Building a Conditioned Mechanical Room
I live in zone 3. I have a 1915 2 story house with a simple hipped roof. The attic is ~1800 sq.ft with 12′ ceilings at the ridge. It is unconditioned and vented. It has an unconditioned large storage room at the top of the stairs (18’x28′) with an 8′ ceiling (so there is 4′ of vented attic space above the room). The room has a plywood floor and plywood walls and plywood ceilings. There is a small raised weatherstripped access door in the storage room to access the rest of the attic. There is a gas fired furnace with ducts in the unfinished attic space (outside the storage room). I want to replace this with a heat pump and have it moved into the large storage room at the top of the stairs and make this room conditioned so it can serve as a mechanical room and storage room. I will then seal where the ducts penetrate the plywood walls in the storage room, remove the old batts at the attic floor, install blown insulation at the attic floor, and spray foam on the ducts that are in the unconditioned attic space. I will leave the access door but hope to never enter the unconditioned attic again.
I have 2 questions.
#1: Do you see any problems with this plan?
#2: How should I insulate the conditioned storage room walls and ceiling? I’m planning to replace the plywood at the walls/ceiling so I open to all options.
Thank you for any advice/comments.
Jennifer
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Replies
Jennifer,
There is additional information here: https://www.greenbuildingadvisor.com/article/should-i-build-a-mechanical-room-in-my-attic
My not-an-expert opinion is the room is probably large enough for what you are describing. But I wonder if the steps required to convert it will be cost-effective compared with having the roofline insulated with R30 of open cell foam.
Thanks Steve. I've read this article a couple of times and found it helpful. I think I am in the minority in that I have existing walk-up stairs and a huge amount of space so a mechanical room is a reasonable idea. You are right it may be cheaper or similar cost to insulate at the rafters with open cell foam but I'm leaning towards not using foam and creating a conditioned mechanical room for a few reasons : the houes has true 2x4 rafters and 1x decking and I'm hesitant to spray foam that can't be removed later, the coverage area is very large so it will be expensive, the existing furnace is in a bad spot so it needs to be moved anyway, the ducts are a mess and need to be replaced/relocated anyway, I'm worried about moisture issues, and I'm worried about environmental/health implications of spraying so much foam. Do you or does anybody else have any ideas on the best way to insulate the mechanical room? Can I use batts on the walls/ceiling? What r-value should they be? Do I have to cover the outside of the room with a barrier? I plan to cover the inside with plywood or t&g flooring that I have saved and need to use. In general, the houes is leaky. The walls are insulated (poorly) and the basement is sealed (well) and the original double hung windows have storm windows. The doors have new weather stripping and thresholds. The attic is the biggest problem I think and I want to improve it when I replace/relocate the furnace/ducts soon. Any advice is appreciated. Thanks! Jennifer
I would be tempted to leave the plywood walls and ceiling as is and cover the inside with a rated rigid insulation such as Thermax (2" to 3" depending on budget). Tape the seams of the rigid for a solid air barrier. This insulation can be left exposed as the interior finished surface. For extra insulation, you can also blow loose fill above the mechanical room ceiling.
You would still have to air seal around the ceiling joists and the attic wall intersection. You can follow the recommendation here:
https://www.greenbuildingadvisor.com/article/what-architects-need-to-know-about-attic-kneewalls
Since you have a spray foam person there, you can also foam in that area to air seal.
Thanks Akos for the reply. Leaving the plywood in place works for me. There are a bunch of shelves attached and I'd like to leave those too. Can I install batts (walls are 2x4, ceiling is 2x6) and taped foam board to the outside? What r-value should I try to achieve at the walls and ceiling? I like the idea of loose fill above the mechanical room ceiling but how do I get it to stay up there? I can add something to hold it in place on 2 of the walls easily but the other 2 walls meet the rafters. I guess I could block at those rafters. I like the idea of having the foam people spray the intersection of the attic floor joists and attic walls since they will be foaming the ducts anyway. Is there a problem with them using closed cell foam in this spot? The plan is for them to use closed cell on the ducts so I'd want to use the same material.
Thank you!
Jennifer
If you are installing batts in the walls than you want the foam on the outside to hold the insulation in place. In this case it wouldn't make sense to put foam on the ceiling as it is much simpler to just pile more insulation up top. To hold the insulation above the ceiling in place you can make some baffles out of scrap pieces of plywood and nail them to the edge of the walls. Since your attic space is vented, there is no problems piling up insulation against the rafters and roof deck on the other sides.
Without the Thermax insulation on the inside, I would check with your building official first as the room might need 1/2 drywall over the plywood as a thermal barrier.
You are in warm enough climate that any amount of rigid foam outside the walls will work, more is always better but you quickly hit the law of diminishing returns above 2".
With the this type of setup, you still need a solid warm side air barrier, so in this case, tape up the seams of the plywood with a quality tape (Zip, 3m 8067, Tesco Vana). Make sure to tape the ceiling to wall intersections as well as all the plywood seams on the ceiling. Also seal around any device boxes in the walls or on the ceiling.
Closed cell foam is fine for sealing the ceiling/floor joist to wall intersection. Make sure they carry the spray foam up across the utility room wall bottom plate to the back of the existing plywood. Your goal is to create an air tight box starting from the ceiling of your house, up the utility room walls across the utility room ceiling and back down.
While the spray foam guy is there, ask them to flash over all wall top plates and any ceiling device boxes of the floor bellow. This is the simplest way to seal up the ceiling of the house which is typically a big source of air leaks in most older houses.
It's also important to have the rigid foam to act as an air barrier for the batts. Batts really need air barriers on all sides to maintain rated R value. If for some reason you can't do that, mineral wool batts are less effected by lack of air barriers than are fiberglass batts. Ideally, mineral wool batts should have air barriers on all sides too though.
You mention you'll have ducts leaving the conditioned space. Ideally you want the ducts in the conditioned space too. If you can't do that, you will want to make sure all the ducts are well air sealed AND insulated.
Bill
Thank you Akos and Zephyr7. I appreciate the info!