House Insulation

[jeepinmatt]

In the next 2 weeks, they will be doing insulation on the house we're building, and I'm tossing around a couple options. I currently have quotes of about $6000 fiberglass and $8500 for open cell spray foam. I plan to go with the spray foam because I really like the idea and see the benefits. Our current house is fiberglass batts and, in my opinion, has thermal performance issues because of improper batt installation.

The general layout of the house is a fairly simply 2 story plus attic above the entire 2nd story and full basement below the entire first story. All exterior walls are 2x6, except half of the basement is poured concrete walls.

I've read that there are different densities of spray foam with the higher densities having higher R values. I'm also aware of the concept of diminishing returns. Are there any advantages to getting it done with medium or high density closed cell foam?

Another question is to do the attic floor, attic roof, or both? And if both, what else needs to be considered?

The biggest decision I'm deliberating is how to insulate between the basement and the first floor. I will have a wood stove in the basement, and plan to use that as a semi-primary heat source in the winter. I will be fully insulating the basement stud walls. I haven't yet decided whether or not to do the poured walls. But it seems like using really good insulation between basement and 1st floor would be a bit counter intuitive for the sake of heating.

Here's a couple pics and the general layout of the plan.

@trailhugger Feel free to upload more detailed plans if that's ok. Not sure if you want that level of detail on the internet.

 

[Bushwhacker]

Looking good Matt!!

I’m no expert but my thoughts are no insulation in the first floor system if you will have a wood stove as a primary heat source in the basement. That’s the way my parents last 2 houses were that we built in Virginia and wood stove was our main heat source.

 

[kaiser715]

I had a bad case of "analysis paralysis" on insulation when we built. Looked at every option from fiberglas batt, mineral wool, XPS outside the wall, open or closed spray foam, etc, etc, etc. The more I read, the more I decided that old school was better.

You can build a house too tight, which brings on it's own problems, which we wanted to avoid. You have to look at cost vs savings payoff. And you do quickly reach a point of diminishing returns. A lot of the various insulation types/methods pay off in much colder (or hotter) climates. Not so much here in NC. And half the year the vapor barrier is on the wrong side.

So far I am happy with our decision. We get a daily report from CEMC (our power company) showing energy cost for previous day Never been more than a few cents over 5 bucks (July thru December (current)). Average cooling cost for July and August was $2.83 per day. That's an all-electric house, 3000sf one level, conditioned crawl. Haven't figured average for December yet, or seen the bill, but looking back we've had a handful of well under $2 days, and a handful of $5 days, so I don't expect any great surprises when the coldest of Jan/Feb gets here. We kept thermostat on 77 for ac, 72 for heat.

If that $2500 difference in insulation type saved you an average of twenty bucks a month (spring and fall, it wouldn't do much for you, winter/summer would save more), that's 125 months payoff, just over 10 years. Stick that cash in the market, and you'd do better than the few dollars saved in energy. Especially if you are heating with your own wood (other than the time/expense of cutting slightly more wood to burn one way vs. the other).

 

[Fabrik8]

Are you doing any sheet foam between the siding and the sheathing, or any other methods to thermally decouple the framing? If you're not, then the insulation will quickly run to diminishing returns. Remember that the house is a combination of thermal sinks/sources, and using the best insulation is barely going to give any extra gain if it's in parallel with things like headers and studs/splines that are causing thermal loss.

I agree with the installation problems with fiberglass batts. That's really the biggest downside; fiberglass is a decent insulator but is so sensitive to installation quality because of how floppy and shapeless it is. Finding a good insulation crew is the only thing you can do, and you'll pay a lot more because the amount of installation labor is so much greater when it's done properly. It's kind of sad really.

Mineral wool is awesome, but it's pricey and it's slow to install as I'm sure your research has told you. But you'll usually not find a company that will do a shitty job with mineral wool because they aren't dealing with the same types of jobs as the quick-and-dirty fiberglass guys. A good mineral wool installation is a thing of beauty, and the guys that do it are like craftsmen.

Be careful with roof insulation, especially with spray foam. It often has to be done as a coordinated system with the roof deck and whatever type of sealed/ventilated attic to make sure you don't end up with trapped condensed moisture and a rotten roof. It usually comes down to the type of attic (sealed or ventilated).
Just be aware that there may be more to it than just spraying a bunch of joist/rafter/truss bays in the attic area with spray foam and calling it good. Honestly, because the house has already been roofed, your option is probably a ventilated attic (insulation on the adjacent/kneewall and the floor, and not the roof) without a lot of additional work.

Almost no one insulates the exterior rim joists unless doing a serious high efficiency build, but that's another thing to think about. It can keep the floors warmer too.

 

[mbalbritton]

I’ll say this much.

My dad went in and spray foamed the bottom side of the roof that included a walk in attic storage area that is unconditioned. Used to be hot as blue blazes in the summer heat. Now, it’s feels the same as the conditioned areas of the house when he walks in.

Knowing the room he’s talking about, I’m sold on spraying the underside of the roof. I haven’t asked him about his power bill since he did it, but it has to be lower.

 

[mbalbritton]

Are you doing any sheet foam between the siding and the sheathing, or any other methods to thermally decouple the framing? If you're not, then the insulation will quickly run to diminishing returns. Remember that the house is a combination of thermal sinks/sources, and using the best insulation is barely going to give any extra gain if it's in parallel with things like headers and studs/splines that are causing thermal loss.

I agree with the installation problems with fiberglass batts. That's really the biggest downside; fiberglass is a decent insulator but is so sensitive to installation quality because of how floppy and shapeless it is. Finding a good insulation crew is the only thing you can do, and you'll pay a lot more because the amount of installation labor is so much greater when it's done properly. It's kind of sad really.

Mineral wool is awesome, but it's pricey and it's slow to install as I'm sure your research has told you. But you'll usually not find a company that will do a shitty job with mineral wool because they aren't dealing with the same types of jobs as the quick-and-dirty fiberglass guys. A good mineral wool installation is a thing of beauty, and the guys that do it are like craftsmen.

Be careful with roof insulation, especially with spray foam. It often has to be done as a coordinated system with the roof deck and whatever type of sealed/ventilated attic to make sure you don't end up with trapped condensed moisture and a rotten roof. It usually comes down to the type of attic (sealed or ventilated).
Just be aware that there may be more to it than just spraying a bunch of joist/rafter/truss bays in the attic area with spray foam and calling it good. Honestly, because the house has already been roofed, your option is probably a ventilated attic (insulation on the adjacent/kneewall and the floor, and not the roof) without a lot of additional work.

Almost no one insulates the exterior rim joists unless doing a serious high efficiency build, but that's another thing to think about. It can keep the floors warmer too.

Are you doing any sheet foam between the siding and the sheathing, or any other methods to thermally decouple the framing? If you're not, then the insulation will quickly run to diminishing returns. Remember that the house is a combination of thermal sinks/sources, and using the best insulation is barely going to give any extra gain if it's in parallel with things like headers and studs/splines that are causing thermal loss.

I agree with the installation problems with fiberglass batts. That's really the biggest downside; fiberglass is a decent insulator but is so sensitive to installation quality because of how floppy and shapeless it is. Finding a good insulation crew is the only thing you can do, and you'll pay a lot more because the amount of installation labor is so much greater when it's done properly. It's kind of sad really.

Mineral wool is awesome, but it's pricey and it's slow to install as I'm sure your research has told you. But you'll usually not find a company that will do a shitty job with mineral wool because they aren't dealing with the same types of jobs as the quick-and-dirty fiberglass guys. A good mineral wool installation is a thing of beauty, and the guys that do it are like craftsmen.

Be careful with roof insulation, especially with spray foam. It often has to be done as a coordinated system with the roof deck and whatever type of sealed/ventilated attic to make sure you don't end up with trapped condensed moisture and a rotten roof. It usually comes down to the type of attic (sealed or ventilated).
Just be aware that there may be more to it than just spraying a bunch of joist/rafter/truss bays in the attic area with spray foam and calling it good. Honestly, because the house has already been roofed, your option is probably a ventilated attic (insulation on the adjacent/kneewall and the floor, and not the roof) without a lot of additional work.

Almost no one insulates the exterior rim joists unless doing a serious high efficiency build, but that's another thing to think about. It can keep the floors warmer too.

Oh, you SOB, now you’re making me question everything I said... bastard!

 

[shawn]

Looks like the roof and a roll air barrier have already been installed, so there are a bunch of options that are now closed to you.

The open cell isn't bringing any extra R/in to the party. The benefit is - if it's installed correctly - you should have some insulation behind receptacle boxes and stuff like that, and it's an air barrier (fiberglass isn't).

I would not spray the underside of the roof sheathing without also having installed XPS under the shingles. Spraying the underside of the roof sheathing moves the dew point into the sheathing, causing wetting of the wood and rapid deterioration (starting primarily at the ridge). This can be mitigated by installing a continuous ridge vent with an air barrier.

If you go fiberglass, make sure you get blocking at the soffits to prevent wind wash through the insulation, do R-15 in the first floor walls and R-19 or 21 in the second floor knee walls, and be prepared to fix a bunch of it yourself (particularly in the attic). I would do XPS foam installed tight to the concrete wall in the basement with insect inspection gaps at the top plate as required by code.

 

[rockcity]

Hmm. This has me thinking as well. I should start building a house in the next year, I’m following so I can plan now.

 

[shawn]

I'm going back and forth on the roof assembly. I think the thermal performance of fiberglass is going to be less than desired, but haven't convinced myself that the water vapor can be controlled with other insulating systems.

Where is the mechanical equipment?

 

[awheelterd]

Following as well.

I'm a few weeks away from having to pull the trigger one way or the other. I'm pretty set on foaming the roof deck.. or at least I was. What needs to be done to the roof to foam it safely? My builder is good so I trust him, but I'm a trust but verify type of guy. Especially when we're talking about the amount of money I'm sinking into this house.

I have pretty much ruled out foaming the entire house. The cost/benefit just doesn't seem worth it. I'm doing crane board siding so there will be a bit of a thermal break with the 1/2" of foam backing.

 

[kaiser715]

Here is a graphic I posted a while back:

source: The Diminishing Returns of Adding More Insulation

 

[rodney eppes]

So Much info. No wonder it gets confusing! I pretty much agree with everything said. Wish I had known this, before you shingled! More than 20 years back, a couple materials came out for heat barrier, in the attic. Basically, rolls of material with reflective backing. Just like you might insulate the floorboard of a Jeep! One material could be used on top of sheathing, & shingle over it. The other, would be stapled inside, under the sheathing. You might could still go with the inside one. Only, I've never seen anyone actually Use these barriers, not even "This Old House". As for basement ceiling/first floor, I always heard you loose 10 - 20 % through the floor. My floor is Not insulated, & the original owner had a wood stove, in the basement. Personally, I like it the way it is, because I feel it helps that 10% to heat & cool my basement, which is also my workshop! i worked a house many moons back, built Energy efficient, to 1970s knowledge! Had 2x6" walls & doors, tongue & groove roof sheathing, & full basement. The Extra wide stairs, were in the middle of the single story house, so the heat from the basement wood stove would rise centrally. Man also figured his duct system would/could circulate the air, by forced fan. And, being built on Lake Wylie, he had geothermal heat pumps! Only effect I've seen of un-insulated floor, is I can get smells in the house! Like grass, from the lawn tractor, paint, ect.

 

[awheelterd]

Here is a graphic I posted a while back:

source: The Diminishing Returns of Adding More Insulation

This was the graph that convinced me not to spend the extra money on sprayed walls.

 

[rabb11d]

We sprayed our house, for the size of our house I think our power bills a pretty good. Im glad we did it. Hope it doesn't screw me at some point later from some moisture issue.

I recommend insulating the basement but not the ceiling/floor. Ours isn't and it has no HVAC, stays a couple degrees cooler year round with no moisture issues.

Im no expert though, just a smuck that built a house 5 years ago.

Are you going to have a door between the 1st and 2nd floor?

Sent from my SM-G950U using Tapatalk

 

[shawn]

Vented attic best practices:

https://buildingscience.com/sites/default/files/migrate/pdf/PA_Crash_Course_Roof_Venting_FHB.pdf

Conditioned attic reference:

https://buildingscience.com/sites/d...pray Foam Insulation Under Roof Sheathing.pdf

Check out the case study house in Coquitlam, BC. It's in climate zone 4C, but used 5.5" of open cell polyurethane on the underside of the roof sheathing and had no wetting issues.

I haven't found a good answer on whether or not you need two coats of latex on the interior side of the polyurethane. It seems clear that you will need to maintain 40% interior RH, though. And I'm a bit concerned with the air tightness of the house if you go this route - do you have plans for an ERV?

 

[Fabrik8]

I'm going back and forth on the roof assembly. I think the thermal performance of fiberglass is going to be less than desired, but haven't convinced myself that the water vapor can be controlled with other insulating systems.

I was just about to post about the continuous insulation chutes and saw you had posted the PDFs of the same stuff. Site built baffles made from plywood or OSB and spacer strips of 1x2 or 2x2 work well and are cheap. The one PDF has the ISO foam method, and it's easy to cut, but OSB is cheaper and you can do the entire thing with a saw and a finish nailer. You put a strip of 1x2 or 2x2 against the rafters on either side to make a spacer block, finish nailing into the rafter, and then rip a piece of 3/8 OSB to close out the chute (doesn't have to be perfect, because the edges will be caulked). It makes perfect 1.5" chutes, and they're really solid. Time consuming if the roof has any amount of area though.

The pain in the ass with any wooden site-built baffle (if the shingles are already installed) is that you have to nip nails or notch the spacer to work around the roofing nails which will be in the way. Easy, but adds another time consuming step. There's no reason the spacer needs to be continuous though, because it's just a spacer and nailer for the OSB, so you can gap the spacer to miss any nails as well.
Normally you would seal the OSB edge to the rafter with canned foam or caulk if using batt-type insulation, but that's totally unnecessary with spray foam for obvious reasons of self-sealing.

Anyway, that's a really good way to do a traditional vented roof with spray foam, and not have to worry about vapor control. At that point, it's no different than a cathedral/vaulted ceiling in the conditioned space of a house, but with much more effective vent chutes than any normal house I've seen (our insulation baffles suck on our vaulted ceilings, but we have fiberglass batts and probably problems with wind washing too).

You're still going to have issues with that valley with ANY kind of ventilated roof system, because of the deadhead rafter bays in the valley. There's just no effective way around that problem. The only way to solve that (after shingles are installed) is to keep the attic unconditioned. Or drill a bunch of cross holes in the rafters, which is sketchy as hell and probably completely ineffective after the first rafter bay.

Looking at the plans and the pictures, you're really not going to have much attic left above the bedrooms though, right? Just a little wedge...?

 

[R Q]

I am no expert whatsoever. Use fiberglass and make sure they do a proper installation. Consider doing internal walls for sound deadening. I did in mine and it’s wonderful. Don’t do the roof rafters, let it breathe. Put your money and energy towards the best windows money can buy. You can spend 2x on insulation and counteract it all with budget windows.
Do Not overthink it.

 

[49willys]

Insulating interior walls is a very neat trick that most people overlook, lived in a house that had them, you don't notice it at first, makes for an incredibly quite house ever with hardwood floors or kids. Friend of mine purchased a wood furnace for his house, replaced an old oil furnace, reused the duct work to circulate the air. I had lived in a place with the stove in the basement, had to cut return vents in the floor so the air would move. It's frustrating having the basement at 90 and 50 upstairs! It was an old house, before plywood, everything was rough cut locust, when doing a bathroom remodel discovered the R11 fiberglass stapled "flat" to the exterior wall!

 

[Mac5005]

Spend more on insulation where the greatest temperature difference will be. These will be the areas of greatest effectiveness.

Locate your fresh air intake(s) for hvac in a porch ceiling as far away from the ground as possible.

Good time to rough in those and the filters for them as well as the ERV if you go that route. Only use fiberglass where moisture vapor can’t get to it.

It loses 36% of R value if the moisture content goes up 1.5%.

Only insulate between conditioned spaces for sound dampening.

Get the 2nd floor hvac system and ductwork into an insulated area, not in an uninsulated part of the attic.

I assume you are insulating the basement walls, so that should cover the 1st floor HVAC being in an insulated area.

Not sure how to achieve it at this point, but I’d do a sealed attic with spray foam and a dehumidifier, same with crawlspace/basement.

 

[paradisePWoffrd]

I would not insulated between basement and 1st.

Unless it is planned to be conditioned also, I would plan to run a dehumidifier in the basement.

I would also suggest running duct from a hood over the wood stove through the house with a small blower. It adds some cost, but makes the stove more effective. I think most ppl tie into HVAC ducting. My house is separate, and I like it, but it's also only 1 story.

 

[shawn]

I was just about to post about the continuous insulation chutes and saw you had posted the PDFs of the same stuff. Site built baffles made from plywood or OSB and spacer strips of 1x2 or 2x2 work well and are cheap. The one PDF has the ISO foam method, and it's easy to cut, but OSB is cheaper and you can do the entire thing with a saw and a finish nailer. You put a strip of 1x2 or 2x2 against the rafters on either side to make a spacer block, finish nailing into the rafter, and then rip a piece of 3/8 OSB to close out the chute (doesn't have to be perfect, because the edges will be caulked). It makes perfect 1.5" chutes, and they're really solid. Time consuming if the roof has any amount of area though.

You're overthinking it. The chute only needs to extend to a point above the insulation (8-12" should do). It's just to prevent wind washing of the batts. You don't want to extend them all the way because it creates a restriction.

You can also build them out of the prefab baffles.

 

[moldman05]

When I think about insulation there a few things I try to remember. Put the most insulation where there is going to be the biggest temperature difference. That would dictate highest R value in the ceiling adjacent to attic or roof, then the exterior walls,and then the
floor over ground, basement or crawlspace.
Most thermal insulation performs by trapping atmospheric air. This is when moisture in the air affects performance. Dry air is a good insulator. But as moisture is added the Rvalue decreases. I have a study that states “increasing moisture by 1 1/2 % in fiberglass insulation in an attic decreases The R value by 36%. So wet air is not a good insulator. Couple this with rising dew points the last 3 summers in NC I question any open cell insulation where it is exposed to atmospheric air.
Fiberglass is very good if dry but it has to be installed very carefully in a cavity such as a wall. Voids in the insulation becacause of pipes , wires, or blocking leaders to air movement and thermal transmission.
Closed cell insulation and I would include thermal pane windows in that category do not depend on atmospheric air so they perform better in constantly changing and increasing moisture conditions.
How we build has to evolve to keep up availability of materials, cost of energy, efficiency of HVAC, labor costs, and changes or as I say climate cycling.
Most buildings can survive small changes in moistures levels but when buildings become wet they deteriorate. Good roofs and leakproof water and drainage systems protect us. Condensation of the moisture in the air can provide a huge supply of water during many of the months in NC. Condensation occurs when air loaded with moisture is cooled by colder air ( rain ) or the colder air and surfaces in and around our air conditioned buildings. This is where insulation can help by preventing a temperature difference across a surface.
What do I like? Closed cell under roof deck with a non vented attic. Walls , anything installed properly. My house built 30 years ago with exterior walls insulated with R 11 fiberglass sandwiched between 3/4 closed cell sheathing on the outside and 3/4 closed cell inside with all seams taped with foil tape. Floors don’t need insulation in the summer if they are over the ground such as a closed crawl space or basement. Let the 55 degree ground help keep us cool. In the winter we might need some if it’s over a closed crawlspace. More if it’s over a vented crawlspace. Fiberglass would be good because it works when kept dry and the crawlspace should be dry in the winter when atmospheric air is drier

 

[shawn]

Insulating interior walls is a very neat trick that most people overlook, lived in a house that had them, you don't notice it at first, makes for an incredibly quite house ever with hardwood floors or kids.

It makes a lot less of a difference than you'd think. In order to get any benefit, you need to putty pad the receptacle boxes, avoid back to back penetrations, caulk the GWB to the floor sheathing, etc. And you're only going to see an STC jump of a few points (34 to38??? Assuming mineral fiber). For the work involved, you'll get better results by adding a second layer of GWB to one side of the wall and/or using resilient channels on one side. But I don't trust the average GWB contractor to correctly install the resilient channels. Even if they do, the home owner will come in and hang a TV or some shelves and couple the GWB to the studs, ruining the assembly.

Edit: if you want to make a big difference in sound privacy, buy solid core doors.

 

[shawn]

So wet air is not a good insulator. Couple this with rising dew points the last 3 summers in NC I question any open cell insulation where it is exposed to atmospheric air.

The open cell spf he'd be using on the underside of the roof sheathing should act as an air barrier at a min 3" installed thickness. Whether it's vapor permeable depends on the specific product. Closed cell is "better", but may cost more.

 

[Fabrik8]

You're overthinking it. The chute only needs to extend to a point above the insulation (8-12" should do). It's just to prevent wind washing of the batts. You don't want to extend them all the way because it creates a restriction.

You can also build them out of the prefab baffles.

Yes, I'm talking about a way to insulate the roof deck in its existing condition, not insulate the attic floor.

What you're talking about works well; I did that a few years ago in the roof bump-out above our master bath, with OSB baffles. I have pictures somewhere.