Insulate eaves in attic? - No soffits


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Old 05-13-16, 01:32 AM
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Insulate eaves in attic? - No soffits

Insulating small and old home (1880s) home in Los Angeles. Attic area is approximately 25x25'. There has never been insulation in the attic, I am adding fiberglass R30 between the joists, and then rolling R30 perpendicular.

Question: There are no soffit vents. When I am in the attic, I can see the top plates of the exterior walls. Then for about 12-14" beyond that the eaves extend past the top plates. The roof has an extreme roof pitch, and there is easy access to the eaves. Should I lay a roll of R30 into the eaves?

My concerns were:
1. Since it is beyond the top plates - will it really do anything to keep the ceiling of the room beneath cool/warm.
2. Even though I have no soffit vents, does this affect air flow in the attic? Would I still somehow develop moldy or wet insulation over the years?
3. It looks like people today do insulate their eaves - but they use those baffles so as not to obstruct their soffits. I guess that means I can do it and it'd be easier since i have no soffits?
 
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Old 05-13-16, 03:08 AM
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The soffits need to be vented along with either gable, ridge or roof vents to get good airflow which will keep your attic cool.
The reason people use baffles is to allow air transfer from the soffit vents into the attic.
Not sure exactly what is required for your climate but a vapor barrier is normally installed on the inside surface of the insulation which would be the ceiling side for an attic.
 
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Old 05-13-16, 05:39 AM
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It sounds like you do have soffits, just no soffit vents, is that correct?

Since the top plates are very exposed it is a great opportunity to air seal that area. Being an 1880 home are those actual top plates or open cavities headed down to the basement? Many older homes were built with balloon framing. Pictures would be good.

If you have soffits and just no vents, it would be best to add vents. Then install your insulation full height over the top plate with the baffles maintaining an air gap between the insulation and the bottom of the roof.

I have more but will wait for confirmation of what we are dealing with. Below is a link about air sealing.
https://www.energystar.gov/ia/partne...ide_062507.pdf

Bud
 
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Old 05-13-16, 03:50 PM
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Correct, soffits but no soffit vents.

There is a gable half circle mouse hole on front of house and then a gable fan back (about 10' from table vent to table fan). This create a nice short distance for the fan to suck the air out. Ceiling head space is high - 15'.

Yes, they are top plates, cannot get to to cavity between the siding and lath and plaster.

Was skipping the vapor barrier part. Between the path and plaster ceiling, with drywall atop that, and 135 years of paint, I have sufficient vapor barrier.

I'll try and get photos. I want to avoid drilling holes for soffit vents - have t been there for 135 years... Don't want to cut into my redwood.
 
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Old 05-13-16, 04:14 PM
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You are in a warm climate so are not dealing with the moisture issues and ice dams that we have here in the north. I can't say I'm well versed on southern climate ventilation needs, but if the house hasn't fallen down in 135 years then what you have is probably working.

As for adding insulation into the soffit area the only benefit would be a better cover over the top plate. In other words, going past the top plate increases the insulation thickness in that zone.

Not sure from your post where the incoming air is being provided. A limited supply of inflow will limit the outflow and increase the negative pressure in the attic. Not sure if that is relevant in your case if the fan is set back from the vent.

Bud
 
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Old 05-13-16, 06:28 PM
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Thanks for the replies!

I may flip flop on soffit vent. I just feel nervous drilling into something that has been there 135 years. Dunno if suddenly more inflow would make problems. We are a very dry climate, and only get 15" of rain a year.

Concern, my soffits are flush - they look very different to more modern soffits. It looks like modern soffits dip down and cover a portion of the exterior wall of the house. My attic joists literally extend 14"-16" beyond the top plate, and then a 14-16" wide board was nailed to the underside of the attic joists. Pretty simple construction. If I drill holes and install vents... will I get a lot more dust and dirt in the attic, especially since my soffits would be relatively flush with the attic floor?

I do like the idea of increasing air flow. I will have to look at the link you provided and maybe share some images. Is there a formula for how many square feet of soffit vent I need? Attic is probably 600-700 square feet.
 
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Old 05-13-16, 06:59 PM
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If we assume your many layers of paint provide the needed vapor barrier then the guidelines are 1 ft² of net free vent area for every 300 ft² of attic floor. Lets say that's 2.5 ft² total. That then gets divided half high and half low. So 1.25 ft² id 180 in². typical for vents to only have 50% NFA (net free area), so 360 in² of soffit vents would be the target. The little round pop in vents wont do it as they are as low as 20% NFA.

Many ways to go, but a continuous strip down the middle with a series of 3" holes under it is one option. If you used 4" round pop-in vents a rough calculation says 72 total. Ouch!

The rafter extending out to form the soffit is called BCO or bottom chord overhang and is the modern day preferred way to gain dept above the top plate.

None of the suggestions I have offered have considered the aesthetics, which means you need to do some searching for something you like. Google soffit vent options.

The natural air flow is not strong, but dust will be dependent upon your area. I have seen fresh air options for inside the home that use a filter.

Bud
 
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Old 05-13-16, 07:57 PM
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Yeesh, that sounds like a lot more holes than I had expected.

I read somewhere to divide square footage by 150 and that tells you how much ventilation you need. Not sure if that meant soffit and gable ventilation. But say I have a 600 square foot ventilation, then I need about 4 square feet of vents. Was thinking maybe 4 of those long rectangular vents on each side.

Good idea, bad idea, re-check my math? Where's a good link with numbers?

Again thank you.
 
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Old 05-14-16, 04:38 AM
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They use the divide by 150 when you have a lot of leaks between house and attic with no vapor barrier. Then they use the divide by 300, which will give half of the divide by 150, when you have the good vapor barrier and good air sealing. I used the divide by 300 in my post above because you are in a hot dry climate and it will give you a smaller number to meet.

All vents will have a NFA (net free area) rating. I estimated 50%, but you can check those numbers before you buy. What the NFA means is, a 1 ft² vent with a NFA of 50% will only provide 1/2 ft² of your needed vent area.

Your math is correct but I still feel you can get away with the divide by 300 as explained here and the prior post. And the resulting number is total vent area so it should be divided by 2 for half high and half low.

If you do not have the required high vent area, then yes, you need more low vent area, but try to get the two numbers to add up to at least the divide by 300 result.

Bud
 
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Old 05-15-16, 11:31 AM
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When I do the math for my soffit vents - should it match the square footage of louvre/gable vent I have?

I guess what I'm getting at is this - should inflow match outflow.
 
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Old 05-15-16, 12:35 PM
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You try to reach the total of the calculation with inflow equaling outflow. When that can't be achieved you oversize high or low to compensate for the other vent area being too low. Technically, when necessary, it is preferred to have the low venting larger than the high.

BUT, these guidelines are for natural air flow not fan powered. I'm not clear whether your fan is blowing directly to the outside or just blowing at the gable vent. The latter would be more circulating the air than exhausting it.

But, having limited high vent capacity just means you need more low vents to try and compensate.

Bud
 
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Old 05-15-16, 08:29 PM
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Thank you for the replies - very helpful and lots to think about.

While I have your attention, any negatives to R60 insulation? From what I am reading, this is extreme.

For the Los Angeles area, it is recommended to have between R30-60 in the attic, but everyone says once you get past R40-ish, it is little gain.

Between the (1) bulk deal at the big box store, (2) a discount with the credit card, and a (3) rebate from the local gas company, I spent $350.

They were practically giving the R30 rolls away; I laid one row with the joists, and the other perpendicular.

I could have gone cheap and rolled R19 perpendicular... but it was a difference of like $50, so I went big.

I guess I am just looking for reassurance that the R60 wasn't a bad deal.
 
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Old 05-16-16, 05:00 AM
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I'll give you a quick example of costs savings for r-60 vs r-30.
Q = U x A x HDD x 24 heat loss equation from Residential Energy by Krigger and Dorsi.
Q= energy in BTUs
U = 1/R
A = Area in ft²
HDD = Hearing Degree Days
The 24 converts it from hours to days

I'm in a northern climate and rarely even look at cooling degree days, but my results will exceed yours, thus you would save less.

1,000 ft² at r-30 with 7,000 heating degree days gives me = 56 gallons of oil
If I double the r-value to r-60 that halves the results to 28 gallons
At $3 per gallon, the extra r-30 saves me $84 per year. If I were paying someone an extra $2,000 to install the extra insulation the simple payback would be 24 years, making the increase a marginal investment.

AND that is what it all boils down to. Does the additional cost of the insulation result in a reasonable payback period. In your case, it's your labor and the material costs were very low. If I ran your numbers for CA using cooling days and your energy costs, you would see something like a $20 or $30 per year savings for the added insulation. Personally, I wouldn't pay $2,000 for that as there are other places where that investment would provide better results. But since you are there and your costs are low, well worth the time and materials.

If you want to run your numbers I can do that, but those suggesting the savings go down are correct. Once you have good insulation, upgrading to great has limited savings. For the record, I have and am continuing to increase all of my r-values to well above the code "minimum" requirements.

Bud
 
 

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