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I'm looking for help in insulating my 1950s San Diego house. A second story was added in the 70s and a gas furnace + A/C was added about 10 years ago in the unconditioned space above the downstairs bedrooms. This space is also right next to my master bedroom.

I'm in IECC Zone 3B/C. Highs here are between 60-85F year round and it never gets below 50F at night (rough I know). The problem is that when it does get to 85F, the vented attic can get to 120-130F since the roof gets a lot of sun. That means at midnight, the outside air temp is 70F and my master bedroom is 90F+. The air conditioning can run all night and not get it below 80F. Then again the next day, the master bedroom is a comfy 75 all day until around 4:00PM.

The wall between my bedroom and the unconditioned attic space is R13 fiberglass (crappy piecemeal) with the insulation faces pointing both directions. The other side is exposed to the attic. I have air sealed all electrical boxes with spray foam. The attic only has pull-down stairs to get in, so a full 4'x8' board will not get in without major destruction. The attic has multiple soffit and gable vents and no fan besides a plug-in stand fan I've been using.

Attic Images - A) Wall to Master Bedroom B) 2nd Wall to Master Bedroom C) HVAC in Unconditioned Attic

I've done my research and asked around and come up with these options:


  1. Put up a perforated radiant barrier on the attic rafters and gable walls.

  2. Fix the insulation so all faced batts point towards the living space and put up additional, rolled fiberglass insulation. I was thinking unfaced R30 horizontally across the studs.

  3. Put up an air barrier (Tyvek / house wrap) behind the fiberglass insulation on the attic side of the wall.

  4. Spreading loose cellulose around the attic to bury the ducts, while staying away from the furnace.

  5. Install a temperature-controlled attic fan.


My gut usually tells me to overdo things and to do #s 1-5 on this list, but what would you do here?

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  • My recommendation would be to find out whether your energy utility companies offer a free efficiency evaluation service. Many do, and that will bring in someone who knows what tends to work in your climate and with your construction. (For example, in my area improving the house's air sealing and dumping several feet more insulation into the attic were the most cost-effective first steps.) These programs often provide a substantial discount/subsidy on these services too, so definitely better than trying to guess!
    – keshlam
    Commented Nov 11, 2022 at 6:34

4 Answers 4

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I think your best bet is two-fold. You can insulate against the temperature delta between the spaces. I strongly recommend spray foam. It will provide a airtight and high R-value insulation layer. You may need to sightly adjust some things in the attic to do it properly but foam is "best in class" for this.

The second is reducing the temperature delta itself. Usually this is done with a ventilation fan which ejects the hot air out a gable, ridge, or similar vent. Fresh air is drawn through eaves vents typically. As you said, it's cool at night so why not fill your attic with that cool fresh air?

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  • Spray foam may be best, but to get back to R-13, I'd need 2"+ and $700+ of spray foam. Is it worth that much over putting up an air barrier behind the fiberglass? Would you skip the radiant barrier and go straight to the fan? I'm using a stand fan on a WiFi plug that turns it on whenever the temp in the attic reaches 80F and it doesn't do very much.
    – pennstump
    Commented Sep 24, 2019 at 21:59
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    Your fan doesn't do much because it isn't directly ejecting hot air to the outside. The appropriate fans tend to be either embedded into the roof or attached to a gable.
    – Matthew
    Commented Sep 25, 2019 at 1:41
  • To answer your second question, I would do both a radiant barrier and install a vent fan, but if your budget only allows for one, I'd guess that moving the air out of the house would be more effective. The radiant barrier could be added later to improve energy efficiency if nothing else. To answer your first question, IMO the cost of spray foam as a primary insulator is rarely worth it, unless you live in a terribly humid environment and mold in fiberglass is a serious concern. Plus there are some horror stories about odors caused improper curing from poor application - Google it.
    – Nate
    Commented Sep 25, 2019 at 4:07
  • @pennstump Also, foam can be messy.
    – Yehuda_NYC
    Commented Nov 17, 2020 at 14:25
  • Closed cell spray foam is R7/in thickness. R24 in a 2x4 wall is superior to anything else and it seals EVERYTHING. Your bedroom is 90F+ so the extra expense will be worth the money. We used spray foam on our house and the utility bills are amazingly low.
    – RetiredATC
    Commented Oct 18, 2022 at 1:33
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I am not clear at all from your description how your rooms are laid out. One thing I'd try is not on your list I'd install an attic fan on one of the gable vents with a thermostatic control. This will keep the attic from getting much over the outside ambient temperature. This will probably be money well spent even if you have to add insulation etc. as well.

Outside the box - solar on the roof might give actually take that radiant energy and turn it into electricity and money, rather than nuisance heat in your attic. I don't know what the current situation is in California but I think at one point the subsidies / incentives made solar a good buy for the homeowner.

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    Psst... I think the "temperature controlled attic fan" is #5 on OP's list.... ;)
    – FreeMan
    Commented Nov 17, 2020 at 17:50
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I've never lived in a climate like yours, so I probably am not the best person to give advice for you, but:

If you do #4, I'd recommend you box it in with plywood and 2*4s. But unless your AC is spitting warm air into the room, this is probably not necessary.

A radiant barrier between the rafters to reduce the heat in the attic may help, especially if you have a conventional asphalt shingle roof. Installing a vent fan would also help cool the attic at night, although not as much during the day. Reducing temperature in the unconditioned space should have a significant effect.

The insulation in picture A looks horrible, I would replace it. The insulation in B looks not great but okay, since any air gap caused by the bulging out should be on the room side. If you think air may be leaking around the insulation, nailing up OSB or plywood and sealing the seams (ie with spray foam) may help, although the common recommendation nowadays is to let your insulation breath (not sure if you have any humidity concern there).

Although adding additional insulation around the structure may help slightly, it's not going to make a night and day difference. The relationship between heat flow and R-value is an exponential decay function, and R-13 (when properly installed) is already past the "knee" in that curve - additional insulation would still reduce heat flow, but less than the R13 already is. It makes more sense if you're concerned about long-run energy efficiency than current comfort levels. I've also read that excessive insulation can even exacerbate cooling difficulties - although this most likely applies to situations with multiple layers of R-30.

And another important consideration may be the insulation above the room.

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  • Thanks. So, can I sum up your advice as install the radiant barrier since I do have an asphalt shingle roof, fix the R-13, and air seal around the insulation? I have an IR laser temperature gun and the ceiling above the room has never shown a temp higher than the ambient air temp in the room, but I guess it could be radiated heat.
    – pennstump
    Commented Sep 24, 2019 at 22:03
  • Its probably fine then, I only mentioned it because you hadn't. Heat will travel down through an uninsulated ceiling cavity faster than it would through a properly insulated wall, but we don't usually think of heat travelling down.... Air sealing should not be necessary in a properly insulated wall, but I know from experience working on older houses that sometimes you wind up with weird gaps that are difficult to properly fill. As long as the insulation makes good contact the entire length of the stud, as well as at the bottom and top of the void, full air sealing should be unnecessary.
    – Nate
    Commented Sep 25, 2019 at 1:04
  • But to summarize my post I'd fix the insulation and try to reduce the temperature of the unconditioned space... Additional air sealing shouldn't be necessary with proper insulation installation, but shouldn't hurt if you have any doubts about it. I'd recommend tyvek housewrap so that humidity can escape, if necessary, over using rolls of plastic. Tar paper is cheaper and has acceptable vapor permeability too, but has the downside of absorbing heat because it's black. If you air seal, tyvek housewrap is the way to go.
    – Nate
    Commented Sep 25, 2019 at 4:16
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Install a small window a.c. to fight the problem directly while lowering the main a.c. temp to offset the cost

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    That's probably just going to make the OP's power bill worse, compared to yanno, actually fixing their envelope issues
    – ThreePhaseEel
    Commented Oct 18, 2022 at 3:30

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