Image Credit: Justin Brown
More Q&A Spotlight
In coastal Massachusetts, Justin Brown is looking for ways to upgrade the energy performance of his very old house. It sounds as if previous owners had taken some steps to tighten up the building envelope, but they didn’t go far enough with either air sealing or insulation. Now, Brown wants to complete the job.
One area of particular concern is the attic. It’s insulated with a mix of fiberglass and cellulose, he writes in a Q&A post, but a cold snap this winter produced some frost on the underside of the roof sheathing.
Completing the insulation of the foundation walls also is on Brown’s to-do list (see the floor plan and images below). The foundation is a mix of poured concrete and fieldstone, and while Brown has added 3 inches of rigid insulation to the concrete foundation walls, the fieldstone walls remain to be done.
“I spent hours digging through cellulose and air sealed some wall plates and open wall cavities that the previous homeowner’s air-sealing and insulation contractors clearly missed,” Brown writes. “There is one 30-foot exterior wall plate in the eaves I cannot reach to air seal from inside the attic.”
The 10 recessed lights in the attic floor have been upgraded with LED airtight retrofits, but are not covered with any insulation. In the basement, the rim joists have been insulated with 2-inch extruded polystyrene and spray foam. But the inside of the foundation walls, a mix of poured concrete and fieldstone, have not been insulated.
Brown has found an ally in his efforts to upgrade. Technicians from Mass Save, a utility energy efficiency program, have visited the house and are offering to remove the existing attic insulation, seal any air leaks that have been overlooked, and add insulation to bring the entire…
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14 Comments
Great project.
I really enjoy these member case studies. Great job GBA.
While looking at Justin's photos, the crawlspace ducts caught my eye. Should Justin add these to his to-do list? The ducts are not insulated, and I did not see any mastic on the joints. Is this a minor issue overall?
Ducts
The duct seams have been sealed with foil tape. I've weighed insulating them, but decided against it since it's tough to do with rigid foam. And ultimately, if I spray foam the fieldstone walls, I'm not sure it's necessary?
Spray foaming the fieldstone walls
Some points to consider when looking at the old stone foundation:
1) Adding foam directly to the stone surface will change the hygrothermal balance, and thus the mortar composition and potential hydrostatic pressure.
2) Anything below the frostline (3 ft.) has a minimal delta T.
3) Once you add spray foam, you cannot remove it without great difficulty.
4) Is the delta T in this area great enough to justify that type of work?
5) The stone foundation is rather aesthetically pleasing, and had been structurally sound for literally hundreds of years.
Response to Shane Claflin
Shane,
In the middle of a cold, snowless winter, when the ground has frozen to a depth of 3 feet, you're right that the soil that is 3 feet 1 inch below grade hasn't frozen yet. But that unfrozen soil can still be at 33°F or 34°F.
If your basement is at 65°F, is that "a minimal delta-T"? You decide. I think that the soil is cold enough to justify wall insulation. (Lots of people agree with me, including energy modelers and code officials.)
I agree that a fieldstone foundation is aesthetically pleasing. But homeowners rarely set up a sofa in the basement facing the foundation, so that they can invite their guests downstairs to admire the stonework while sipping Cabernet Sauvignon and listening to Bach. It's one of those aesthetic features of a house that, for better or worse, is rarely appreciated.
Follow-Ups
Scott, Peter, Martin & Dana, thanks so much for your feedback, it's been invaluable to me.
Radon: Since we last spoke, I added a sub-slab depressurization system with an RP145 fan, tied into my lower basement French drain. My slab was super leaky I couldn’t believe it! I spent days filling small cracks, and had to invert the fan from suck to blow (no jokes) to find the last few air leaks. System has been on 6 days and radon has dropped from 6 to 2.9pCi/L and dropping. Humidity has dropped from the mid-50% range, to the mid-40% range. Basement temp has held steady so I don’t think I’m losing conditioned air. This is without tapping into the crawlspace or upper sump/drain. Since radon is typically highest in winter, and I’ve got 24” of snow on the ground, I’m hoping my number will keep dropping to the low-2 or high 1pCi/L range with the current setup.
Basement framing moisture: I will begin monitoring my sill and basement framing as I assess spray foaming in the fieldstone – is 1x per week for 6 months or so good? Across all seasons I assume?
Capillary break: I will add gutters and try to manage exterior water. I prefer not to add exterior insulation. If I opt to use closed cell spray foam on interior fieldstone:
1) Any benefit to stopping the spray foam at or just below the sill to allow for some inward sill drying?
2) Spray the foam right on the rock, or add a plastic sheet/barrier on which to spray the foam?
3) Assuming the wood moisture issue is solved for, do you have any concerns about entombing the fieldstone, in terms of the mortar and stone itself? I realize we don’t have hundreds of years of data on this…
Regarding basement ceiling insulation, I currently have faced batts. Basement temps are hovering 55-59 since I’ve insulated the poured concrete with 3” Thermax. If I turn the heat off, temps in my “vented cathedral ceiling wing” drop to 50-52. Should I remove the batts from the basement ceiling? Any downside (colder floor?), or will that help basement heat migrate up?
And relatedly, I can’t tell you how much I don’t like my vented cathedral ceiling. Am I correct that my only real option for solving that would be to rip down the drywall ceiling, spray foam under the rafters, ditch the vents, and re-drywall? If so, seems hard to justify cost…if I remove an outlet in the wall in that section of the house, on a windy day I can feel the wind wash as the builder obviously didn’t air seal the wall top plates, so everything is communicating.
Thank you everyone.
Justin
Response to Justin Brown
Justin,
Q. "If I opt to use closed cell spray foam on interior fieldstone, is there any benefit to stopping the spray foam at or just below the sill to allow for some inward sill drying?"
A. This is a judgment call. If you decide to leave your sill uninsulated, at least use caulk for air sealing.
Q. "If I opt to use closed cell spray foam on interior fieldstone, should I spray the foam right on the rock, or add a plastic sheet/barrier on which to spray the foam?"
A. If your foundation wall has a history of water entry, you may want to install dimple mat between the fieldstone and the spray foam. The dimple mat should lead to an interior French drain that leads to a sump. If your foundation wall has no history of water entry, you can apply the spray foam directly to the stones.
Q. "Assuming the wood moisture issue is resolved, do you have any concerns about entombing the fieldstone, in terms of the mortar and stone itself?"
A. No. Closed-cell spray foam adds structural strength to the stone wall, and can help hold loose stones in place.
Q. "Basement temps are hovering 55-59 since I’ve insulated the poured concrete with 3-inch Thermax. If I turn the heat off, temps in my 'vented cathedral ceiling wing' drop to 50-52. Should I remove the batts from the basement ceiling? Any downside (colder floor?), or will that help basement heat migrate up?"
A. If your basement is warmer than the room above, and you want more heat to rise to the room above, go ahead and remove the insulation between the two rooms.
Q. "I can’t tell you how much I don’t like my vented cathedral ceiling. Am I correct that my only real option for solving that would be to rip down the drywall ceiling, spray foam under the rafters, ditch the vents, and re-drywall?"
A. That's one approach. You can also install spray foam between the rafters from above, when it's time to re-roof. Or you can install a thick layer of rigid foam on the exterior side of your roof sheathing when you re-roof.
Response to Martin
Q. "Basement temps are hovering 55-59 since I’ve insulated the poured concrete with 3-inch Thermax. If I turn the heat off, temps in my 'vented cathedral ceiling wing' drop to 50-52. Should I remove the batts from the basement ceiling? Any downside (colder floor?), or will that help basement heat migrate up?"
A. If your basement is warmer than the room above, and you want more heat to rise to the room above, go ahead and remove the insulation between the two rooms.
JB: Okay, and before I do this...is there any potential downside? Will this cause heat from my first floor to migrate to basement, or is that not really of concern given the basement is warmer in that section of the house?
And once I sprayfoam the fieldstone/totally insulate the basement walls, should I remove all fiberglass insulation from basement ceiling?
What may be confusing me, is that a family member had an energy company spray foam their foundation, but then also install thermax underneath the subfloor between the ceiling joists, which I thought was unnecessary if basement walls are insulated? That house does have radiant flooring however, which could be the difference?
Thank you, just want to be sure before I go rip it down.
Response to Justin Brown
Justin,
When you basement is warmer than the room above, heat will flow from the basement to the room above.
When the room above the basement is warmer than the basement, heat will flow from the room above the basement to the basement. That's physics.
In general, when a basement has insulated walls, there is no need for insulation in the basement ceiling, unless (a) the insulation is part of a sound control strategy, or (b) the room above the basement has in-floor hydronic heating, and the homeowner wants to keep the basement cool.
Response to Martin
Martin,
Thank you.
The area where I want to remove the basement ceiling insulation has a vented cathedral ceiling on the 1st floor above (and that 1st floor area is the only part of house that gets colder than the basement).
I just want to be sure encouraging that warmer basement air to migrate through a floor to an area under a vented cathedral ceiling won't cause a net heat loss. Forgive me if this is a silly question...so holding that heat in the basement provides no net benefit (thermal mass or otherwise) in the face of potential heat loss out the cathedral ceiling?
Just want to confirm -- thank you!
Oh and, do you also agree on removing unvented crawlspace fiberglass ceiling insulation, when the walls are insulated? And, if the wall on one side is more a "slope," and not a vertical wall, should the spray foam run down some of that slope/floor? Or just insulate the vertical?
Thanks again.
Response to Justin Brown
Justin,
I don't think that removing the insulation in your basement ceiling is going to make a huge difference in the temperature of the room upstairs. But it will make some difference. The laws of physics require this to be true.
To the extent that you end up raising the temperature of the room above the basement, of course your rate of heat loss through the walls and ceiling of the room above the basement will increase slightly -- because the rate of heat loss through these walls and the ceiling depend on the delta-T. If you make the room warmer, the room will lose heat to the outdoors faster, and you will use more fuel. Again, that's physics.
If you want the room above the basement to be warmer, you'll need to burn some fuel to achieve that goal. If you want to save fuel, keep the room as cool as possible.
Q. "Do you also agree on removing unvented crawlspace fiberglass ceiling insulation, when the walls are insulated?"
A. Yes -- because fiberglass insulation above a crawl space often deteriorates and is hard to maintain in good condition. But if the fiberglass insulation is in good condition, removing it isn't urgent (or even necessary, as long as the pipes aren't in danger of freezing). For more information, see Building an Unvented Crawl Space.
Q. "If the wall on one side is more a 'slope,' and not a vertical wall, should the spray foam run down some of that slope/floor? Or just insulate the vertical?"
A. If you have a crawl space wall that leans, I might call up an engineer to evaluate the soundness of the wall. But if an engineer says that the wall is structurally sound, I would insulate all of it.
Response to Martin
Martin,
You are awesome, thank you.
“If you want the room above the basement to be warmer, you'll need to burn some fuel to achieve that goal. If you want to save fuel, keep the room as cool as possible.”
JB: Thank you. I apologize if you feel you’ve answered this and I’m being daft: assuming no changes to thermostat settings, does removing basement ceiling insulation and encouraging (albeit minor) heat migration up from a warmer basement accelerate total house heat loss, or simply change the walls (1st floor vs basement) through which the heat dissipates?
“A. Yes -- because fiberglass insulation above a crawl space often deteriorates and is hard to maintain in good condition. But if the fiberglass insulation is in good condition, removing it isn't urgent (or even necessary, as long as the pipes aren't in danger of freezing). For more information, see Building an Unvented Crawl Space.”
JB: It is in good condition. Crawlspace temp in winter is about 55-59F. I just am not 100% clear on whether, once the crawlspace and basement walls are 100% insulated and the basement is 100% in the thermal envelope, leaving the basement ceiling insulation detracts from, or enhances, overall heating efficiency?
“A. If you have a crawl space wall that leans, I might call up an engineer to evaluate the soundness of the wall. But if an engineer says that the wall is structurally sound, I would insulate all of it.”
JB: It’s not a leaning wall, but rather the crawlspace ground slopes from about 4’ of height clearance to about 1.5’ where it meets the rim joist (there is a picture on the blog). My question is, in a situation where there isn’t a full wall, should I spray foam insulate some of the poly-covered ground of that slope, or only the vertical surfaces?
Response to Justin Brown
Justin,
Q. "Assuming no changes to thermostat settings, does removing basement ceiling insulation and encouraging (albeit minor) heat migration up from a warmer basement accelerate total house heat loss, or simply change the walls (1st floor vs basement) through which the heat dissipates?"
A. To a minor extent, the change you describe would increase the rate of heat loss, because you would be sending heat upstairs to raise the temperature of the room above.
Q. "I just am not 100% clear on whether, once the crawlspace and basement walls are 100% insulated and the basement is 100% in the thermal envelope, leaving the basement ceiling insulation detracts from, or enhances, overall heating efficiency?"
A. Heating efficiency is determined by the type of boiler or furnace you have. I think you mean "overall fuel use." It's unclear whether removing the crawlspace ceiling insulation will change your annual fuel use. It depends partly on whether you have any heating equipment or heating appliances in the crawl space.
Q. "It’s not a leaning wall, but rather the crawlspace ground slopes from about 4’ of height clearance to about 1.5’ where it meets the rim joist (there is a picture on the blog). My question is, in a situation where there isn’t a full wall, should I spray foam insulate some of the poly-covered ground of that slope, or only the vertical surfaces?"
A. Insulating the floor of a crawl space is rarely cost-effective in terms of energy use. Don't bother to install spray foam on the floor.
Response to Martin
Martin,
“A. Heating efficiency is determined by the type of boiler or furnace you have. I think you mean "overall fuel use." It's unclear whether removing the crawlspace ceiling insulation will change your annual fuel use. It depends partly on whether you have any heating equipment or heating appliances in the crawl space.”
Yes, I mean overall fuel use. System 2000 boiler is in the fieldstone basement adjacent to the crawlspace. So it sounds like it’s best to assess framing moisture, and if I proceed with spray foaming the fieldstone, re-assess basement temps and determine at that point whether removing the basement ceiling insulation makes sense?
Thank you.
450 Gallons in oil for an entire year of heat + hot water in a classic ma. colonial? That can't be accurate. I would say 450/month?!? I'm so jealous. What am I doing wrong? I better keep reading.
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