Insulated Garage

[Corey Friedman]

Hello Everyone,

Looking for thoughts and opinions on a case I’m consulting with (non H.I. related).

2.5 car- wood frame detached garage, gable roof, no outside soffit vents, not sure if any roof vents but likely a couple of mushroom vents (now concealed). Walls 2x6 and rafters 2x6. Walls are insulated with R19 batts and drywalled. Rafters R19 with drywalled (ridge to top rafter plate) .

Chicago area climate. 20,000 btu electric hanging heater with the intent to keep temps around 40-50 degree F.

Thanks
Corey

[Jerry Peck]

Corey,

What is the applicable energy code and what are the requirements in it?

If there is no applicable energy code, then it's 'whatever they want to do',

Also, many (most?) energy codes only address 'for comfort conditioning', but do not address energy for processes or other 'non-comfort' uses ... such as keeping the inside temperature above freezing for whatever reason.

[Markus Keller]

What exactly is the question and what are you consulting on? Are you asking if the garage will stay warm enough?
The lack of venting worries me for other reasons beyond the fact that 2 roof vents are required. Buttoning up the garage super tight poses safety concerns depending on what is being stored in the garage.
However, you did not state a couple of very important factors. Are the overhead and service doors insulated; has full weather stripping been applied around doors to reduce air drafts and what kind of window is in place. Is there a decent insulated window or a glass block window that ends up being a cold sink.
At roughly 500 square feet the 20K heater should be feasible to keep the space warm enough. The real question would be what will it cost. You might want to call the manufacturer for efficiency and square footage ratings for the unit. You could also call your heating guy to figure a load calc.
I'd think about adding more ceiling insulation if possible. Sounds like they insulated the rafters rather than installing a ceiling. Not the best option.

[Jerry Peck]

Markus,

I've got a 2-1/2 car garage and it's 625 sq ft.

I wondered about the same things you did - not enough information given if the question was about will that 5.8 kW heater keep it warm.

Open that big garage door once when it's freezing outside and it'll let a lot of cold in ... but you guys know that better than I do.

[Corey Friedman]

Thanks for the reply guys,

Unknown at this time about the overhead door side door. Key concerns are insulating the rafter area and size of heater.

Thanks
Corey

[Markus Keller]

Sorry Corey, I think you are putting the cart before the horse. How can you accurately calculate heat load when you aren't factoring heat loss properly. Great way to either oversize or undersize heating capacity.
I realize insulating rafters is easy enough with batts but investing in higher R value now will likely decrease operating costs over the long term which is a potential big ticket bill monthly. ComEd isn't exactly cheap. You may want to help the client rethink their approach.
Also I suggest you rethink the venting concerns.

[Raymond Wand]

Yup an expensive way (electric) to heat a garage with a concrete slab that will become nothing but a heat sink. Cause no one thought to insulate under the slab.

Dependent on what is stored in garage radiant heat will only heat those objects close to the heat. And stuff in the shadow will not be heated.

Also no mention of height of ceiling which will affect the cost to heat/efficiency.

In my opinion 20K btu is not going to accomplish the job. That is very little heat, and no one has factored in weather conditions, temp, humidity, winds.

[Jerry Peck]

To clarify what I am envisioning:

- 2.5 car- wood frame detached garage ... (of unknown size, 2-8 ft wide doors or 1-16 ft wide door?, separate entry door for people?)
- gable roof, no outside soffit vents, not sure if any roof vents but likely a couple of mushroom vents (now concealed) ... (the 'mushroom' vents, if present, will allow moisture and air to infiltrate and exfiltrate through the 'roof-ceiling system' faster where they are than through the rest of the roof-ceiling system area)
- Walls 2x6 and rafters 2x6 ... (no comment on this item)
- Walls are insulated with R19 batts and drywalled ... (no comment on this item)
- Rafters R19 with drywalled (ridge to top rafter plate) ... (with the rafters being drywalled over {and likely no ventilation above the batts in the rafter spaces} I will presume that there are no ceiling joists - which is not a good thing - unless the ridge board is a ridge beam?)

- Chicago area climate ... (the weather and climate was addressed)
- 20,000 btu electric hanging heater ... (20,000 btu ~ 5.8kW)
- with the intent to keep temps around 40-50 degree F ... (you guys up there would know better about what size would be needed, but I would stay with electric as I would not want an open flame heater - NG/LPG/etc - in the garage)

Anyone else envisioning the same things?

[Thom Huggett]

"- Rafters R19 with drywalled (ridge to top rafter plate) ... (with the rafters being drywalled over {and likely no ventilation above the batts in the rafter spaces}"

Well said, Jerry, but the IRC does allow un-vented rafter bays in R806.5 if you meet all the specific conditions.

- - - Updated - - -

"- Rafters R19 with drywalled (ridge to top rafter plate) ... (with the rafters being drywalled over {and likely no ventilation above the batts in the rafter spaces}"

Well said, Jerry, but the IRC does allow un-vented rafter bays in R806.5 if you meet all the specific conditions.

[Jerry Peck]

"- Rafters R19 with drywalled (ridge to top rafter plate) ... (with the rafters being drywalled over {and likely no ventilation above the batts in the rafter spaces}"

Well said, Jerry, but the IRC does allow un-vented rafter bays in R806.5 if you meet all the specific conditions.

Thom,

Correct - what I was pointing out was that drywall over those insulated rafter bays indicated, to me, that there were no ceiling joists below ... because installing the drywall above ceiling joists would be a lot of work.

And that if there were no ceiling joists to serve as rafter ties, the ridge board would need to be a ridge beam (so there would not be any roof load trying to push the walls apart.

I was describing what Corey's information was describing to me ... sounds like you pictured about the same things?

[Mark Reinmiller]

Run a heat loss calculation to see what the heat loss is. One big unknown is the garage door. Some leak so much air that no amount of insulation makes up for that.