Roof insulation advice (rock wool vs foam board)
McLaughLiam
| Posted in Energy Efficiency and Durability on
$12K Difference for Rock Wool v Rigid Foam Board for Roof Insulation
Live in a leaky house built in 1700s with a uninsulated field stone basement with water issues, decaying fiberglass insulated crawlspace, half the roof uninsulated with the other half Fiberglass Batt and most gutters non functional.
Converted from Oil to a Heat Pump and currently invested in an exterior drainage project with Rigid Foam Board up to grade with gravel on top of perforated pipe which will be fed by updated gutters
Reducing a major draft from a poorly sealed door to open exposure in the basement and tackling the roof next.
Contractor proposing a 12k difference to insulate from the outside with Rockwool vs Rigid Foam Board. Tacking an additional 12k for Cedar Fascia to address the raised roof.
Do I:
a) Proceed to insulate entire roof with Rockwool at 12k additional expense? (Not including additional $12k for fascia)
b) Proceed to insulate entire roof with rigid foam board and save $12k? (Not including additional $12k for fascia)
c) Only insulate the uninsulated portion of the roof from the inside with Rockwool, avoid the need for fascia altogether and update the older batt insulation at a later date?
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Replies
I see no advantage to using rockwool here, so I'd use the rigid foam and save the $12k.
Is there any possibility to insulate from the inside though? If you could convert to a vented attic and insulate the attic floor with loose fill (usually cellulose here), that would probably be your most economical option and also would be a pretty robust way to go with little chance of failure.
Bill
Thanks for your input here. The more I think about it this seems like a more appropriate approach.
The original thought here was to try and eliminate heat loss as much as possible to support the Heat Pump (Mitsubishi Hyper Heat).
Other helpful context about the house:
• Insulated part of the attic is conditioned and the uninsulated part is not. There’s also an air handler and ductwork in the uninsulated part of the attic.
• Very low ceilings (sub 7 foot) across the board.
• There are 5 fireplaces with modern flues and I just sealed one with a high efficiency wood stove insert.
• There are currently 2 open exposure door cavities on the first floor (thin half inch doors on the interior with a 6 inch gap to another half inch wood door that opens to the outside) and the open exposure basement door. The two doors on the first floor will be insulated and sealed entirely and we’re replacing the basement door with something more winterized and sealing the joists.
Do I even need to insulate the unconditioned space in the attic assuming I can eliminate the biggest leaks in the basement and on the first floor? Am I better off spending the money to replace decaying batt fiberglass insulation in the crawl space?
You may want to consider prioritizing your projects and doing them in order from biggest bang for the buck to least. This can help spread out your costs while getting you the most improvement as quickly as possible.
Note that heat pumps don't need super duper levels of insulation to work, but overall efficiency of heat pumps will be better in a well insulated and air sealed home -- just as any other means of heating will use less overall energy in such a home. I don't like people thinking heat pumps can't work without a bunch of other insulating/air sealing work.
Regarding the other projects you've mentioned, I would consider insulating the roofline of the unconditioned attic if it contains a lot of ductwork that you can't relocate or properly seal and insulate. It's always best to avoid running any mechanicals in the attic, but sometimes you're stuck. The important thing is that you maintain your building envelope when insulating, which means to insulate the external boundaries of the home only -- don't, for example, insulate the floor AND roofline in the attic and leave it vented. You need to insulate the floor OR the roofline, and if you insulate the roofline, you need to seal the attic.
Low ceilings in the home shouldn't affect your insulating/air sealing choices, since you'll be doing all the insulating and air sealing on exterior walls, attic roofs/floors, and basements/crawlsplaces. Living areas should be relatively unaffected in terms of the space taken up by insulating materials.
I don't really work with wood stoves so I can't speak to best practices here.
I would make sure you meet any emergency egress requirements when you're considering removing any doors. I would make those determinations first, then work on insulating after you know which doors you can or cannot remove. I would use at least what I would call "standard thickness" doors at all exterior locations. Steel doors with a foam core can get around R6-R8 or so, and they're not that expensive, so I'd use those as a minimum. BE SURE to have good compression weatherstripping on all of these exterior doors. If this is all DIY work, pre-hung doors will make things a lot easier for you.
Yes, you really need to insulate the attic if it's over conditioned space. My preference would be a vented attic with loose fill (blown) cellulose on the attic floor, but ductwork may change that depending on your particulars. Usually a vented attic with loose fill insulation on the floor is the cheapest way to go, and it's also a very robust system with little chance of moisture problems. You will cut down on stack effect by sealing all the low level air leaks, but that works in conjunction with the insulation in the attic to help maximize your overall energy efficiency. I would try to air seal the exterior walls, attic floor, and basement/crawlspace (pay particular attention to the rim joist areas), then insulate the attic floor, then insulate the basement/crawlspace, in that order.
You fieldstone basement "with water issues" is going to be tricky. Closed cell spray foam on the walls is probably your best option, but you really need to fix the water issues first, which means an exterior drainage and/or grading project. That can be expensive. You can insulate the basement floor, but that's not ideal.
Bill
Thanks for the detail Bill. I appreciate the confidence in the Heat Pump with only decent insulation—that will help me sleep better at night.
We're good on egress (plenty of ground floor windows and two main doors plus an escape through the basement door + hatch under the porch) and good notes on door sealing.
Forgot to mention that the unconditioned attic floor has existing fiberglass batt insulation in varying states of quality. I like your approach of insulating the floor of the attic vs the roof. Think I'm going to stick with the batt insulation that's there for now and update areas where there's a clear need for it. Will have the contractor seal where they can on exterior areas and then put my money towards replacing the decaying batt insulation in the crawl space.
Regarding the basement water issues. I'm about to complete an entirely new drainage project around the exterior updating gutters, downspouts which will feed into 6" perforated pipe which is backed by landscaping fabric, plastic and 2" rigid foam board around the exterior foundation up to grade.
My hope is this will eliminate the majority of water issues, but may have to install a drainage system in the fieldstone basement if further water comes up from the ground.
My Contractor is against the idea of insulating the rim joists because of fear of moisture issues and rot—even with the upgraded drainage solutions in place due to his concern around how fieldstone permeates water. What are your thoughts there?