How much spray foam? The excruciating question!

[Dave-H]

Hi all -

I did a 2 car garage conversion in Denver a few years ago, and now I'm doing another one here in California (I've just moved back to the Bay Area.).

So, I am loving this because I'm doing the exact same plan to convert my new house's garage into an office - including spray foam for the walls and ceiling (which is the underside of the roof, cathedral ceiling.)

I furred out the gable walls so now we have 2x4 cavities in all walls and ceilings. The spray foam is expensive, but in Denver I had team come and pack the cavities and trim them, which is a lot of work. The trimming isn't cheap and the installers really try to avoid it around here.

But now I keep hearing ' you only need 2 inches around here'. And it's true, the Bay Area isn't cold like Denver. But it still gets chilly and can get really hot around here, too. But, it's so pricey that I'm considering just going 2 inches all around and calling it a day.

But then, I remember the joy of being able to heat my last garage (with 3.5 inches all around) with just a little heater when it was 5 degrees outside, plus the air sealing and the sound reduction - so good!

Any opinions on the value of going past 2 inches of spray foam in a situation like this? Maybe 2 on the walls and 3 up top is a compromise - I just cant decide.

But I am addicted to the idea of spray foam after seeing how well it performed last time!

 

[yeldogt]

What was the framing previously that you furred out to get a 2 x 4 cavity?

Depth is normally a question of code -- What is the requirement?

I did a flash and batt with 2" in NJ. Very impressed before any of the conventional fiberglas batt went in. It was a very hot week -- upper 90's and the 2" just eliminated the heat gain through the roof. But, I agree -- my typical is at least 3 in the wall and 4"+ in the ceiling to hit the code in PA.

So you have prices and 2" is the norm?

 

[iced98lx]

Where you are at if you're going to go further than 2 inches do it on the roof to avoid solar gain through it.

Do a heat loss calculation with 2 and 3 and 4 inches of foam if you're really curious and I think you'll see that once you hit 2 inches and get the air barrier (and reduced infiltration rate) from the foam you're not going to see payback on 3/4+ inches down there. My educated guess based on hot but dry summers and warmer winter days in the Bay area.

I understand your impulse though, maybe you don't care about payback and you want the MOST COMFORTABLE SPACE you can get, in which case I think you'd see some difference, just might be throwing money into you don't really see back.

 

[dcg9381]

There are climate based recommendations for R value. They assume you're living there full time. Likely you won't meet them in Denver with 2x4 framing and open cell.


For 2x4 framing the answer is simple to me - open cell them full.
Ceiling, I'd refer you to recommendations based on climate zone.

What's not calculated generally is that there is a matrix of how often you use the space, cost of more efficient HVAC, and the cost of insulation. We can all build a Yeti cooler to live in and cool it with a 1/2 ton HVAC, but when the cost of insulation vastly exceeds the power costs and HVAC costs over 20 years, I'm not sure that the most efficient home makes sense.

The other advantage to foam is that it tends to seal the envelope. It's not just R value, it's all those nooks and crannies that you'd otherwise miss.

 

[nadogail]

IMHO, You should get a proposal from a contractor who is familiar with your local requirements. I am sure that the various municipalities in the Bay Area do not all have the same requirements.

There are 3 things that are relevant about Bay Area building codes and they all start with L, as in location, location, and location.

 

[meathooker]

I’d bet you’d be happy with 3” ceiling and 2” walls.