Our 100 year old Craftsman house in Los Angeles is completely uninsulated, and the 2nd story gets awful hot in the summer. The asphalt shingles hit 177 degrees, and a lot of that heat makes it into the house. There are large gable vents but no soffit vents.

The roof is way old and wiggly, and we want to install a PV solar system, so we're reroofing.

To make the house more livable, we're:

• using energy star shingles
• air sealing the attic floor
• insulating the attic
• possibly improve venting in attic
• weatherstripping the windows
• replacing the 2nd story furnace with a heat pump

Advice has varied about how to insulate the attic. Most people say blow in cellulose; our Old House Guy suggests the wind from the gable vents would blow that around too much, and prefers batts; our energy auditor says we should put in Wind Baffles to prevent that.

http://apps1.eere.energy.gov/buildi...-homes.pdf suggests that batts are not a crazy idea, and even suggests installing them kraft paper side towards the installer so it can be carefully sealed to the structure (to solve the wind-washing problem); and because code requires a flame barrier, laying another layer of unfaced batt on top of that! Sounds touchy, but I'll think about it and see if I could do it well.

Our OHG also suggested solar powered roof vents, but every building science guy I know says NO; and make me think about adding soffit vents instead (which might be practical because the reroofing will involve restoring raftertails, so there will be scaffolding up, making it easier for the roof guys to drill vent holes in the soffits).

(I was worried about mixing vent styles, but http://www.greenbuildingadvisor.com...ting-roofs suggests it's ok to mix soffit vents with gable vents.)

Any opinions about how to get the best bang for the buck here?

Since you are re-roofing, is a vent along the roof peak out of the question? Those usually go quite well with soffit vents as they promote flow up along the sheathing which helps lower it's temp and lower overall radiation into the rest of the attic space.. If the peak vent area is great enough(they usually are), you could probably even block off most of the gable vents thus solving the blown insulation issues they might cause... Then it wouldn't really matter what type insul you choose. Blown seals a little better IMO, but it tends to settle more over time

Sure, a ridge vent is probably an option. But I hesitate to mix gable vents with any other high-up vents, so many people warn about problems that can cause.

Our OHG reiterated today that adding soffit vents would be a waste, and that our giant gable vents were perfectly good at letting cool air in, but not good enough at letting hot out. I can't quite follow.

Posted By Dan Kegel on 20 May 2015 06:11 PM
...Advice has varied about how to insulate the attic. Most people say blow in cellulose; our Old House Guy suggests the wind from the gable vents would blow that around too much, and prefers batts; our energy auditor says we should put in Wind Baffles to prevent that...

Google [stabilized cellulose] for another option to consider.

thanks for the tip about stabilized cellulose!

I may be using the wrong word here. Soffits are horizontal, I think. We have no horizontal anything at the eaves. Instead, there's a vertical board about six inches high at the wall of the attic where the roof almost meets the aatic floor, and that same board is visible on the outside of the house. I bet I could just drill a hole in it and cover it with wire mesh.

I wonder if I could do an experiement to measure the effect of adding a single vent.
Maybe measuring whether it draws air in, or measuring whether the temperature of the underside of the roof is lower there than on adjacent cells without the vent...

Attic venting is for purging moisture, and is not a cooling strategy (a common misconception.) If there are no moisture issues apparent in the attic now, leave the venting as-is. A 100 year trial period is a long enough test periods, eh? :-) The amount of power used to power-vent the attic typically exceeds the amount of air conditioning power use it might save, and passive venting with soffit to ridge venting won't change the attic temp much- not enough to matter from a comfort point of view, as long as you have insulated the attic floor to code min. (This has been shown in all sorts of careful in-situ testing on houses over the past 30-40 years.) If you want to dig into it a bit, start here: http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf

If you're going to ignore that advice and go ahead with soffit to ridge venting, never mix gable vents with soffit / ridge venting, since gable vents short-circuits the flow, limiting the flow through the soffits. (Even though bungalows don't have true soffits, venting at the boards between the rafters is still referred to as "soffit venting".) The total cross sectional area of the soffit venting should be at least 50% more than the ridge venting to avoid depressurizing the attic relative to the condtitioned space ceiling. If the ridge vents are bigger than the soffit vents (or even the same size) the stack-effect of the height difference to the ridge will pull air from the conditioned space into the attic through any air leak it can find at a much higher rate than if the soffit vents are oversized relative to the ridge.

A self-contained solar attic vent would deliver a very modest reduction in cooling power use, but had nowhere near the comfort improvement you'll see when you put R38 or more of cellulose on the attic floor. Depressurizing the attic with ANY mechanical venting scheme carries some risk, since it pulls conditioned space air into the attic when operating (even more so than an improper soffit to ridge approach.) In an L.A. climate a bit of induced air leakage would not be a huge risk, but when looking at it from a total bang/buck point of view there is no good case for it, on either the comfort or energy use end.

With PV shading the south facing pitches your roof temps would be much lower even without the cool-roof shingles, but I doubt the Title 24 code enforcers would accept that on it's own. Cool roof shingles certainly won't hurt though, and the combination will knock at least 10F off the peak attic temps, maybe even 25F, depending on how much roof is shaded by the PV, and the SRI of the shingles.

+1 on stablized cellulose approach if insulated at the attic floor (after air-sealing, and verifying the air tightness by pressurizing/depressurizing the house with a blower door or large window fans looking for leaks.) Even standard cellulose won't blow around much if you use a gardening sprayer to wet the top surface near any vents, but the moisture activated adhesives of stablized forumla are even more assured. Also, stablized cellulose does not use sulfated fire retardents, whereas a lot of dry-blown goods do. Sulfate fire retardents are corrosive to metals if they ever get wet, and are to be avoided if possible. Specify "borate only, sulfate free" for any requests for bids. The material cost difference is miniscule compared to the installed price.

Blown insulation fills all voids, and has no gaps. As long as it's raked smooth and hits the prescribed depth everywhere, with no dips or compressions it's performance is VERY good. By contrast, it's nearly impossible to install (or inspect) multiply layered batts, and fiberglass is translucent to infra-red radiation, which makes the insulation an inch or so in from the top hotter than the attic air temp(!), which means you're insulating against a higher temp with less total insulation. If you MUST go with a batt solution, use rock wool (which is opaque to infra-red), or high density "cathedral ceiling" fiberglass batts, either of which are sufficiently air-retardent to perform pretty well from a wind-washing or convection point of view, even without a top side air-barrier, eliminating the whole fire-barrier issue as well.

If you have ducts and air handlers in the attic you may be better off insulating at the roof deck (despite the expense), and the time to decide that is BEFORE you re-roof, since it's far cheaper to put the requisite amount of exterior insulation (R5, for Los Angeles, US climate zone 3) that would allow an all-fiber solution between the rafters, eliminating the need for a more expensive spray foam solution on the underside of the roof deck.

wow, thanks for all the tips!

The heat pump and ducts will alas be in the attic. Worse, latest hvac guy wants to raise ducts up a bit rather than bury them, saying "Oh, they come with R-8 built in these days".

R5 in the roof deck, eh? Is that 3/4" polyiso? And then also insulate the attic floor?