Garage Insulation Advice

stagger19 · 03-02-2012, 07:13 PM

Hi, I have a detached 16x24 garage with roof trusses.

The Garage gets very hot in the summer time, too the point its unusable.
If its 90 deg outside, it will be 95 to 100 inside and thats on the floor.
Up in the gable area its prob 10 to 20 deg hotter.

I have gable vent on each end but they don't do much.

I was thinking of insulating with R30 and then dry walling the ceiling.

If I was to put insulation in the ceiling, but not the walls, would I still see a big gain in reducing the inside temp?

Thanks...

joecaption · 03-02-2012, 07:17 PM

You need a ridge vent not gable vents and full soffit venting. Insulate the ceiling not the roof.

stagger19 · 03-02-2012, 07:20 PM

Hi Joe, thanks for the quick reply.

Yes I was going to insulate the ceiling. My ceiling is 8 ft tall.
I wasnt going to bother putting in ridge vent, just go with the insulation.

So you think insulation in the ceiling will make a big difference then?

Windows on Wash · 03-02-2012, 07:43 PM

Quote:

Originally Posted by stagger19

Hi, I have a detached 16x24 garage with roof trusses.

The Garage gets very hot in the summer time, too the point its unusable.
If its 90 deg outside, it will be 95 to 100 inside and thats on the floor.
Up in the gable area its prob 10 to 20 deg hotter.

I have gable vent on each end but they don't do much.

I was thinking of insulating with R30 and then dry walling the ceiling.

If I was to put insulation in the ceiling, but not the walls, would I still see a big gain in reducing the inside temp?

Thanks...

Blow in R-50 as it is not that much more expensive.

Soffit and ridge venting is ideal as Joe said.

If not, get a thermostatically controlled fan that mounts in the gable and blows air across.

If you are not conditioning the space, see how it feels with the insulation to start and you can put a couple of roof vents near the ridge.

Something like a whole house fan will blow up ambient temp air into the attic and flush out the heat and cool off the garage at the same time.

**Gary in WA** · 03-03-2012, 01:56 PM

If trying to cool the garage space, insulate the walls, doors, windows, and ceiling. Add radiant barriers if your location warrants it. If cooling the attic storage, r.b. the rafters or joists, air seal http://www.jlconline.com/cgi-local/v...170a32100a05c7 and insulate the ceiling below and don't worry too much about the ventilation: http://www.professionalroofing.net/a...2/feature2.asp

Cost vs. value on the insulation: http://www.enersavesystems.com/pdf/E...Insulation.pdf

Gary
P.S. You could double drywall the ceiling to slow the heat intake but the thermal mass storage would keep it warm into the cooler night. You could build a solar chimney......

Windows on Wash · 03-04-2012, 01:34 PM

Quote:

Originally Posted by GBR in WA

Cost vs. value on the insulation: http://www.enersavesystems.com/pdf/E...Insulation.pdf

Gary,

Is this a marketing piece for Icynene? I am not sure I trust several of their numbers and where do they specify what material they are measuring?

This was a quote from their "study":

Based on this observation, it is very difﬁcult to justify the additional cost of adding insulation thickness beyond 5”

Really??? R-19 is now considered effective R-Values?

**Gary in WA** · 03-04-2012, 09:43 PM

Hard to swallow the first time I read it a few years ago, also. You'll need to look up that ASTM test yourself, I did quite a while ago. A different slant, same principle- diminishing returns: http://www.buildings.com/ArticleDeta...1/Default.aspx

An interesting read: http://www.greenbuildingadvisor.com/...cience-podcast

I enjoyed that comment-- Temp. difference across wall increases—there is a greater and greater proportion of heat flow transported…….

And with air movement, it's even worse with high R insulation thickness- R-20 to R-60 in a new house, pp. 40: http://www.buildingscience.com/docum...-climate-zones

Shoot me a PM to discuss it further....

stagger19, where are you located, or did I miss that?

Gary

Windows on Wash · 03-05-2012, 09:03 AM

Quote:

Originally Posted by GBR in WA

Hard to swallow the first time I read it a few years ago, also. You'll need to look up that ASTM test yourself, I did quite a while ago. A different slant, same principle- diminishing returns: http://www.buildings.com/ArticleDeta...1/Default.aspx

An interesting read: http://www.greenbuildingadvisor.com/...cience-podcast

I enjoyed that comment-- Temp. difference across wall increases—there is a greater and greater proportion of heat flow transported…….

And with air movement, it's even worse with high R insulation thickness- R-20 to R-60 in a new house, pp. 40: http://www.buildingscience.com/docum...-climate-zones

Shoot me a PM to discuss it further....

stagger19, where are you located, or did I miss that?

Gary

Gary,

No argument from me that air movement is the primary culprit of inefficiency in an overwhelming majority of homes.

As far as the ASHRAE study and the non-linear reduction in BTU via R-Value, that is also well accepted.

I think for the DIY reader though, it needs to be a bit of data that is taken with a grain of salt. Most homeowners with any housing stock of the last 20 years will have blown in fiberglass which doesn't work in any capacity and they will need at least 14" to perform near that 7" capacity.

Additional insulation is also cheap when you figure the cost of getting the machine or professional to blow in 4" is not half as much as blowing in 8".

Go higher on the R-Value will still give you a reduction in heat flow and also ensure that any low spots or other areas will still have proper depth.

The reason I took umbrage with the study is that 5" of what is the problem. Put a 5" batt or blown in fiberglass and it is not going to perform and the customer will not be comfortable. Even 5" of cellulose will allow for some convective looping in a portion of it and the resultant de-rating of the theoretic R-Value to some extent.

That testing data is also in a static environment as far as I can tell which does not take any convective looping into account.

5" of Icynene will perform quite well but will be far more expensive at the end of the day than will 14" of cellulose and some air sealing.

**Gary in WA** · 03-05-2012, 04:23 PM

The study quoted showed that adding more than R-18 (5"), doesn't hardly pay for itself when you are at 95% there. The further gains (5%) could be sacrificed there and used for air sealing, weatherizing, etc. that would give more "bang for the buck". At 7" (R-25) or 97%--- you really need to assess the value/cost in my book.

They based it on R-3.6: "This equation is used to calculate the benefi t of increasing the thickness of any type of insulation as
long as there is no air movement (convective heat transfer) through the insulation.
As an example, consider 1000 ft2 of insulated area with a temperature differential of 40 degrees F.
Let us include the outside air fi lm at R-0.2 and the inside air fi lm at R-0.4. The total R-value before the
application of any insulation is 0.9. Increasing the insulation thickness by 1” increments at R-3.6/inch
provides the following heat fl ow rates as shown in Figure 1.1 and 1.2."

I encourage everyone to insulate to minimum code for your location. Anything further in R-value, weight the cost (labor/materials) against the 2-3% increase in heat flow reduction. R-50--60 in the ceiling.......yet walls are still R-15-21.... go figure, I wonder who's pushing that, LOL.

Gary

Windows on Wash · 03-06-2012, 08:12 AM

I agree with you on walls.

I think that posting 5" is enough insulation is quite misleading by this study and is aimed at promoting foam for certain.

5" is no where near enough when you figure in convection and many homes will have 3-4" in them so if you come in as a contractor and say you are going to put 1" more...you are probably going to get laughed out of the home.