Insulation and HVAC questions

[kogerat]

I bought a home last year with a 24'x36'x9' stick frame detached garage. It was built in 1989. The walls have been insulated and drywalled. It is roofed with asphalt shingles and the underside is just bare sheeting with no insulation. I want to insulate the underside of the roof deck and install a HVAC unit.
After doing some research here, it looks like my best options are to use spray foam insulation on the roof deck and go with a split mini HVAC unit. I'm a bit out of my element here and would appreciate any help or guidance.
Thanks, Jim

 

[MagKarl]

Welcome. Before you get too far, make sure your shop roof or ceiling will have proper ventilation. If you post some pics you will get more suggestions.

 

[DC73]

Go to BuildingScience.com, click on the "Information" tab and you'll find plenty of information on how to insulate this area properly.

Another good source is GreenBuildingAdvisor.com. You can ask questions in their Q&A section.

Between the two sites, you'll find enough information to make a good decision. Note that your local climate is an important factor.

Spray foam can be a good choice.

DC

 

[kogerat]

Thanks for the replies. I'll post some pictures tomorrow and check out the links in the mean time.

 

[St-rider]

Spray foam directly on the underside of the roof deck may be a bad idea as the deck needs airflow to keep cool during hot weather.

 

[DC73]

Spray foam directly on the underside of the roof deck may be a bad idea as the deck needs airflow to keep cool during hot weather.

They've begun to change their thoughts somewhat on this and roof decks are now routinely sprayed with foam. Depending on the climate, you may have to use closed cell foam and there are ways to calculate the foam thickness to avoid condensation on the underside of the roof deck. In my climate, we can get away with open cell foam. To save costs, some are using a technique called "flash and batt" which consists of a layer of closed cell foam just thick enough to avoid condensation issues and then the remaining rafter cavity is filled with standard insulation (fiberglass, mineral wool, etc.). Some people recommend a ventilation channel between the roof deck and the beginning of the insulation but that's done more often when spray foam is not the insulation of choice.

DC

 

[kogerat]

Some people recommend a ventilation channel between the roof deck and the beginning of the insulation but that's done more often when spray foam is not the insulation of choice.

This is the reason I'm looking into spray foam. My garage is an existing structure and there is no easy access to the soffits, no vents in the soffits, and no vent in the roof ridge to create a ventilation channel. I'm still working on getting pictures to help visualize my situation. Been a busy week so far. To bad I have to work to support my hobbies!

Jim

 

[DC73]

This is the reason I'm looking into spray foam. My garage is an existing structure and there is no easy access to the soffits, no vents in the soffits, and no vent in the roof ridge to create a ventilation channel. I'm still working on getting pictures to help visualize my situation. Been a busy week so far. To bad I have to work to support my hobbies!

Jim

Sounds like spray foam would be a perfect fit for you. You'll probably need to use closed cell foam but your local spray foam contractors would know for sure. There are charts and resources on the two websites I mentioned that can tell you the minimum levels of closed cell foam to avoid condensation issues.

DC

 

[kogerat]

Here are some pictures of the soffits and roof deck.

There are charts and resources on the two websites I mentioned that can tell you the minimum levels of closed cell foam to avoid condensation issues.

DC

I've been working my way through both web sites. There is a wealth of information there. Thanks for the links.

Jim

 

[MagKarl]

Looks like standard framing to me, should be no reason at all you could not cut in soffit and ridge vents that would allow you to use inexpensive, standard building materials to finish it off. Sheet the ceiling and blow in insulation. Do it yourself and save big.

 

[kogerat]

Looks like standard framing to me, should be no reason at all you could not cut in soffit and ridge vents that would allow you to use inexpensive, standard building materials to finish it off. Sheet the ceiling and blow in insulation. Do it yourself and save big.

There are 2x4s between the rafters blocking off the soffits. Do the 2x4s serve some structural purpose, or can I just remove them? They look like they would block air flow from the soffit vent.

 

[DC73]

There are 2x4s between the rafters blocking off the soffits. Do the 2x4s serve some structural purpose, or can I just remove them? They look like they would block air flow from the soffit vent.

Obviously if you're going with spray foam, you'll just leave them be. But if you are considering a vented attic like MagKarl suggested, you'll have to find out if they serve some kind of structural purpose (maybe as nailing blocks for roof deck??) and whether or not they can be easily removed.

You could just drill holes in the middle of the blocks. I would think you could get enough holes in each block to provide adequate attic ventilation. There are ways to calculate how much area you need for soffit and ridge vents. Just make sure the holes drilled provide the needed area and do the same for the soffit vents you install to the outside.

Double check your trusses and make sure they can support the weight of the sheetrock. Apparently, not all trusses are designed for a sheet rock ceiling load.

DC

 

[MagKarl]

The blocks normally serve no purpose other than to keep birds out. Typically they have holes covered with screen and come in a bundle with the trusses.

 

[kogerat]

The blocks normally serve no purpose other than to keep birds out. Typically they have holes covered with screen and come in a bundle with the trusses.

Thanks for your reply. I removed a block and it looks as if it does give some support to the rafters as well as keep birds out. I could bore holes in the blocks and cover the holes with screen, put screen vents in the soffits, and install a full length ridge vent to ventilate the roof deck. This would be more labor intensive than spray foam, but my labor is cheap. The picture on left is before I removed the block, picture on right is after block is removed.

 

[kogerat]

There are ways to calculate how much area you need for soffit and ridge vents.

Thanks for your help. Is there a website where I can find out how to calculate the vent area needed to get the volume of air necessary for proper ventilation?
Edit:
I think you already gave me the place to look!
http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

 

[Platonic Solid]

Just another option: Sheet and insulate the ceiling, then install gable vents.

 

[earthworks]

Just another option: Sheet and insulate the ceiling, then install gable vents.

Yep.. I was about to chime in with this one as well. Gable vent at each end, sheet the ceiling, blow in or batting. Easiest and cheapest and perfectly acceptable as long as you have sufficient airflow in there.

 

[kogerat]

Platonic Solid & earthworks,
I was wondering about that. Even if I had to put a thermostatically controlled fan in one of the gable vents. Its not like I'm living in there.
Jim

 

[earthworks]

Platonic Solid & earthworks,
I was wondering about that. Even if I had to put a thermostatically controlled fan in one of the gable vents. Its not like I'm living in there.
Jim

I think even the fan is overkill. There is a specific formal to calculate airflow, and vents are sized accordingly. 300 sqft of attic space to 1sqft of ventilation. Ridge vent preferable, but a gable vent would work if at the peak.

 

[Platonic Solid]

If you do Gable vent (at peak like earthworks said), don't do ridge vent.

 

[DC73]

If you do Gable vent (at peak like earthworks said), don't do ridge vent.

Just curious, why do you say that? If gable vents are sufficient, you don't necessarily need any additional vents but I don't see why a combination of ridge vents and gable vents is a no-no. I know of two houses built without soffits that use gable vents down low as intakes and ridge vents up high for exhaust.

DC

 

[kogerat]

Thanks guys for the replies. If I were to go with gable vents, it seems to me that with a vent in either gable, a path for air flow would be created through the attic. Hence the idea of the fan, although it may be unnecessary.
I could also see gable vents mounted low in the gables with a ridge vent creating a convection flow as hot air rises. What I'm not clear on is does the ridge vent run the full length of the ridge, or there individual vents mounted along the length of the ridge at regular intervals.
Also, I thought that the idea of vent channels running from the soffits to the ridge was to create a sweeping action on the underside of the roof deck. Is this sweeping action unnecessary in a structure that is not continuously occupied?
I don't see this sweeping action occurring with gable vents, or a combination of gable and ridge vents. I do however, see a turning over of air with these venting strategies. Is this enough to vent a structure that is not going to be continuously occupied?
At the end of the day, I don't want waste money and time on venting and insulation solutions that are way overkill. But, I will invest the money and time necessary to do the job right to avoid problems down the road. Again, thanks guys for your input.
Jim

 

[kogerat]

Just curious, why do you say that? If gable vents are sufficient, you don't necessarily need any additional vents but I don't see why a combination of ridge vents and gable vents is a no-no. I know of two houses built without soffits that use gable vents down low as intakes and ridge vents up high for exhaust.

DC

I got the impression Platoinc Solid is saying vents at the top of the gable and ridge vents would be redundant due to their proximity to one another.

 

[Platonic Solid]

I got the impression Platoinc Solid is saying vents at the top of the gable and ridge vents would be redundant due to their proximity to one another.

Correct, thus reducing air flow in the center of the attic. I think it's a common mistake that people let their roofers talk them into.

I had to research this when my roofer offered to put ridge vents on my gable vented attic.
The air will enter the gable and exit the nearby ridge vent leaving the air in the center of the attic stagnant.

 

[DC73]

I had to research this when my roofer offered to put ridge vents on my gable vented attic.
The air will enter the gable and exit the nearby ridge vent leaving the air in the center of the attic stagnant.

Makes sense. Around here we don't have to ventilate for humidity reasons, just to exhaust heat so you see a lot of different methods, all of which seem to work.

What I'm not clear on is does the ridge vent run the full length of the ridge, or there individual vents mounted along the length of the ridge at regular intervals.

If you are going with soffit vents, I like a continuous ridge vent. If you are going with low mounted gable vents, given Platonic Solid's advice, I'd install some centrally located ridge vents so the air has to move from the low mounted gable vents to the center of the space and then out.

DC

 

[Platonic Solid]

... I know of two houses built without soffits that use gable vents down low as intakes and ridge vents up high for exhaust.

Must be a regional thing as I haven't seen that kind of setup around here in New England.