insulation in crawl spaces

Is it correct to have :a vented crawl space, ductwork in it, insulation installed between the joists w/ the kraft face installed towards the crawl space, not the floor side, which is the heated side. And can you leave the facing exposed (combustable)

I presume from your profile that the crawl space is in Michigan, a cold climate. I also presume that, because the crawl space is ventilated, that the ductwork has no supply register in the crawl space. If my presumptions are true, then the insulation should, as you surmise, have the vapor retarder up against the subfloor (on the winter warm side of the assembly), not down toward the crawl space, and the insulation need not be covered.

Installation of insulation incorrectly in a crawl space is common, and if you were to ask the insulator, you’d probably hear “Oh, we do it that way all the time”.

I presume from your profile that the crawl space is in Michigan, a cold climate, and I also presume, since the crawl space is ventilated, that there are no supply registers in the ductwork in the crawl space. If my presumtions are correct, the insulation should, as you surmise, be installed with the vapor retarder up against the subfloor, on the winter warm side of the assembly.

The ductwork should also be insulated in the crawl space, but it may be insulated by an internal liner.

vapor barrier installed next to the conditioned space, flamable paper not to be left exposed per manufactures instructions (printed on the vapor barrier).


What you describe is work done by someone who has little conception of how heat, air and moisture (HAM) flow through a house and what good energy conservation work and building science is all about. Better energy efficiency and conservation is just about a whole new trade specialty as would be electrical, plumbing, cabinets, etc. The vast majority of carpenters, contractors, handymen and even insulators have incomplete knowledge of the new specialty.

For the above described location, you’ve got to decide where the thermal and air barrier is going to be. That will be influenced by such items as plumbing, ducts, cost, ease of floor insulation versus outer support walls. If you insulate the floor, you should airseal at the rim (or band) joist and any other floor penetrations for wiring, plumbing, etc.

With the style of insulation mentioned, the vapour barrier location is not that critical. In certain cold locations, there is a situation where it would be better to have a sealed plastic vapour barrier under the whole floor (just opposite to the common knowledge but that’s another story).

You then have to seal the duct joints and then add at least R12 commercial grade duct insulation (note: From 20-30+% of the designed air flow through unsealed residential duct systems leaks)

As for ventilation of the crawlspace, more recent research shows you have to have very big vents (well above code) and many of them to prevent condensation/mould/ mildew in the space due cooling of the space from the soils. The other technique being brought forward by many is:

Seal the outer walls of the space to prevent the inward movement of high humidity summer air which will condense as it cools. Also install an effective ground moisture barrier to stop evaporation from the soil which also may condense.

If you decide to insulate the outer walls, an airtight vapour barrier has to be part of the thermal barrier and the ground has to have an effective soil moisture barrier. The ducts should also be sealed and insulated as mentioned above as you don’t want to lose conditioned air to the unconditioned space or heat or coolth through the thin metal walls of the duct.


It’s unlikely that residential ducts have duct liner- an insulation product used for acoustical purposes. There are rigid fiberglass duct systems for energy conservation though.

Here’s a good website that someone on this board was nice enough to send to me.

I know I’m a bit late with a response (just back from vac) but hope you can still use the info. Like Michigan, Mass & NH (areas where I’ve worked as a lic. Const Supv/GC) have cold winters and insulation is require/necessary. Code (both engery & building) requires insulation in unconditioned areas. So allow me to presume the area you’re describing is unheated threrefore, insulation is required against conditioned areas (such as between the floor joists as you decribe).
This is a correct application (insulation between the joists) however, it is improperly installed if the paper (kraft face) is not against the subfloor. To answere your question “And can you leave the facing exposed” - NO, and you’re right it is a fire hazzard. As to having duct work in the crawl space - it’s no different than an attic so long as the ducts are properly insulated, on the outside is how I’ve always seen them.
hope it helped


"As to having duct work in the crawl space - it’s no different than an attic so long as the ducts are properly insulated, on the outside is how I’ve always seen them"

If you’ve never seen them sealed at all joints/ boots before being insulated, then you’ve been seeing poor, energy inefficient work. Where the ducts penetrate the building’s thermal envelope should be sealed for air leakage also!

The issue with the paperfaced batts is just about that- it’s paper in books…theory and code…but in this application, the vapour barrier is a dead issue, in my opinion. Most subfloors are plywood or OSB board which are close to VB’s themselves. The issue in houses with moisture getting into walls, attics, etc is not vapour diffusion (1-2% of moisture problems and the main theory being bandied about) but air leakage through and around unsealed vapour barriers (98-99% of moisture problems and well proven) so the existing vapour barrier here is not an issue. Have you ever seen a vapour barreir in this location and position cause a problem? I haven’t.

Is the paper a fire issue? What about the untreated wood joists, studs, rafters, truss members, wood furniture, urethane foam pillows/cushions, newspaper left lying around houses (I start a fire a day in the winter with newspaper)

I hope this helps, when I insulate a crawl space I rip down 1" rigid foam insulation that fits between the floor joists and is held in place by strapping, I then install r-30 fiberglass on top and then the sub floor.

This application is generally for crawl spaces that do not have duct work in them.


What type of foam and do you airseal the foam to the joists to prevent air leakage from getting up into or through the fiberglass batts?

I use Dow 1x2x96 and yes I caulk all the seams and butt joints which not only prevents air infiltration but stops the bugs as well!

PS, The crawl space should be vented as well.

I was just making a general comment about ducts in the crawl space as Eric had mentioned it in his original thread. I didn’t see the need to get into a full length document about the proper installation of a HVAC systems, vapor barriers and the entire theory of moisture and air penetration. His concerns was with the installation of insulation “Is it correct to have…w/ the kraft face installed towards the crawl space,…And can you leave the facing exposed (combustable)”, just to paraphrase. My reply was to confirm his concerns and because of his concerns, I made the assumption he was referring to paper faced insulation. It’s a mis-conseption for people to refer to paper faced insulation as kraft faced, kraft faced refers to insulation with a moisture barrier applied.
As you seen to know something about construction, you also know that codes require a vapor barrier between conditioned (heated) and unconditioned (unheated) spaces and those same spaces require insulation and the most common to use (right, wrong or indifferent) is kraft faced and the most popular is paper (least-wise here in the U.S.), therefore fire codes require all paper faced insulation be covered with a fire-retardant material, be it plywood or drywall or some similar material but in no way is it to be left exposed (period). Also, Eric describes the area as being a crawl space, I will make another assumption the floor is soil and not concrete. I’m not an expert in moisture but IMHO paper facing soil would deteriorate over time causing failure in the moisture barrier, therefore wouldn’t the proper application be to have the paper against the subfloor?


I can’t believe you made this comment. Where paper has a much lower ignition threshold construction lumber is used as a *fire-retardant *as required by codes. ie: 2 x’s on the flat and plywood are used and sometimes required as fire blocking between wall studs and other construction areas where fire could penetrate an area and move through the house. When a fire breaks out these materials are only to slow the fire down enough to give firefighters more time to fight it.

From the DOE website mentioned above:

  1. Close crawlspace vents after making sure the crawlspace is dry
    and all construction materials have dried

I generally rely on my 22 years of construction experience more than a companies web site or installation instructions. I advise my clients to open the vents during warm seasons and close them up in the fall.


See this at DOE:

From Energy Design Update (EDU) May, 1994: (A pricey little newsletter- $385 US/yr- for 15-20 pages/month but it keeps you years ahead of the curve)

“New research evidence and field experience are leading to the conclusion that even in cold regions, polyethylene vapor retarders are probably overkill and that ordinary paint should provide adequate resistance to moisture diffusion through walls and ceilings”

“Some of North America’s leading building scientists are already discussing how building codes should be changed to reflect the new findings. And closer to the building community is a new report , written by the National Association of Home Builders (NAHB) Research Center in Maryland, which concludes that: the level of understanding regarding the movement of moisture, air and heat through the building envelope at the research level is not yet reflected at the applications level.”

From the same EDU article:

-In a study published by ASHRAE, Kumar Kumaran of the Canadian National Resarch Council says “Contrary to previous assumptions that only special vapour barrier paint should be substituted for polyethylene, his study shows that ordinary latex paint is adequate, even in climates as cold as Winnipeg, Manitoba.”

-Three extensive field studies by George Tsongas, professor of mechanicl engineering at Portland State University, found virtually no relationship bewteen vapour barrier presence and moisture content in walls. A fourth study, conducted by Tsongas last year, did find a correlation between air leakage sites in walls and moisture problems.

My comments:
As I mentioned in another post in this thread, air leakage is by far the reason for moisture problems from condensation in walls, attics, etc. We need to concentrate on the air leakage; vapour diffusion is so little that only latex paint is needed to stop the 1-2% of the total. The original theory that vapour diffusion caused all the moisture condensation was so far off the mark that it was almost wrong! Airseal! Airseal! Airseal!

Building codes have clauses that allow for innovation and changes from the norm if experience, research and logic/common sense are presented. (If we didn’t have them, nothing would change and we’d still be living in huts!!!) We should make an effort to educate code officials on these items (Note: I find that in my area inspectors are not sent to enough re-training sessions due to budget constraints; that has been confirmed by a supervisor I know in the building inspection dept.) I have had houses passed that do not meet code in some items related to moisture, insulation, etc. and have had another supervisor call me for advice on a few items he has seen in the field. By the way, I just did a pre-purchase inspection for another member of that office last week.

I’m going to copy a comment from the senior editor of one of the most popular building magazines in response to those questioning my posts on their web forum: (Note- my online moniker was “Experienced”)

“I’d like to second everything that experienced said. His knowledge is cutting edge, and his advice right on the money. It may fly in the face of some orthodoxy, but that’s 'cause we in the building trades tend to be conservative, not because he’s wrong.”

Here are some guidelines to follow for insulating a ventilated crawl space:

  1. Carefully seal any and all holes in the floor above (“ceiling” of the crawl space) to prevent air from blowing up into the house.
  2. Insulate between the floor joists with rolled fiberglass. Install it tight against the subfloor. Seal all of the seams carefully to keep wind from blowing into the insulation. Also, adequately support the insulation with mechanical fasteners so that it will not fall out of the joist spaces in the years to come. DO NOT just rely on the friction between the fiberglass and wood joists to secure it in place.
  3. Cover the insulation with a house-wrap or face it with a vapor barrier.
  4. Install a polyethylene vapor retarder, or equivalent material, over the dirt floor. Tape and seal all seams carefully. You may also cover the polyethylene with a thin layer of sand or concrete to protect it from damage. Do not cover the plastic with anything that could make holes in it, such as crushed gravel. Be sure the headroom of the crawl space meets local code regulations if you are considering pouring a concrete slab.
  5. The orientation of the vapor barriers depends on the home’s location or climate.