Attic Insulation for an Existing House
GadBuilding
| Posted in Energy, Heating & Insulation on
I live in a standard, early 1970s house in Southern California.
I have access to the attic, which has old, pink, thin insulation between the joists.
I want to add insulation to the attic, as the 2nd floor rooms get very hot in the summer.
1. Should I remove the old insulation first?
2. Should I lay the new batts between the joists, or parralel to them?
3. Do I need a vapor barrier? Is so, should I be on top of the news battas, or on the bottom?
In my area we get very little rain, and the temperature rarely drops below freezing.
Thanks,
Gary in So Cal
Replies
Most of the heat we get here is radient heat, so making sure you have enough insulation in the attic is important. Pulling the old stuff is only good if you plan on sealing any penetrations, which is also a good thing.
Where is your ductwork located?
Get
an insulation contractor to remove the old, air seal the attic floor completely, and then blow in a bunch of cellulose.
You don't need to pull the old insulation first, unless you need to air seal, and then only in those areas needing sealing. Blow new cellulose (best) or chopped fiberglass over the existing. Be sure to install chutes or dams around any eave vents, etc, to keep them from getting clogged by insulation.
In southern CA you don't need to be obsessive about air sealing -- just plug around pipe/flue penetrations and make sure the areas above any soffits or drop ceilings are sealed.
No need for a vapor barrier.
Also, upgrade your attic ventilation system
The less temperature differential there is between the conditioned and unconditioned space, the less heat there is to transfer. So, be sure to do as much as you can to limit the heat buildup in the attic.
For background: My house, and my parent’s house, in the Las Vegas area used to get extremely high, (180-degrees), temperatures in the attics.
We started by installing an additional 10-inches of cellulose, blown in over the existing fiberglass batt insulation. That helped a lot but only cut the power consumption by about 15%. The next summer we added thermostatically controlled, electric ventilator fans, which were twice as large as the manufacturer recommended, and got the temps down to just 10 degrees above ambient. That made a huge difference in the amount of heat transferred from the attic into the conditioned space. Installing the fans dropped the power usage by a third at my place, and about 25% at the folks house.
At the folk's house Dad had a 5,000-cfm swamp cooler that he had installed to cool off the garage so he could work on sports cars during the summer. When they were out of town one summer, I started leaving the door from the garage into the house open, and running the swamp cooler into the house and out through the upstairs windows. That cut the power bill by about 80% for the first few months of the cooling season. The swamp cooler doesn't work well once monsoon season starts, and the humidity goes up. So it didn’t do much for the August power usage.
The next year Dad installed ducts from the upstairs bedrooms into the attic, so that the attic was getting the cool air discharged into it. That cut the power usage by another twenty percent.
Dad also built trellises the full height of the south and west facing walls. The shade and evapotranspiration of the plants did a really good job of keeping the stucco from gaining heat, and helped to keep the cooling costs down.
Would recommend blown cellulose in general. Inexpensive, you can do it yourself if you want ... blowers are often offered to rent when you buy the insulation. Cellulose provides better coverage and is inexpensive ... maybe cheaper than fiberglass ... and the better coverage is well worth it. Can install right over the fiberglass.
Two issues: 1) sealing penetrations in e.g. wall top plates. Now's the time to do it ... albeit not much fun to do if the access is limited. 2) installing baffles for the insulation at the eaves of the house. I assume you have eave vents? It's important to maintain a ventilation space from the eave up to the the main attic (and the ridge, gable end, and/or 'mushroom' attic vents). Also not much fun, but an important aspect of insulation installation (fiberglass or cellulose).
The insulation is easy, baffles and sealing penetrations are the hard part. Be patient. Try not to cut corners. Leaving this step out is like plugging small leaks in a dam while leaving the big ones in place.