Insulation Question

[Gofor]

My shed reaches temps around 104 - 109 during the summer mid-day hours. With fans, windows and doors open I can get it down to the high 90's most days. It has a white metal roof over a plastic membrane on OSB sheathing. I plan to put in R-19 single faced fiberglass roll insulation, but due to the design of the roof truss structure, not sure which method would be best. The roof does have a ridge vent. I am hoping to get the heat load down in the summer but also will have to heat in the winter.

This is the roof configuration:

.

Which would be most effective: A - Running to the peak while leaving the vent open; B - running over the cross truss leaving the top section uninsulated; or a combination.?

I do plan on AC/heat of some sort but would like to get a better idea of the temperature load I will be dealing with before deciding what I really will need.

Thanks for any insight you may have on this issue.

Go

 

[Roy G]

My house has a sunroom that has similar framing. They insulated along the rafters and over the flat part, but used some spacers so the eave vents would function properly. It's some cardboard bent into a shape to provide a channel for the air to flow.

Roy G

 

[Gofor]

My house has a sunroom that has similar framing. They insulated along the rafters and over the flat part, but used some spacers so the eave vents would function properly. It's some cardboard bent into a shape to provide a channel for the air to flow.

Roy G

Good point on the spacers. If this had a shingled roof, I would just go with R-15 at 4" thick instead of the 6" R-19 I am considering. (The shed does not have any eaves nor vents). I am hoping it won't cause problems with the metal roof. Over the flat will reduce the total air volume in the working area, a plus for AC/heat, I guess.

Go

 

[gmakra]

From a R value either will have the same insulating effect.
So the question is are you going to cover the ceiling?
If you are design B gives you a pipe chase for duct work, compressed air lines.

 

[Skymaster]

B your heat load is going to zoom if you insulate the peak. Plus any insulation in the rafters MUST use the foam channels made to allow air to flow. No need for treated air above the trusses. As previously stated this also gives you "attic: space for ac,lines etc.

 

[gmakra]

Jack please explain how this would be?

B your heat load is going to zoom if you insulate the peak

 

[JimD]

As has been stated, you want to do B. The two story great room in my house is somewhat like this, it starts at 8 foot at one outside wall and goes up to 16 feet where it flattens out and is at that height at the other wall. It was insulated like A. It totally did not work. The insulation fell down, there is a ridge vent that became ineffective because there was insulation over it and the insulation didn't work because what air came through the ridge vent went on the back side of the insulation. We had electric bills as high as $500 and temperatures that were not what we wanted.

I blew in fiberglass - a very easy way to insulate - and things were much better. I had the floor spray foamed this spring and bills on hot months are about $125 - and I can hold any temperature I've tried.

You are doing things in the right order. Insulate is first priority, then HVAC. Going the other way can be expensive and frustrating.

 

[sawman101]

Dennis is there no way you can put vents in the gable ends? I would prefer plan B with vented gables, and foam air channels under the roof, and the insulation. There will be no sweating under the roof and between the insulation that way. Although not apparent in the Wintertime in the south, it's very apparent in the north where huge icicles form on the eaves of improperly vented buildings.

 

[Jeff]

I prefer plan B and adding soffit vents instead of gable vents so that the hot air rises by convection and out of the roof peak vent.

 

[Skymaster]

Gmakra Lets presume your ceiling height under the trusses is 8', space above the trusses is 4'. the heat/cool load for the 4' space is 50% of the 8' room. You really cant use it for anything but storage and or mechanical space so why spend 50% more to insulate it, heat/cool it? you bill will be 50% more for 0 net gain.
The BEST thing you can do is install a automatic attic fan in that space. Set "attic" temp, fan will maintain that temp and reduce the "hidden" load caused a scorching hot attic space.

 

[Gofor]

Thank you all for your insights. I knew that air flow under a shingle roof was necessary to keep the shingles from frying from the bottom side. Had not realized the condensation issue also applied to metal roofs. So, the baffles are installed. Have to wait on the insulation to get drop-shipped to the store (Lowes). They don't keep R19 23" wide rolls in stock.

Go

PS. I will be going with plan "B".

 

[Skymaster]

A hint for you; get a bunch of 2x4 the length of the bottom chord,where you have you sheetrock joints, apply 2x4 along the length of the cord, this way you have 1 1/2 for each side of the joint to attach to NOT crappy 3/4. dont have to split a joist, :gar-Bi:gar-Bi. You can do this while awaiting the rock, lay ceiling out, then all u need do is hang the rock, better yet hire some rockers,get out of the way, chaise lounge and a cold one, supervise