For Valerie & friends: It’s cold! What do I do?

It’s cold!  What do I do?

 Ok, the house doesn’t have a basement. It has a crawlspace and the floors are cold. There is a dirt floor, which is dry now, but there is efflorescence so it has been wet in the past or maybe seasonally.

Air heating ducts pass through the area. They are incompletely insulated and there is likely plumbing and wiring present as well.

The good news is there are no evident structural problems.  So what can be done?

Quick and dirty: heat the crawl space.

Get an electrician to put electric baseboard heaters around the perimeter about 16” down from the floor joists. He’ll tell you how many and what size. Wire them to a thermostat mounted near the floor hatch. (Have him run in some lights with a switch by the hatch if you don’t already have it.)

Set the thermostat (55F to start, wait 2 days, if floors aren’t warm enough raise to 60F, wait 2 days…repeat as required.)….and sit back to// ops no, go out and get a second job to pay the hydro bill.

Venting? Put in small hatch type screened vents (1 sq ft of screened venting split between hatches on exterior walls  – opposite sides for cross drafts. Insulate and close the hatches for winder. Open them for summer.

Good advice in 1960.

There had been a prior halfhearted attempt to insulate the floor with ‘the pink’ fiberglass batts in some joist spaces.

A buyers’ agent had recommended adding additional insulation, plus vapour barrier (where possible) and protecting the ducts from cold. It was well-intentioned advice and basically the right approach, but not enough information.

Vapour barrier (VB);

The governing factor in the choices of insulation type and placement is the vapour barrier.

It has to be on the warm side of the insulation no matter what, and it has to be a continuous membrane barrier. No leaks. No bleeds. No pass throughs. No Matter What.

Anything less will cause problems (mold and wood rot).

In a crawlspace you can either insulate the floor area (ducting included) or the perimeter walls. The existing floor was not built with insulation so it has no vapour barrier. So you have to add it.

To (1) insulate the floor you have to VB the under side surfaces before you fill the spaces with insulation batts.

You’re not going to rip up the flooring to do it like new construction so you have to wrap it from underneath.

At a minimum that means 6 mil polyethylene plastic fitted up the sides of joists, across under the floor to the next joist, down that side, around and up the other side and on and on till you’ve done all surfaces. Along the way you must cut and fit around every pipe, wire, duct, cross brace, blocking and beam. Every time the plastic is cut or holed it has to be taped and overlapped for 12” and taped again. VB tape (red) is specialized and not cheap. You CAN NOT substitute. The whole job has to be perfect.

 Now where can you find that anal retentive worker whose obsessive compulsive disorder is focused on the perfect vapour barrier retrofit and who likes to work in a cramped area with slippery materials and who has a helper just like him.

Practically, you can’t do a perfect enough VB job to insulate the floor that way.

(2) Insulate below the floor:  Remove all insulation present and any paper, foil or plastic sheathing.  Seal and reinsulate the ducts fully (increase the insulation ‘R’ factor as much as possible.)

Apply VB plastic to the underside of the floor joists, stretched across for bottom edge to bottom edge and lap and tape all edges seams, cuts an holes as previously mentioned. That OCD guy could be handy here but any patient crew can do it.

Next thing would be to apply rigid insulation.  A double layer with joints lapped would be ideal.  Some products have lap cut or stepped edges to accomplish the same thing and then a single layer of sufficient R-value would be sufficient.

Most people don’t have that guy working for them, so what else can we do?

(3) Insulate the perimeter walls and rim joist areas (Rim joists are the space from the walls up to under the floor). You don’t have to insulate the air ducting, as the whole space now becomes a conditioned (heated or cooled) zone.

There are many ways you can do this. Stud frame and batts. Glue or screw on rigid insulation. Or spray foam installation.


1.     Stud framing and batts just needs a VB cover. Then it’s done.

2.     Spray foam fills all voids, can fill stud cavities, and skins over to form it’ own VB surface.

3.     Rigid insulation can be of many types including fiberglass and foam.  Foam board joints have to be taped. Other types require the same VB covering as batts in stud framing.

4.     All foam boarding or spray foam HAS to be covered with an approved fire grade material (gypsum board, cement board, stucco or concrete parging -gunnite). THIS IS A SERIOUS FIRECODE DETAIL due to lethal off gassing properties at near burning temperatures. The gas will migrate through walls and floors. You just won’t wake up.

5.     All foam insulation in a basement or crawlspace has to be ‘closed cell ‘ type, both spray or board forms. “Open cell’ types can harbour molds.

Instead of VB and batt insulation mentioned above in the first option ((1) insulate the floor) some people would consider (4) spray foaming the underside of the floor instead and just from the point of view achieving the desired insulation and VB this would work.

However, if the whole floor is not finished with a layer of concrete or covered with stone, marble or ceramic tiles this is not recommended. See the third option (3) Insulate the perimeter walls and the fourth item in the facts list. It is a serious risk. (Sadly we know this because people have died.)

The dirt floor is acceptable in a crawlspace, but here we must put a VB just under the surface to act as a moisture barrier. We use the same 6 mil plastic as before, cover the whole floor wall to wall, lapped seams, taped edges etc. Then it is covered with a few inches of earth (rarely), sand, or crushed stone usually pea gravel. This is to hold it in place and let you cross it with out disrupting it. This moisture barrier is designed to block the migration of ground moisture into the crawlspace from the earth below.

Venting: Most of the continental US used to require this space to be vented (screened hatches to the outside) but this is changing because the air ducting now often also carries cooled air seasonally as well as heated air.

In most of Canada where we have a boreal climate the old mode of vents closed in winter and open in summer often causes unwanted condensation. If the crawlspace is insulated at the perimeter then the interior is like a mini basement and that type of venting is not desirable. The zone is now inside.

Heating: Lots of time homeowners do not add heating to an insulated crawlspace but it is a smart practice. A few electric baseboards set at 40F will only come on during extreme conditions, just when you want them to. So it is not a hemorrhaging expense like I described in the Quick and Dirty paragraph.

Alternatively the existing air ducting can be modified to supply and pick up return air from the crawlspace zone.

Having discussed this to this point, most people want to know what I would do? What is my recommendation?

After closing all vents to the outside and doing the covered moisture barrier on the dirt floor (pea gravel is my preference – it’s cleaner) I would do the third option and spray foam the perimeter walls to R30 or R40 insulation values.

Then I would have the foam covered with a sprayed on concrete layer. Both spray operations are guaranteed to fill all voids and get 100% coverage. And they are comparatively easy and faster for workmen in cramped spaces. The covered moisture barrier on the floor would actually take the longest.

As for heating I would have the ducting modified to introduce some air and heat here but not to fully heat the space. It would be mostly to create some air circulation. I would then install a few electric baseboards (as back up – set on minimum levels).

I also would have all the old insulation in the floor and on the ducts taken away. The ducts (after getting them cleaned) would then be checked and sealed (taped) at all joints and openings. The ducting will transfer some of its heat into the surrounding air of the crawlspace but that’s ok, even desirable. The crawlspace is now part of the heated envelope of the building. It is just not maintained at as high a temperature as the floors above. The floor overhead will be at a comfortable temperature as the heat transfer between floors will be low.

We are a ways away from adding a few batts, and placing some VB where accessible. Neither are the ducts cold now.

But we’ll have invested in this home, permanently solved the cold floor problem and controlled the crawlspace humidity. It will have been done with consideration. There should be no negative impacts from the changes made and although they weren’t cosmetic (none of it is visible to visitors) these changes will have increased the value of the home.

A Crawl Space in Need of Insulation This is the blog that triggered me to write this one.


Original blog post on ActiveRain: Link to Blog Post