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What materials meet code for "flash and batt" on my ceiling?

I have an unfinished basement and I am looking to add about 5" of insulation to fill up the joist bays on the ceiling. The bays are 7 inches deep and will soon have around 2" of foam.

I am wondering if I would be able to fill the bays with Roxul and cover the entire ceiling with house wrap.

The reason for the roxul is that if necessary it would still be possible to access what's in my ceiling (wiring and some pipes), without the negative effects on air quality of the fiberglass.

I understand I can trim the roxul with a knife to match my very uneven 80 year old ceiling joist bays.


Would this arrangement allow the roxul to deliver its full R value?
And would using house wrap maintain the fire retardant property of my insulation covering so I can meet code? If not house wrap, is there another insulation holding canvas material that would work?

Asked by user-6993560
Posted Jan 12, 2018 3:03 PM ET


10 Answers

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First, can you tell us your name?

A few comments:

1. It usually makes more sense to insulate your basement walls and leave the basement ceiling uninsulated than it does to insulate the basement ceiling.

2. If you proceed with your plan, you will end up with a backwards flash-and-batt job -- with the spray foam on the interior side of the assembly instead of on the exterior side of the assembly (as usually installed).

That said, your backwards approach shouldn't cause any moisture problems.

3. You should consult your local code authority to determine whether exposed housewrap is permissable or is considered a fire hazard.

Answered by Martin Holladay
Posted Jan 12, 2018 3:15 PM ET
Edited Jan 12, 2018 3:15 PM ET.


Sorry I updated my profile but it didn't show up in the post. Robert Wan is my name!

Also I understand that I can use insulation supports (i.e. http://www.energyconscious.com/ado-wire-batt-supports-20-pack-500-bs200.... ) to hold the insulation in place. However that wouldn't prevent air from circulating through it and making it dirty. Hence the reason to add an additional layer.

Answered by user-6993560
Posted Jan 12, 2018 3:21 PM ET
Edited Jan 12, 2018 3:23 PM ET.


Most homeowners with your dilemma install a suspended ceiling with removable ceiling panels.

Answered by Martin Holladay
Posted Jan 12, 2018 4:01 PM ET


A suspended ceiling would cost like $8000, wouldn't it? Why would we put that much money into a basement used for cold storage of things like bikes and lawn mowers and lawn furniture? Also I only think we would need to access those wires once or twice in a 10 year period.

The thought of using house wrap would be a quick and dirty solution and would cost only a few hundred dollars. So I am wondering if anyone else has done it. Since it's white it would also have the advantage of making the basement brighter! (Of course it would be nice to find a housewrap that did not have the brand name written in huge letters all over it!)

Answered by user-6993560
Posted Jan 12, 2018 5:01 PM ET
Edited Jan 12, 2018 5:21 PM ET.


If you don't have moisture issues in your basement, would you be open to insulating the walls (as Martin suggested in his first response)? In many parts of the country, you can buy reclaimed foam for a fraction of the cost of new material. I would imagine this approach would be easier than cutting and cobbling foam into your joist bays.

Answered by Steve Knapp CZ 3A Georgia
Posted Jan 12, 2018 5:36 PM ET


I would think we would need to put up studs and then cover the foundation with spray foam. Cost of that is around $5000, maybe $6000 for the size of our basement. Then put drywall or some kind of fire retardant paint on top of that? That could run $7500 to $8000. The foundation is uneven, so no way to put foam board on it.

Cost of putting in a thin layer of spray foam is around $2200 on our ceiling and also covering the rim joists/ sill area. Then tyvek plus the insulation, maybe around $1800? The latter part is a DIY project... though we could probably get someone to do most of the labor for $800. Total cost $4000 to $4800.

I guess a good question is, what is the R value we would get from 1.5 inches of closed cell plus 4 inches of roxul plus one layer of tyvek in a joist bay that is a total of 6.5 to 7 inches?

How do we calculate this and how "tight" does the house wrap layer have to be (the spray foam will stop most of the air movement but staple holes in house wrap in an interior basement (no wind) may allow some minor air flow in the assembly.

Answered by user-6993560
Posted Jan 12, 2018 5:41 PM ET
Edited Jan 12, 2018 5:51 PM ET.


I eventually removed my cellar ceiling insulation, dried up the moisture and insulated my basement walls; where we have spray foam with fire retardant paint, we've left it and not covered it. It's pretty tough. My investment not only gave me a much more comfortable house, but a whole lot of slightly cool, but comfortable space for a shop and hobbies.
Insulating the joists with spray foam and Roxul is a bandaid; you'll still have a cold cellar with drafts up the stairs and through the electrical and plumbing holes. Assuming the basement isn't tight, you could end up wth frozen pipes.
One of the main principals in Passive House is that the house envelope encloses all the living space, even semi-used basements. We've seen in our home as well as other older homes that conditioning the basement is the most underrated and unappreciated ways to improve comfort in your home.

Answered by Bob Irving
Posted Jan 12, 2018 6:53 PM ET



Do you have a stone foundation, or just a poured concrete foundation with some irregularities? Are your numbers for the spray foam based on recent quotes?

Perhaps your code enforcer would allow intumescent paint over the foam or an outer layer of Thermax. Stuffing the joist just seems like a pain, and it won't do anything to moderate temperatures in your basement.

Answered by Steve Knapp CZ 3A Georgia
Posted Jan 12, 2018 6:54 PM ET


Bob: no drafts up the stairs. There are no stairs from the living space, just one stairway going outside. The electric and plumbing holes are well sealed. The problem is that the first layer of flooring is 1x6 planks with gaps. That is what we want to seal with the spray foam.

I doubt we would get frozen pipes when the walls of the basement are 2 feet thick; many houses are far more vulnerable to a cold snap than ours.

Steve: Intumescent paint quote is $1500. Spray foam going 4' down on foundation (not really enough) quote is $3100. I bet it would be $5500 to cover all the basement walls. In parts of the basement there is 4' of stone exposed outside so the cold is going to be transmitted to part of the basement floor. The basement floor is divided so that most of it is not connected to a cold wall. Some modifications in grading on the exterior should reduce that exposure.

I really have no need for a 60 or 65 degree basement, and talk of a "passivehaus" is impractical because the cost of sealing the envelope on the 1st floor and where it meets the second floor is going to be well over $50,000.

So my goal is to eliminate the flow of air from the basement upwards and from sill area into the basement.

My question is, for the money I am going to spend anyway on the intumescent coating for the foam, why not fill the bays with a fire retardant product?

Covering all of it with Thermax is an interesting idea. Though do I run into any condensation issue, with thermax on one side and my warm side spray foam on the other? If it was possible to use just poly sheeting then I could use a fire retardant version: https://www.uline.com/Product/Detail/S-20069/Plastic-Sheeting/Standard-F... but I don't know if that would stand up over time, where the house wrap probably would last.

Answered by user-6993560
Posted Jan 13, 2018 10:54 AM ET


There is no need to use (more environmentally and cash expensive) closed cell foam in the joist bays. For the amount you would pay for 2" you can do the whole 7" of half-pound open cell foam, and it would be using less polymer, yielding about R26.

If the point of the foam is merely to air-seal, ~3" (R10-R11) would be enough at half the cost of 2" of closed cell foam. An R19 compressed to fill the remaining 4" or blown cellulose would come in around R14-R15, for an R25-R26 totl

In climate zones 1 & 2 IRC 2015 code minimum for floors over unconditioned basements is R13, in zones 3 & 4A/4B it's R19, zone 4C, 5, & 6 i'ts R30, or a complete cavity fill (R19 minimum), zones 7 & 8 R38 or complete cavity fill (R19 minimum).

There are essentially no condensation issues during any season in unconditioned basements as long as they are air-tight to the outdoors. Thermax cap-nailed or strapped in place with furring through screwed to the under side of the joists inserts a vapor barrier, but the insulation fluff or open cell side of the vapor barrier will have a dew point that tracks the seasonal average dew points of the fully conditioned rooms above. In the summer even if the rooms are running 65% RH 80F (barely air conditioned, or not air conditioned) the top-side facer would have to be 67F or cooler to accumulate moisture. When it's that hot & humid out it's likely that even the basement would be north of 67F. In an air conditioned house dew points are generally in the 50s or lower in summer.

In the winter an air-tight mostly below grade basement will track a bit lower than the average subsoil temperatures but in most cases will average north of 40F (if it's lower than that there's a real freeze-up risk), and when it's 40F in the basement the dew point upstairs will usually be south of 40F (~35% RH @ 69-70F), and the facer on the top side of the Thermax will be a few degrees warmer than that, again, no moisture accumulation risk.

But for the record, what is your location (for climate and deep subsoil temperature purposes.)

Answered by Dana Dorsett
Posted Jan 13, 2018 11:31 AM ET

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