How much attic insulation? R__??

masraum · 04-12-2010, 07:23 AM

I live an a 2 story home in the Houston area that was built in the late 60s. The attic is about 900sqft. It gets up to at least 130 in the attic during the summer. I've already seen temps over 100. How much insulation do I need? I had a guy come out to give an estimate the other day. He said R38. I was thinking more than that, but I don't know.

What says the braintrust?

How much do I need? R38, R50 or 60, R1000...?

Porsche-O-Phile · Porsche-O-Phile's Garage

38 is awfully high. Check the code requirement for your area. While more is better in terms of energy efficiency, you do have to consider the implications of paying for something you're not going to realize any (or much) benefit from. Also consider how long you're going to live there and whether or not you'll gain benefit on a life-cycle cost basis.

A residential job I did recently required R30 between rafters or R23 continuous. And this is a pretty cold climate. It also varied based on the compsition of your building envelope (more glass = higher roof insulation requirement, at least in my case).

That said, also keep in mind that codes are MINIMUMS. You can (and should) exceed them if it's to your benefit to do so.

island911 · 04-12-2010, 08:48 AM

I would expect that you would have diminishing returns beyond R38 (if not sooner). Attic insulation is a bit bigger player in cold climates --hot (room) air rising. In your case, you want to keep a pool of cold air in the house - meaning floors and walls need to be well sealed w/some good insulation.

Also, for the attic, a good radiant barrier (foil-backed insulation) should prove worthwhile.

masraum · 10:41 AM

The guy that I spoke to Sat said R38.

I spoke to 2 guys today. 1 said R30, the other said that for this area, with an electric furnace, R49, or with a gas furnace, R38.

So that's 2 votes for R38.

The big difference is that around here, the insulation is more about keeping heat out than keeping heat in.

I guess based on what I've heard, I probably don't need to go above R38.

The guy this weekend quoted IIRC, $640 to do both the attic and the garage (plus another 20% off). And then $1300 to put in a solar powered attic fan.

1 guy today quoted $936 to insulate both the garage and attic, and then $950 for a commercial solar attic fan

The second guy that I spoke to today, said that he doesn't recommend insulating the attic over the garage. He said that insulating the garage will actually make it hotter since it is not conditioned space, and that will cause problems with the paint and drywall. He quoted $710 for the attic insulation. He mentioned something about vents in the rafters (I believe to make sure that they still get the intake air from the soffits). He said that they can do some sort of passive square vent for $125 a pop or they can do a solar powered attic fan.

So, I've got these bids

$640 to do the insulation (R38) for both garage and attic. $1300 for a solar attic fan

$936 for the garage and attic insulation (R30). $950 for a solar attic fan

$710 for just the attic (R38) (plus some extra small stuff), recommended against doing the garage, and will do a solar attic fan, but didn't get a cost.

Out of curiousity, looking at these quotes, do you have any thoughts?

I'm sort of leaning towards the guy that recommended against the garage. What he said about insulating the garage made some sense. I'm sure some of the heat is coming from the attic over the garage, but I'll still get heat into the garage, and with the insulation, that heat will take longer to dissipate. I guess what I really need is some sort of vent to move air out of the garage.

island911 · 04-12-2010, 09:55 AM

Quote:

Originally Posted by masraum

...
I'm sort of leaning towards the guy that recommended against the garage. What he said about insulating the garage made some sense. I'm sure some of the heat is coming from the attic over the garage, but I'll still get heat into the garage, and with the insulation, that heat will take longer to dissipate. I guess what I really need is some sort of vent to move air out of the garage.

n'no. you have to look at where the big heat producers come from first.

On a hot summer day, the attic temps will be MUCH higher than ambient. You want to block that big heat source. Walls that see direct sun are next up.

Also, consider, that if your garage is on a slab, that you have a nice heat sink, right there.

RWebb · RWebb's Garage

ditto

you need VENTS!

A930Rocket · 04-12-2010, 10:20 AM

We insulate with a minimum of R30 in SC, NC and GA. I'd ask the price difference between R30 and R38. Wrap your pull down stair open with a 2x6 or 8 to keep the blown insulation from falling in. Insulate your pull down stairs with fg insulation or blueboard.

I always insulate the garage walls and ceiling/attic. Keeps the room above or next to it cooler. An insulated g door helps as well.

Solar powered roof vents are fugly. Use turtle backs and or ridge vent (if you have a gable roof), but make sure you have enough soffit vent to get good air circulation. If you have a hip roof, you may need a power vent, but use a 120v on a thermostat. a new one at HD or Lowes is $100, plus install, $50. if you have any power in your attic, you can jump off that. Don't use any type of power vent with a ridge vent. It will just pull air from the nearest place, the ridge vent.

masraum · 11:54 AM

Quote:

Originally Posted by A930Rocket

We insulate with a minimum of R30 in SC, NC and GA. I'd ask the price difference between R30 and R38. Wrap your pull down stair open with a 2x6 or 8 to keep the blown insulation from falling in. Insulate your pull down stairs with fg insulation or blueboard.

Yes, I want to insulate the Attic hatch/door

Quote:

I always insulate the garage walls and ceiling/attic. Keeps the room above or next to it cooler. An insulated g door helps as well.

The garage door has 3/4"-1" styrofoam on each door section, so semi insulated. They face West too. The garage is otherwise uninsulated. The walls are not insulated, and the ceiling/attic is not insulated. The garage is on the north side of the house and is connected.

Quote:

If you have a hip roof, you may need a power vent, but use a 120v on a thermostat. a new one at HD or Lowes is $100, plus install, $50. if you have any power in your attic, you can jump off that. Don't use any type of power vent with a ridge vent. It will just pull air from the nearest place, the ridge vent.

Yes, I have a hip roof with about 5' of ridgevent. THe entire circumference of my roof is perforated hardie soffit, probably 80-100' of it.

I suspect that's enough intake, but I think the 5' ridge vent is inadequate to move the hot air out of the attic.

The first company that I spoke to said that when they add a solar powered fan, they block off most of the ridge vent to mitigate that direct transfer as a problem.

Right now at the house it's sunny and 75°, and in the attic it's 102°.

Why a utilities powered vent instead of a solar powered vent? Seems like using Utility power to save electricity via the AC defeats the purpose a little. Seems like the solar (free) powered vent would be better. I can put the vent on the back of the roof (faces east), but put the solar panel on the south facing portion of the roof, and the front facing portion is still nice and clean.

Thanks for the food for though folks. Keep it coming.

Joe Bob · Joe Bob's Garage

R-19 in CA.

s_wilwerding · s_wilwerding's Garage

If I were in a primarily warm to hot environment, I would put about 12" of insulation on the floor of your attic, and radiant barrier against the rafters. Also, ideally use a ridge vent with soffit vents, and maybe even include a thermostatically controlled fan.

dad911 · 04-12-2010, 01:16 PM

Hot air rises. You need more ventilation near the peak. 5' of ridge vent is nowhere near enough.

Also make sure that the insulation in the ceiling is not blocking the soffit vents.

Mark Henry · 04-12-2010, 01:31 PM

Put a white roof on your house and your cooling cost will drop to about half.

Porsche-O-Phile · Porsche-O-Phile's Garage

Another thing to consider is whether the R value is a total assembly (continuous) or just between the joists/framing. Big difference. A lot of codes only prescribe a net assembly performance which means you might "need" a big R number (like a 38 for example) to realize a lower R number (like 30, for sake of example) over the assembly - accounting for the fact there's no insulation where the framing is.

There are all kinds of ways to get high R values and efficiency. As has been stated, light-colored roofs are a good choice and there are a bazillion kinds of "cool roof" assemblies out there.

tcar · 04-12-2010, 03:01 PM

If the insulation has a vapor barrier, put it on the OUTSIDE, not the inside like you do in cold climates.

masraum · 04-12-2010, 03:49 PM

I've been thinking about the garage. I guess that since it's essentially a sealed box, it should probably be "cooled" similar to an attic. I should work out some passive cooling/venting. I need to get some way for the air to flow through the garage. I've seen "vents" put into the bottom panel of the garage doors, and then either something that goes to the outside, or something that goes out of the ceiling. And then I need to make sure that the garage attic is vented well enough. It also has perforated soffit and ridge vent. I suspect it also needs more exhaust vent.

A930Rocket · 04-12-2010, 04:54 PM

With a hip roof, 5' of ridge vent isn't doing anything. It's like breathing through a straw. You need either a lot of turtle backs or a power vent of some kind as mentioned.

For power vents, the solar powered ones just look too big and clumsy. Maybe it's the fact that I've seen people install them in really bad places seen from the front of the house. I like all on the back and hidden (as well as any plumbing vents).