QA-spotlightheader image
0 Helpful?

Upgrading a Crawl Space

A homeowner wonders how to heat a crawl space and worries about risks from radon

Posted on Dec 19 2016 by Scott Gibson

David Meyer's Seattle-area home is built over a crawl space, and after stripping out the old insulation and vapor barrier he is ready to re-insulate and seal the area. After looking into his options, Meyer is leaning toward "encapsulation," meaning the crawl space would be sealed (unvented), with the insulation on the walls, not between the floor joists.

He'll have 18 inches of clearance to work with. The floor framing consists of what he calls the "old style post and pier construction method" with 4x6 joists on 48-inch centers, followed by 2x6 tongue-and-groove subflooring, and hardwood finish flooring on top of that.

"After cleaning up the crawl space I started doing research into how to insulate the area, and found the encapsulation method," Meyer writes in a Q&A post at GreenBuildingAdvisor. "I like the method due to not having to run insulation in the floor joists, giving me more space to work down there, and also in keeping the area cleaner for rodent inspection purposes and for when I do plumbing or other type of work."

There are, however, a few details yet to be worked out.

The house is currently heated with electric baseboard heaters, but Meyer is considering a switch to a gas-fired forced-air system. If he locates the furnace in the crawl space, it would heat the space and the ductwork would not have to be insulated. Is that a better option than putting the furnace into the attic and running a duct to the crawl space? Or heating the crawl space by placing an exhaust fan in one of the crawl space vents and opening a floor register between it and the house? Would the exhaust fan be enough to mitigate radon hazards?

Meyer's questions are the starting point for this Q&A Spotlight.

You don't need to heat the space

Don't worry about the heat, GBA senior editor Martin Holladay advises, because you don't really need it.

"A sealed, unvented crawl space in your climate will not require any heat to stay warm," Holladay writes. "Once you have sealed the air leaks and insulated the walls, it will never freeze."

Many crawl spaces do incorporate an exhaust fan and a register in the floor above, he adds, but those features have nothing to do with heating the space. They are included to control moisture.

Holladay adds that Meyer could install a register from his forced-air heating system to the crawl space. In that case, he should keep the floor grille, but not bother with an exhaust fan.

Here, too, the object is not to heat the crawl space but to provide humidity control.

As for radon control, he adds, start with a radon test to determine whether it's a problem that must be dealt with. If so, Meyer will need to do more than rely on a crawl space exhaust fan alone.

A gas appliance will probably be too big

Dana Dorsett sees trouble ahead if Meyer goes forward with his plan of installing a gas-fired furnace in the house, particularly if he puts the furnace in the attic as the HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. installer wants to do.

"Almost any gas-burner is going to be ridiculously oversized for a 1150-square-foot rambler in Fall City, especially after tightening it up a bit," Dorsett says. "That would be mistake #1. Putting it in the attic rather than in [an] insulated, conditioned crawl space would be mistake #2."

With an outside design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year). of between 20° and 22°F, and assuming Meyer performs some air-sealing work, the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. is likely to be less than 15,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. per hour — and that puts him within range of a 1.5-ton ducted minisplit.

There are two models that might work, Dorsett says: the Mitsubishi MVZ-A18AA and the Fujitsu AOU/ARU-18RLFCD. "With either of them you have the benefit of modulated ultra-quiet output (more so with the 18RLFCD than with the A18AA), and at your average wintertime temps you would beat their HSPF efficiency numbers if the ducts are all extremely short, and inside a conditioned insulated crawl space," Dorsett says.

Under the right conditions, a 1.25-ton or 1.5-ton ductless minisplit also might do the job, and cost less to install, he adds. There might even be local rebates available to help pay for the conversion from electric baseboard to a heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump..

"Even the smallest gas furnaces are likely to be 2x oversized for your actual loads," Dorsett says. "If you're committed to going with natural gas, a condensing gas water heater and a suitably sized hydro-air handler is probably a better solution than an oversized gas furnace + water heater."

What if the house got bigger?

Meyer replies that he plans to expand the house into part or all of the garage in the next year, enlarging the house to a total of about 1,600 square feet.

"We are in the process of converting to natural gas for cooking, water heating (tankless gas water heater), space heating (forced-air furnace), and possibly clothes drying," Meyer says. "The baseboard heat and the old electric tank water heater are costing us a fortune. The house is not too open, and has 4 bedrooms and 2 baths ('1.75 bath' in realtor speak)."

Even so, Dorsett says, the design heat load will be less than 25,000 Btu/hour, making only the very smallest gas furnace a reasonable option.

"Replacing the electric baseboard with hydronic baseboard, running off a condensing tank hot water heater would allow you to micro-zone with impunity, and never have to suffer the limitations of a tankless water heater," Dorsett says. "It's a lot easier to find places to route pipes than it is to route ducts (which is how ducts all too often end up outside of conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. ). The 50-gallon all-stainless HTP Phoenix Light Duty is a good candidate for this type of combi-system, but there are others."

A ducted HVAC system would only make sense if Meyer were planning on central air conditioning, Dorsett says, "in which case heat pumps (ducted minisplit or traditional) make a lot of sense."

What comes first: insulation or vapor barrier?

Meyer's plan to encapsulate the crawl space will include both insulation for the walls and a vapor barrier to seal the floor. Which comes first?

"Most people install polyethylene on the floor and walls before insulating the walls, but frankly, it doesn't matter very much," Holladay tells him. "If you insulate the walls first, with either rigid foam or closed-cell spray foam (and no polyethylene on the walls), everything will still be OK. You can do the floor last if you want."

If Meyer chooses polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. foam insulation for the walls, however, Dorsett suggests it would be better to lap the ground vapor barrier up the foundation at least a foot before installing the insulation to prevent the cut edge of the insulation from touching soil directly.

"Holding the polyiso in place with 1x4 furring through-screwed to the foundation puts a few minor holes in the vapor barrier," he says, "but on the walls that doesn't matter."

Insulate the exposed footing, too

The photo Meyer has provided of the crawl space (see the image at the top of the page) shows that some of the concrete footing has been exposed. Holladay suggests that Meyer insulate the exposed areas with closed-cell spray foam, available in a two-part kit.

Meyer seems concerned about cost of adding a required thermal barrier over the two-part foam, but Dorsett says painting it with an intumescent paint would be enough in most jurisdictions.

Alternately, Meyer wonders whether he could use rigid foam to cover the exposed parts of the footing.

"There's nothing wrong with your plan to cover the horizontal and vertical parts of the exposed concrete footing with rigid foam," Holladay tells him. "And, as Dana Dorsett pointed out, there's less heat loss through concrete components that are well below grade than through concrete components that are near grade. So you're fine. "

Our expert's opinion

GBA technical director Peter Yost added these thoughts:

On the West coast, it sure seems that there's a preference for venting crawl spaces and placing the air and thermal barriers or control layers on the underside of the first-floor assembly. I have found it really hard to get continuity of the air and thermal control layers there, so I have a strong preference for installing them on the crawl space perimeter. It does mean you are conditioning or semi-conditioning an extra volume, but that penalty is overcome by what you gain in energy efficiency and moisture control by placing your crawl space firmly inside your building.

You should check with local HVAC folks about the 18-inch clearance in your crawl space, but getting a furnace into this space, much less getting it well installed, seems highly unlikely to me. I'm betting that the space is just too tight.

When in comes to testing for radon, of course you test at the lowest inhabited level of the house. But when do you test? If you test before your work is complete, there is some uncertainty concerning whether you will improve or worsen radon levels on the first floor. And don’t rely on just what your neighbors might say or know; research has shown there is very little correlation of radon levels in buildings that are right next to each other.

Another question is: What type of radon testing do you do? The most accurate radon test is the alpha tracker test, but to get useful numbers you need to test for up to three months when the building is closed up, as it would be during the winter. I have found that the Siren Pro Series 3 electronic meters correlate pretty well with alpha track testing, and they give results in just three days (although testing over longer periods is much more valuable). I like the fact these meters can be set for short-term testing (three-day averages that after three days drop the first data and add in new — a running average) and set for long-term cumulative data collection.

In the photo provided, there is evidence of water wicking up the concrete piling on the right, but also what appears to be a sheet capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. break between the concrete and the wood post. Make sure that you indeed have a capillary break underneath each post.

Finally, on the question of whether you need a thermal barrier or ignition barrier to protect spray foam in crawl spaces: The model building codes allow for spray foam thermal barrier exceptions in attics and crawl spaces where entry is limited to repairs or maintenance. So in this type of crawl space you can use one of these ignition barriers prescribed by the codes:

  • Mineral fiber insulation, 1 1/2 inches thick.
  • Wood structural panels, 1/4 inch thick.
  • Particle board, 3/8 inch thick.
  • Hardboard, 1/4 inch thick.
  • Gypsum board, 3/8 inch thick.
  • Corrosion-resistant steel, 0.016 inches thick.

Alternative ignition barriers — such as intumescent paints or coatings — can be approved by code officials based on testing that follows ICC-ES Acceptance Criteria 377, Appendix X. Be careful: even if someone claims a material qualifies as an alternative ignition barrier, you should really insist on the appropriate test information.

Finally, you may be able to leave spray polyurethane foam (SPF) exposed, so long as the SPF has been tested and approved per AC 377 Appendix X or ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. E 970. (For more information on this topic, see Thermal Barriers and Ignition Barriers for Spray Foam.)

Tags: , ,

Image Credits:

  1. David Meyer

Dec 19, 2016 1:10 PM ET

Seimic risk?
by Steve Knapp CZ 3A Georgia

David. Have you considered doing some seismic work on your foundation while you still have unobstructed access? It might be work investigating (especially if you are west of the I-5 corridor).

Dec 20, 2016 3:23 PM ET

I also have a crawl space to
by Alan B

I also have a crawl space to dig out and encapsulate, not there yet but planned for the future, i wonder if i should test for radon before or after encapsulation?

Dec 20, 2016 4:00 PM ET

The answer is "yes" @ Alan B
by Dana Dorsett

You should test for radon both before & after, but clearly the before is of higher importance, since if it reads in the "remediate" range some cheaper-better measures that can be taken for radon abatement might be ruled out if it is already encapsulated.

Even if it came in OK before encapsulation doesn't guarantee it won't need some radon work after, especially if the crawlspace is fully vented or very air leaky. In cases where it only tests high after encapsulation, an exhaust-only venting scheme or a tiny HRV like a Lunos should make a big dent in the levels without using a lot of power.

Dec 20, 2016 4:05 PM ET

Response to Alan B
by Martin Holladay

Remember, you will be testing the lowest habitable floor of the house: that is, the level above the crawl space. You won't be testing the crawl space.

Dec 20, 2016 4:59 PM ET

@ Dana & Martin
by Alan B

@Dana, that is very interesting, thanks for your answer
It is very air leaky right now, i assume gaps to the outside i can't yet reach plus the house is balloon framed and i call it a chimney to the attic (i can see the paper faced fiberglass with a mirror from the basement).
I have never heard of a Lunos HRV, i would like to HRV the entire house when i get air leakage down of which the crawlspace is contributing (15ACH50 right now).

@ Martin I have 2/3 basement and 1/3 crawlspace so i figure i would be testing the basement.

Dec 20, 2016 5:06 PM ET

@ Dana
by Alan B

Can you give me some examples of what might be ruled out if already encapsulated, i was planning to dig out enough to crawl in throughout, put down 6 mil vapour barrier and attach at walls somehow (tape, foam, whatever).

Dec 20, 2016 5:07 PM ET

Response to Alan B
by Martin Holladay

There is no reason to test the basement unless you consider it "habitable." To my mind, you would need to have a finished room -- a bedroom or a TV room -- in your basement to make it worth testing for radon.

Dec 20, 2016 5:58 PM ET

With air leakage between crawl & 1st floor you need to test!
by Dana Dorsett

It's not as if radon-burdened basement or crawlspace air somehow stays in the basement, especially in a house that leaks air. Stack effect drives bring it upstairs. You might just test upstairs, I suppose, but knowing the concentration at the worst-likely locations is a good indicator of how much mitigation efforts might be needed. Interrupting the radon path between source & living space at the higher concentration point would (in theory at least) be the best bang per energy-expended buck.

If it tests high before encapsulation, a passive radon mitigation system could be installed below the ground vapor barrier, and configured to be potentially retrofitted with a radon fan if levels tested high later. If the levels don't test high before encapsulation, installing a radon system probably isn't worth the trouble.

A house in my neighborhood tested ~30 pCi/L in the basement and ~10 pCi/L in a first floor bedroom, well above the EPS action level of 4.0 pCi/L level. A modest amount of basement ventilation, depressurizing the basement (not the slab, which was on clay soil) slightly relative to the first floor was enough to bring the first floor levels under 4.0 pCi/L everywhere, even though the basement levels remained above the 4.0 pCi/L. With more air-permeable soil and a sub-slab depressurization approach it probably would have fixed the basement levels too (albeit at higher fan power), but it is a low headroom basement that will never be converted to living space, so...

Dec 20, 2016 6:50 PM ET

Edited Dec 20, 2016 6:53 PM ET.

@ Dana & Martin
by Alan B

@ Martin Its an unfinished basement with laundry, furnace and ductwork in the basement and pipes/wires/ductwork in the crawlspace. I do spend some time downstairs but mostly for repair, laundry and storage purposes. The headroom is not enough to ever be finished.

@ Dana If you ever come to southern Ontario i would love to pay you for a couple hours of your time to come by and give your opinion on all the efficiency and radon issues this house has :)
Right now i have the crawlspace and attic to handle, the crawlspace the digging and encapsulation and attic air sealing for stack effect, electrical wiring for putting lights in rooms and venting/insulation (current is paper faced fiberglass between rafters only).
My original plan was to encapsulate then test, because the stack effect ventilation is probably lowering radon quite a bit right now if its there. I'm also hoping the air sealing plus vapour barrier will reduce the 5-15 pints/day of water the dehumidifier is pulling in spring/summer/fall (house is single story 700 sq ft, basement about 2/3 and crawlspace 1/3)

Dec 21, 2016 8:21 AM ET

Response to Alan B
by Martin Holladay

I stand by my statement. The space you describe is not a habitable space. Standard recommendations do not require, nor do I recommend, testing this type of basement (one with a low ceiling and a washing machine) for radon.

You test the floor where people live (where they spend several hours each day).

Dec 21, 2016 2:31 PM ET

Edited Dec 21, 2016 2:32 PM ET.

@ Martin
by Alan B

@ Martin Fair enough, i will follow your advice
Thanks to both you and Dana :)

Dec 24, 2016 11:04 AM ET

What about insulating the ground?
by Justin Smith

I am in the same position, in Southeast Alaska, near Juneau. We have an 18" stem wall foundation that I plan to insulate. What are the consequences of not insulating the ground in the crawlspace? I assumed we would need at least 2" of foam there, or the ground would become a huge heat sink.

Jan 17, 2017 1:06 AM ET

Rising damp
by Colin Allison

Hello Martin: I'd appreciate advice on how to work with a probable situation of 'rising damp', brought on by a high water table in a concrete foundation wall. Some back ground information first. We live 400 miles up coast from Seattle;(the top end of Vancouver Island) a temperate climate with low temperature being +22 F, high about 75 F. Rain fall 9-10 ft. annually. A flat property of natural beach gravel 26 ft above sea level, 1/4 mile from the coast.The land swallows water beautifully never any surface water puddling but winter weather where consecutive days of average rainfall (3/4-1 in) plus 18 foot high tides and strong on shore wind will bring the water table to within 8 - 12 in.of the bottom of the footing. The house is early 1970's built. 1900 sq.ft. area, crawl space and main floor plus 1/2 floor bedrooms. The base of the footings 20 in. approx. below grade total concrete assembly 36 in. with no capillary break below the mud sill/top of the foundation wall. At this point we have a roofing felt gasket laid dry no caulking or sealant was used back then. I presume there will be continuous rising damp happening in that wall, the amount being variable according to the water content of the sub-grade? Some time ago the entire outside of the building from footings to soffits was foam boarded (1 1/2" EPS Type II) rain screened and resided. The concrete work was covered with a dimpled vinyl membrane to hold the back fill off the foam board. This membrane is not UV stable so above grade (about a 12" band) was covered with flashing. I'm thinking this wall face doesn't breath any more? For 40 years the foundation wall was bare, but at the time of foam boarding I neglected to remove the first run of ship lap which would have exposed the last 1 1/2" of concrete+edge of mud sill+bottom half of the rim joist to check for moisture damage. I could then have also done a decent job of air sealing all those joints. Mr. Hindsight strikes again... We are now an all electric house, so the prevailing attitude regarding energy use doesn't do. I want to EPS (Type II foil faced) to R16 (2"x 2 ) the inside of crawl space/foundation walls also rim/floor joist/mud sill. Am I correct in thinking this would be semi-impermeable and risking the rising damp accumulating under the mud sill? Could I vent this wall successfully to the outside by implanting lets say 3" dia midget louvers (Norwalk Conn). through the flashing, dimpled membrane and part way or completely through the EPS? The 3" dia. has 4 sq. ins. N.F.A. on their specs and with enough of them.......How would you quantify rising damp? The weather being ugly just now I could be doing the R 16 job but I'd like to have the breath ability issue resolved first. Your thoughts will be appreciated. As an aside to this I have a quantity of XPS 3"x 2'x4' tongue& grooved cement top (1/4"layer) panels, enough to cover the crawl space floor area. How significant would be the value of this to the heating costs of the crawl space (5,150 cu.ft; 26'x66'x3')- If worth while should they be caulked together or laid dry? Some tongues have been damaged and the 2' ends are cut square. Either way they'd be over a 20 mil of 'Cleanspace' liner sealed to the 6"edge of the footing.

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!