Crawlspace Wall Insulation
What's the story on adding rigid foam board to the crawlspace block walls?
I'm looking into sealing my crawlspace. Currently there is 6 mil plastic on the ground and the vents are closed. I want to add plastic up to about six inches from the top of the blocks. Before I do that, I'm thinking about adding rigid foam board to the blocks.
Will check the county building code for requirements.
Is adding the foam boad a good idea before adding the plastic over the blocks?
Is foil covered better than straight foam? If so, which direction for the foil?
Recommended foam board thickness?
Termite treatment company is on-board with the idea.
sounds like you are generally in control. greenbuildingadvisor.com has several articles on this. j
Earnie, here's what I would recommend:
1. Seal the crawl space vents and doors with airtight covers.
2. Install rigid foam board on the crawl space floor.
3. Install a 10+mil vapor barrier on the floor and 1' up the walls.
4. Install spray foam insulation on the walls.
5. (Optional) Install a self-draining dehumidifier in the space to keep it dry so pests won't be an issue.
6mil vapor barrier's only good IF its never walked on again imo,,, we like 15mil stego,,, i wouldn't worry about insulation of the floor so much as the walls depending on your geographical location - ambient earth temp's usually about 67f as i recall.
Conc, I've heard the ambient earth temp is about 50... am I wrong?
Personally, I use a 20mil vapor barrier and encourage the homeowner to use the crawl space as storage, but few people are willing to bid on the extra expense of that kind of material nowadays.
Here, I found a source that says it's about 50-60F year-round. See page 2 of this PDF:
Many thanks for the replies and information.
After doing a bit more reading and calling the County building department, here is a tentative plan.
- Cover the existing 6mil poly with a 12 - 15 mil reinforced poly.
- Install Dow Thermax White Finish polyisocyanurate board on block walls up to six inches from the top of the blocks. Allows termite company to inspect for activity.
- Possibly upgrade existing house dehumidifier that is in the crawlspace with a commercial unit.
- What should be installed first, the poly or the foam board? Install poly one foot up the wall then install the foam board, or install foam board and lap the poly one foot over the foam board?
- What about Radon gas? I am in a low (zone 3) area but it's still a concern. Will the complete sealing of the ground help to keep radon out of the crawlspace?
Anytime, Earnie. :-)
That Dow Thermax White Finish polyisocyanurate board sounds like a pretty good plan. Just be sure that you install a continuous, unbroken sheet of insulation along the walls-- any gaps will significantly affect its insular value. That's why I usually recommend spray foam instead. :-)
The poly sounds great as well. I'm also very on board with the commercial dehumidifier -- I recommend the SaniDry or a Santa Fe model.
One word of warning -- if you have crawl space vents, it's extremely important that you seal them. Check with local building codes, of course, but you're trapping outside moisture, rain, and wet air inside your crawl space if you don't -- and the insulation won't be much help either if you have open vents. Also install a plastic airtight door. I can't emphasize enough how important that step is.
I would personally install the foam board first, then the poly.
As far as radon gas goes, a test kit costs $15-25 at your local hardware store. Now, before you seal your crawl space, is the time to find out if that's going to be an issue -- you never know. That said, crawl space encapsulation will create a passive radon mitigation system in your home that will hold the gas back and help things out significantly.
Microsoft Word - RE_06_64_07
Cover the foam as you don’t want foil faced as previous article.
2.1. Continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm (0.47 L/s) for each 50 ft2 (4.7 m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.8;
You may get by with an ignition barrier: http://www2.iccsafe.org/cs/committee...E_06_64_07.pdf
The Thermax board is available with a ship-lap seam. That will make sealing a bit easier. I'll proabaly go with either 1.5" or 2" thickness.
I closed off the wall vents this past spring but there are still air gaps that need sealing.
Thanks for the BSC link. Only issues I have is that I will not be able to install the foam board down into the ground along the block wall. Also, I don't think applying the foam board all the way up an over the top of the block wall is a good idea. What prevents termites or other bugs from crawling behind the foam board up to the wood?
I still need to look at the radon issue. I noticed mitigation systems that can be installed under the vapor barrier.
Earnie-- try treating the wood with boric acid, or borate. That should deter pests and stop mold.
Pool liners make for great vapor barrier in the crawl space.
You can often pick them up quite reasonable form pool shops, that were made the wrong size.....or free if you catch someone tearing out a pool. Some buy and sell papers/websites will also have them.
For taping up the vapor barrier seams, house wrap sealing tape or Tuck tape works good.
CT - Nice idea on the borate. Just did some reading on it. I forgot about "20 Mule Team....." Might be a good idea to place some borate along the block wall and interior pillars before installing the poly.
Rick - I'll check some local pool stores. Thanks.
To Gary or anyone else that knows:
I'm still stuck on the foam board installation. I've read and re-read the BSC article about installing foam board to the block walls. I still do not understand what they are recommending.
Up to two inches of unfaced extruded polystyrene (R-10), four inches of unfaced expanded polystyrene (R-15), three inches of closed cell medium density spray polyurethane foam (R-18) and ten inches of open cell low density spray foam (R-35) meet these permeability requirements.
In crawlspaces where the insulation material will need to be covered by a fire/ignition barrier, it may be acceptable to use fire-rated foil-faced insulations. However, such requires careful attention to supplemental moisture management strategies. With vapor impermeable facings on interior insulation, it is possible that water may accumulate between the insulation facing and the inside surface of the foundation wall. The airtightness of the assembly is, therefore, extremely important to prevent the exchange of air between this damp interface and anywhere else in the building.
This section has me confused. Per my building code, I need the foam covered by a fire rated material. Thermax has 1.25 mil foil on one side and 0.9 mil foil on the other side. This exceeds the 0.41 mil thickness required by R318.104.22.168 (Corrosion-resistant steel having a base metal thickness of 0.016 inch (0.41 mil)).
- Is aluminum foil equal to "corrosion-resistant steel"?
- Since I also need about 5 inches of inspection area on the block wall to check for termite activity, how would this affect the air tightness or moisture accumulating behind the foam per the last sentence in above BSC quote?
- Can anyone explain what R314.6 states about an ignition barrier not being needed if the foam has been tested? This is referenced in R314.5.4
I'll give you my thoughts here Ernie, and hopefully it will be of some help.......
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