New Addition - Crawl Space

Old 03-05-15, 07:14 AM
Thread Starter
Join Date: Mar 2008
Posts: 30
Received 0 Votes on 0 Posts
New Addition - Crawl Space

I am building a small addition to the back of my house in Ontario.
Excuse the crude drawing (best I could come up with).
I am interested in finding out how to insulate and protect the crawl space.
This space will have sides(not sure if its going to be lattice or PT boards - #2 on drawing).
Either way,it will be partially exposed to the elements.

In the photo attached, it shows the insulation I intend to use between the
16" centre joists. Covered with Comfortboard (#1 on my drawing).
There would be vapour barrier on top of the joists,under the T&G sub floor)
Im not sure if I should then cover the Comfortboard with thin PT plywood or perhaps Aluminum soffit ?
I then want to make sure I can seal all the gaps and spaces (to prevent mice infiltration etc). Suggested method ?

Is there any point in covering the crushed stone ground (#3 on drawing) with Poly sheet ?(or is this only beneficial in an enclosed,conditioned space?)

Any benefit to cover the 6"x6" posts and 2"x10"(x3) beams with Comfortboard and PT plywood ? (to further prevent transference of cold)?

Any assistance would be appreciated.

Name:  Insualtion and Board-1.jpg
Views: 378
Size:  29.4 KB

Name:  Screen Shot 2015-03-05 at 9.46.29 AM.jpg
Views: 494
Size:  44.9 KB
Old 03-05-15, 08:33 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 27,198
Received 942 Votes on 860 Posts
Yes, IMO, I would cover the Comfortboard with standard non-treated plywood and Tyvek. I would then bend some aluminum trim coil on a brake and cap the edges of the Comfortboard and plywood sandwich with an L. The plywood will give your aluminum soffit something to staple to, the L-flashing will cap the Comfortboard to keep mice out.

No, I would not cover the posts or beams with anything...IMO they are too far removed from the heat source to make further insulation valuable. In other words, just covering them with insulation isn't going to make them warm- they are outside!

The only benefit to putting poly down under your rocks would be to help keep weeds down... for that reason alone I would do it. (just be sure the grade slopes AWAY from this area, not toward it, or the soil could get rotten smelling from stagnant water and mold.) With almost 3 feet between the bottom of the room and the grade, there is plenty of room for ventilation.

Treated wood cannot be in direct contact with aluminum, and it should all be out of the weather anyway, so I would not use any sort of treated plywood. Be sure your siding drops down past the soffit as a drip edge.

HVAC would best be run in the attic space of this addition since you can dump way more insulation on top of the ductwork up there. Putting it in the floor compromises your r-value there.
Old 03-05-15, 09:16 AM
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
One of the problems with frost is if it can grab a hold of the sides of a buried column it will lift that column even though the bottom is well below the frost line. Never used the helical piles, but they sound like something the frost would/could lift.

In addition to the aluminum flashing caution and the PT wood, be sure all fasteners attaching to the PT wood are specified for that purpose. having removed some that were not a few years later I wonder how many structural problems are going to surface because this rule was not followed, everywhere.

Old 03-05-15, 01:57 PM
chandler's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,964
Received 7 Votes on 6 Posts
Just to illustrate why aluminum and PT don't dance together.....

Name:  bad flashing.jpg
Views: 257
Size:  26.2 KB

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: