re: Updating insulation in existing house attic

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I don't know why. The insulation is really there to keep air from moving, as air is an awesome insulator. I don't understand why you'd want to move a bunch of hot air around the attic.

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The insulation keeps the air RIGHT NEXT to the space you want to keep conditioned from moving. At those temperatures, convection is the most efficient way to move heat from one thing to another, so it reasons that if you use the insulation to reduce the convection, you reduce the amount of heat that moves into or out of the space you're trying to keep conditioned.

However, there is still conduction through the insulation and convection from the surface of the insulation where it is exposed to attic air. During the summer, the roof of a house is heated tremendously by radiation from the sun (YAY, we've touched on all three modes of heat transfer!). The roof gets stupid hot, way above ambient, and some of that heat gets transferred to the attic air sitting right under it. The hotter the attic air is, the more of a temperature difference there is between the space you're trying to insulate and the hot space above it. Temperature difference in heat transfer is like pressure in piping. The more you have, the more "stuff" you're going to move. So, even with insulation, the hotter the attic, the less effective that insulation will be because the hotter the attic gets, the more "push" there is to drive heat across the insulation and into the conditioned space. The idea behind the vents and whirlybirds is not to move the air around the attic, but to bring "cool" 100 degree air in during the summer and to get rid of the 150 degree air that would otherwise be in the attic. The slight increase in heat transfer due to convection caused by the fans is absolutely swamped by the reduction in heat transfer due to the lower attic air temperatures.

TL;DR: Attic ventilation isn't to circulate air around the attic, it's to bring cooler air into the attic during the summer.

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Recently bought a house that has fiberglass batting in the attic. Electricity bill is ridiculous. Looking to update. Recently got an estimate to blow foam in the rafters, closing off eave vents and whirly birds for $6k.

Which method of insulation in an existing house is most cost effective?

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If you have any gas appliances (water heater, furnace, etc) in the attic, you cannot seal it. My recommendation to you would be to find someone to put about R-30 worth of loose fill on the ceiling.

Also, there are some issues with foam insulation under the roof deck. Mainly, if your roof leaks, the insulation will keep the roof decking moist, leading to rot in many instances.

Some good info on roof venting.

LINK

You need ventilation in your attic, I wouldn't close off vents and whirlybirds. Rent the machine from Home Depot and blow it yourself (or get your ole lady to do it)

Foam!

I had the same situation you have about a year ago. Spent $650 on R-30 fiberglass batting from Home Depot and laid it over the existing shitty insulation myself. Each month since then, my energy bill has gone down at least 17% with a high of 26%. I'm very glad I did it.

Also, I've read some absolute horror stories on foam insulation. Does it work? Absolutely! Is it risky? Absolutely! The common issues with foam insulation is the fumes it emits into the home (this is why it's crucial that a certified foam insulation contractor does the work and even then it's still risky). Put too little, it doesn't work. Put too much and you have major chemical issues inside the home. It's a very fine line that the contractor has to walk.

Blown in insulation is very effective as well, but you have to rent the machine to do the work for you and you need at least two people for the job.

I ultimately went with the R30 bats from Home Depot since I was able to carry each one up into the attic myself. I took a long wooden stick, put a nail at one end of it, and I used that to move and place the bats from a distance. I couldn't have done it without that. Be careful not to tear the insulation with the nail, though. If you're in a hurry, you will cut corners and not be very effective.

Ventilation, ventilation, ventilation! This is extremely important in the attic. Ensure you leave an airway at each space between the roof rafters to ensure airflow remains active from the eaves of your roof to the top of the roof, out the vents. No ventilation means rotten wood and ceilings.

ETA:
The attic foil insulation stuff will work as well, according to most of what I've read on that subject. There are those who disagree with it and those who swear by it. I wanted to go that route BUT I read a lot about bad cell service with it and since I live in an area with already bad cell service, I didn't want to risk it. Sure, you still get service through the walls and windows of your home but lots of it goes through the roof as well and I just didn't want to risk it.

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If you lay it yourself, if your present insulation is between the joists run this layer perpendicular to the first layer.

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Yes, but in my case the existing insulation was so worn down that I had to lay it on top. The existing insulation was almost 40 years old so it had settled quite a bit.

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Yes, but in my case the existing insulation was so worn down that I had to lay it on top. The existing insulation was almost 40 years old so it had settled quite a bit.

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Good point. If the OP is in the same situation, I bet you could get the old sucked out and new blown in for way less than foam.

I still say you can fill your entire attic up with insulation, but if your doors, windows and exterior walls are the main problem you are wasting money.

Try to get someone with a FLIR camera to check out your house. The thermal envelope on the building perimeter may be a bigger issue than the ceiling as far as the insulation issue is concerned. The small leaks and hot spots can sometimes add up to the equivalent of leaving a window or door open.

I swear Entergy used to do this type of energy audit for free????

I blew wool fiber over my existing batt insulation... Looked like Santa's wonderland up there but has worked great.

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closing off eave vents and whirly birds

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closing off eve vents is 100% guaranteed to start mold growth and trap moisture and heat in your attic, that is where 100% of the cool air comes in from to keep your attic cool

OMG i cant even, this is so stupid, whoever told you they wanted to do that learned about ventilation from a comic book ad

if you dont have ridge vents or gable vents then adding whirly birds helps move the heat out, just position two of them equally so they are dividing the roof into 3 equal sections and place them within 4ft of the peak

as someone already mentioned, just buy "unfaced" "unbagged" bat rolls of insulation and lay them accross the rafters so the top layer is not laid in the same direction as the bottom so it reduces gaps. you want unfaced / unbagged insulation so it breathes and lets moisture escape to stay dry.

a big problem with older houses is the walls and not the attics, many houses have little to no insulation in the walls, especially if they are older houses like 40-50 years old

whatever you decide to do, old insulation is still insulation so its just stupid to take it out and buy more to put it right back, just add to whatever you already have.

foam is good for walls but not ceilings, you need a way for moisture to escape so if its all foam then you have to find a way to solve a moisture problem you just created by doing foam ceilings. foam walls and bat ceilings is the smart way to do it