Insulation without HVAC?

[ewmyers]

Hi, I have a workshop which gets too hot in summer (Sacramento, CA). I am looking for information about how to reduce the effect of the summer sun, but I have no plans to include an HVAC of any sort. So basically I am looking at adding insulating to the roof somehow, and probably gable or roof vents.

The building is a 14' x 24' Tuff Shed on a cement slab, with 4 skylights in the roof, 3 windows per side, and a door and a window at each end. Note: The trusses are exposed, and I want to keep them that way. The roof is 4-in-12 pitch, and there are no roof or gable vents. The eaves are boxed, and were built without soffit vents >:-(

I have blinds on the windows, and shade cloth over the skylights during the summer. A shed roof awning along the west side keeps that wall shaded, and made a dramatic difference in shop temperature when it was built. However, it still gets pretty warm in there, and even with doors and windows open, you can feel an oppressive heat by just raising your hand up above your head.

I have looked at insulating between the top chords of the trusses, but they are 2x4s so there isn't a lot of room for insulation + air gap. I have considered mineral wool for R15, but I'm not sure how to cover it up to avoid bugs living in it and dust from falling down into my lungs I have also considered the foil faced bubble wrap stuff but I don't really understand how the radiant shield aspect compares to insulation. In either case, I don't really have anywhere for the warm air to flow to except to the peak. I don't know much about vents as far as type, location, cost, etc. We are probably within 5 years of having to re-roof, so should I consider adding insulation above the current deck at that time instead of inside? Dealing with the skylights will increase the cost of that….

So, what recommendations do you have?

Thanks,
Eric

 

[DC73]

With that many windows and the skylights, you really have your work cut out for you.

I would consider doing away with the skylights and then having closed cell spray foam insulation installed in the walls and under the roof deck. You'll be amazed at the temperature difference.

If you keep the skylights, they will be a major source of heat gain into the building. Maybe if you could shade them somehow, it would help but I really think it would be best to get rid of them if you can live without them. You might could sell them to recoup some of your insulation investment.

Don't waste your money or time with bubble wrap. The first steps to reduce heat infiltration is to seal all air gaps and to insulate properly. After you do that, any type of radiant barrier won't be cost effective.

Any other choice of insulation that gets installed directly under the roof deck will require a ventilation channel between the insulation and the bottom of the roof deck along with soffit vents and a ridge vent.

Good luck,

DC

 

[Ed Devinney]

Sounds like a lot of solar heat gets built up in the roof area and the hot air just stays there?

Not sure that any amount of insulation will be really effective against that. As DC73 says, those skylights are a primary source of solar gain, and the roof itself is a close second.

How about getting rid of the hot air? Maybe a solar attic fan, vents in the gable ends, and/or a big fan in the gable?

 

[ewmyers]

Sorry, I had a longer reply written up but apparently my login timed out before I hit "post" :-( Here is the shorter version.

The skylights are here to stay, but I may consider solid shutters instead of the shade cloth blinds I have on them.

I think my first priority should be getting rid of the hot air with venting. I'd rather go with something I can close off in the cooler months, so a whirlybird or vented cupola are possibly not ideal options. I can probably fit 1 - 2 sq ft of vent in each gable, but it would be done as 2 smaller vents on either side of the king post in the end trusses. Would that be enough? I'd consider a reversible fan at each end too, so I could swap intake/output as the wind changes.

Thanks,
Eric

 

[DC73]

I think my first priority should be getting rid of the hot air with venting.

The best you can do with venting is to have the interior temperature equal the outdoor temperature. And that assumes you'll have no heat gain through the windows or skylights. Maybe that's cool enough where you live. But, for proper venting you'll need both intake and exhaust vents. Ideally the intake vents would be as low as possible to draw in the cooler air and the exhaust vents would be as high as possible.

DC

 

[Dragfluid]

You asked for opinions, so I'll give you mine. Get rid of the skylights. If you want cheap lighting, put in some LED fixtures. Put steel panels on the ceiling and fill it with cellulose. 12 or so inches, preferably 16. Vent the attic.

 

[Fixin'Stuff]

And don't waste any money on a solar attic fan. Their claims are ridiculous and don't stand up to the laws of physics. Many claim to move ~1,500 CFM, but have a 10-watt solar panel. Watts = work, and it takes work to move air. 10 watts is about what the fan in your computer uses. That just isn't going to do much to cool an attic.

A 120 volt attic fan that is rated at 1,500 CFM has 1/4-HP motor. 1/4 HP takes almost 200 watts to run. Those solar folks are using some Marketing Math.

 

[Fixin'Stuff]

One other thought. Once you have your shop insulated, it will still get hot. It will also stay hot, probably most of the night. That insulation doesn't stop heat transfer, it just slows it down. Problem is, it also slows it down on the way out.

 

[47WDXPW]

On a roof that had insulation rolled out before the sheeting was installed(2" maybe ) spray foam can not be sprayed on top of it is that right ?

 

[Dragfluid]

One other thought. Once you have your shop insulated, it will still get hot. It will also stay hot, probably most of the night. That insulation doesn't stop heat transfer, it just slows it down. Problem is, it also slows it down on the way out.

See my post in a new thread about insulation.

 

[Fixin'Stuff]

See my post in a new thread about insulation.

You don't live in a place that sees 90-100 degree days for months at a time.

Might work in Minnesota, but in the south (Or Sacramento, like the OP) you'll just have a well insulated sweat box. Been there, done that. When the overnight low is 80+ and the daytime high is 100+, you aren't going to get into the 70's inside.

The forecast high for Sacramento tomorrow is 108. That's air temp, not the sun shining on the siding. The drier climate in desert locations will cool off more at night, but most folks aren't in the shop at 4AM.

 

[Dragfluid]

You don't live in a place that sees 90-100 degree days for months at a time.

Might work in Minnesota, but in the south (Or Sacramento, like the OP) you'll just have a well insulated sweat box. Been there, done that. When the overnight low is 80+ and the daytime high is 100+, you aren't going to get into the 70's inside.

The forecast high for Sacramento tomorrow is 108. That's air temp, not the sun shining on the siding. The drier climate in desert locations will cool off more at night, but most folks aren't in the shop at 4AM.

Please keep reading over there.

 

[ewmyers]

Thanks for the additional thoughts, everyone. I haven't really made any progress on this project yet :-(