comments on my questions from my post above?

 

[Nealtw]

Thanks everyone who responded.

Bud - I've modified the design given to me by O'Hagin to show the existing ventilation:

View attachment 603231

Their design shows 13 of their low profile vents (it assumes no other vents are installed). I've added the location of the existing two eyebrow vents (red) as well as the gable vents (purple). The gable vent on the very far right is located on the downstairs dining/living room (Area 3 in the diagram). There is no other room above it. I guess it may provide airflow for in between the floors, but I really don't know. The other two gable vents are on the upstairs section of the house.

The section of the house that I outlined in blue has absolutely no ventilation whatsoever (intake or exhaust). If we keep the existing gable vents and eyebrow vents, does it make sense to add attic ventilation to the area in blue? Or will that not provide much benefit as we've discussed?

Thanks.

What are the square boxes that look like low and high box vents?

 

[Bud9051]

Here is some on topic reading: https://www.energyvanguard.com/blog/the-best-way-to-cool-your-attic

The O'Hagin vents shown should help ventilate, but any passive ventilation will struggle to lower the attic temps. Read the link above as it includes passive temperature results from hot climates.

As I recall you intend to add solar panels, how do they fit with this venting approach?

Bud