Insulating and finishing a "loft" in our 39' by 36' pole barn / garage. The space will be used by kids and the climate will not be continuously controlled(although I would like to build properly incase it would ever be continuously controlled). I will be used reclaimed 2.5" polyiso rigid sheets (not foil backed). Roof rafters are 2X6. If I use 2 layers (5") between rafters and have 1/2" air gap to keep a "cold" roof is this sufficient? From a interior air / vapor side, would would be recommended? Thanks in advance, Ian.

Posted By ian_upton on 28 Oct 2016 11:01 AM
Roof rafters are 2X6. If I use 2 layers (5") between rafters and have 1/2" air gap to keep a "cold" roof is this sufficient?

The 1/2" is fine but you need a way to move that 1/2" of air up and out of the space. Vent the soffits and the upper part of the roof. You can use a vented ridge cap or roof jacks for the upper part of the roof. Keep your roof jacks up near the ridge, on one side of the roof only and do not stagger them. If you are ever going to make that a "room" and not just a loft, you should plan now for the"correct" amount of ventilation necessary. There are a number of vent calculators online. I like Lomanco vents, so here's their calculator: http://www.lomanco.com/vent-resources/vent-calculator

Also consider these (where there isn't flow to the ridge). Air seal the interior side well and it takes very little air gap to remove moisture from air that leaks through. OTOH, see here if ice dams are possible. And review code requirements (probably 1").

Thanks for the response. So a little more clarity.

The garage is already built. There are soffit vents and there is continuous vented ridge cap. I will try and attach some pictures later, but the garage has two different roof lines.

Glad to hear that the 1/2" (but I will check code) of space will work. Much appreciated.

Since the garage (detached pole barn) has never been heated, ice damming have not been present. I would hope that the airspace plus 5" of rigid Polyiso would be enough. There will of course be the thermal bridging of the rafters themselves. I will take a closer look and see if it makes sense to simply put the foam sheets on the inside of the rafters.

Again thank you.

You have to make sure that your soffit vents are free from the foam. If the foam is up close and touching you will not get the needed ventilation. I usually cut the foam back 4-6" from the vents and cut it at an angle to get the air flow. Don't let it get too close to the vents.

Also, just because you have ridge cap venting does not mean you fit within the calculator formula. It's best to keep the soffits balanced (sq inches) with the sq inches at the ridge - but if you must increase one over the other, increase your soffit venting.

A half inch of vent space under the roof deck would not meet code- it needs to be at least an inch! See R806.3 (near the bottom of the page:

http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%208.html

If it's a low-pitched roof (3:12 or lower) give it 2", even though code only demands 1".

Thanks. Will make sure I get the full 2 inches. Probably will sister into the rafters to get the additional depth.

If you're going to add only an inch or two of depth to the rafters, use the 2.5" reclaimed foam as a continuous layer (or edge strips) rather than adding R1/inch lumber then filling between it with cut'n'cobbled foam.

Dana - I had thought about that, but how will I attached here the drywall through 2.5" foam over the 2X rafter?

Use 4" deck screws?

Based on Dana's suggestion to add foam as edge strips on my Nashville rehab a couple years ago, I simply screwed 1 x material to the foam as the backer for the drywall to attach to. They are going to use a ton of screws on the drywall. Better to let them use standard 1 1/4" screws for that.

Nashvegas' solution is one of the standard methods of dealing with it. Some people just opt for longer drywall screws for smaller projects (or thinner foam.)

A bit of an update. I have plant to sister as shown in the following picture. There is a gap between the wood that will either naturally fill with dense pack cellulose or I will shoot some foam in between.



Thinking of putting down some baseboard electric heat while it is not yet closed in. I do not have access to do a proper manual J. Here is what I have.

Zip Code: 44262.

This is a loft space in a garage.
The flat of the ceiling will have blown cellulose at R38
The vaulted portion of the ceiling is fully exterior on 1 side and 1/2 exterior on the other side. Loft has the small window shown above. Because of the roof line to the left in the picture, part of the interior of the loft is "inside" albeit unconditioned space. This part of ceiling has 2.5" of polyiso between the rafters + 4" of dense pack cellulose (my best guess is that is an R38 as well.

The walls have 2.5" of polyiso between the studs. (1 end wall is to the outside (visible in picture) the other end wall will be insulated the same, but effectively be an interior wall.) The "knee walls" are 63" high.
The floor (12" floor joists) plan to have R38 fiberglass batting then closed in with 1/2" OSB to the interior of the garage.
Windows and doors: window shown in picture. Similar window on other side. Door is a steel entry door. Nothing fancy on door or windows.

Overall dimensions for the room will be 14' X 35'

In order to have this as comfortable teen space in the dead of winter, what do I need for electric baseboard heaters? 220V is available.

Thanks in advance.

Ian.

I would also add that although I do not plan on doing a blower door test, I have and will continue to spend considerable time with air sealing.

I am coming up with a heating load of just under 7000 BTU.

So 2000W of electric baseboard heat would be sufficient?

Just an update on heating requirements.

The past few days have been in the upper single digits. A 1500 W utility heater has kept the insulated space very warm.

The heater has a dial thermostat without a scale. It does not short cycle but it also does not run constantly.

Very pleased with the ability to keep this space warm.

Ian.

High R/inch foam thermally bridged between studs is pretty much wasted due to the thermal bridging. It's much more useful to use it in continuous sheets over the framing & cavity fill, where it delivers it's full R.

A half ton (or 3/4 ton) point terminal heat pump (PTHP) is still uner $1000 for the hardware, including the wiring, breaker, and the wall sleeve, and is probably a more useful way to condition the space than 1500-2000 watts of baseboard, since it allows you to air condition as well as heat, and it uses only half the amount of power to heat the place that baseboard does when it's 25F outside, even less when it's 35F or warmer.

http://www.energyconscious.com/lg-7-300-btu-packaged-terminal-air-conditioner-heat-pump-2-5kw-lp073hd2b.html

http://www.totalhomesupply.com/7000-btu-class-ptac-heat-pump-20-amp/p/AMANA-PTH073G35AXXX

https://www.comfortup.com/gree-etac-07hc230v20a-a-7-000-btu-12-2-eer-heat-cool-208-230v-3kw-heat-residential-commercial-use