Design detail for kitchen extension on helical piles
We are in the process of having a 15’x5′ kitchen extension built on helical piles and have some concerns about the detailing of the area underneath the kitchen floor. We are located just outside Toronto so we are subject to humid summers and moderately cold winters.
The current design as approved by the local building department is a 6” ¾ gravel base on top of a sandy base (our soil drains well) with the gravel coming up within a couple of inches of the floor joists. The 9” joist space is open to the existing basement for access of services and airflow. Closed cell spray foam will be applied to the new floor joists, cement board walls and on top of the gravel. No concrete slab was poured due to concerns about the helical piles shifting. We will have plumbing and HVAC running through the joists into the kitchen that is outside the existing house foundation. A HVAC duct will also condition the enclosed 15’x5’ kitchen addition. Siding will be wood siding and I believe a scratch coat will be applied to the cement board. No foam insulation will be applied on the outside wall structure.
Questions:
1. Should we seal the space where the 2 sets of joists rest on the existing cement block wall foundation?
2. For air transfer should we have a register in the kitchen floor or one of the basement rim joists so air can move between the existing house and new addition space?
3. Should we pour some cement (2”) over the gravel or put a poly sheet over the gravel up the cement walls to act as a further vapour barrier beyond the spray foam?
Your thoughts, concerns and suggestions about the proposed design?
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Replies
Greg,
We need more information.
It sounds like you are creating a crawl space that is 2 inches high.
You wrote that you will install spray foam insulation "on top of the gravel" -- in other words (I assume that you mean) you will install a continuous horizontal layer of spray foam insulation on top of the crushed stone that we can see in the photo. How thick will this layer of insulation be? Will it reach to the bottom of the floor joists? Higher than the bottom of the floor joists?
You also wrote that spray foam will be installed "to the new floor joists." Does that mean "just barely up to the bottom" or "along the sides of the joists"?
Martin, yes, it appears to be a crawlspace that you cannot crawl in.
The spray foam will be sprayed about 2" -3 " thick on top of the gravel and up the sides of the cement board perimeter. The foam should reach up to just under the floor joist. I need to confirm but I believe there will be a break between the continuous horizontal layer of foam on the gravel and the bottom of the joist.
The foam will be sprayed in between the joists from the bottom of the joist (or just slightly below) to the top of the joist, filling the joist space.
Greg
Also... Are you not installing any insulation on the exterior walls, or just not using foam in those areas?
Greg,
Just to be sure I understand -- you will end up with 11 inches of closed cell spray foam -- 2 inches under the joists, and 9.25 inches between the joists? That's a lot of closed-cell spray foam.
(You wrote, "The foam will be sprayed in between the joists from the bottom of the joist, or just slightly below, to the top of the joist, filling the joist space.")
Martin - yes, 11 inches of spray foam. We are open to alternatives - I know it is expensive and not the most green option.
Steve - no exterior insulation. Our initial plans were mineral wool interior, rigid foam exterior but had to settle on spray foam interior with no exterior insulation. We have been trying to apply the best practices as outlined here on GBA but it is a real challenge to find contractors that will do anything outside what they normally do or know. Even if they will consider something new, you have to worry about them getting the details correct. Exterior insulation on residential homes here is definitely in the minority as well as the use of mineral wool in residential settings. Different story in commercial where I see all kinds of buildings with exterior mineral wool and foam insulation, high tech air barriers, more attention to green details.
Greg,
11 inches of closed-cell spray foam has an R-value of about R-66 to R-70. That's a huge amount of insulation.
No one installs R-70 floor insulation. Who specified that much and why?
Martin, it was my misunderstanding the joists would be spray foamed. They will not be insulated.
The actual depth of the spray foam applied to the gravel floor and cement board walls will be 5" (R31) and not 3".
The spray foam on the gravel floor will have 2" of clearance from the bottom of the joists so the foam will not touch or go into the joist space.
The rim joists on the 15' section of existing block wall will not be blocked off or spray foamed apparently to allow the space to "breath". So we will have an open rim joists behind drywall.
Greg,
OK, we're beginning to get a clearer picture of your plans.
First, you will build a 7-inch-high crawl space.
Then you will install a horizontal layer of closed-cell foam on the crushed stones that cover the floor. The spray foam will be 5 inches thick, so you'll end up with a 2-inch high space between the spray foam and the bottom of the joists. Is that accurate?
Sorry Martin. Yes, that is accurate.
Greg,
Q. "For air transfer, should we have a register in the kitchen floor or one of the basement rim joists so air can move between the existing house and new addition space?"
A. Please describe the air flow paths better. Do you want to encourage air flow from the conditioned space above (the kitchen) to the crawl space below, or do you want to encourage air flow from the crawl space below to the kitchen above? And why?
Q. "Should we pour some concrete (2”) over the gravel or put a poly sheet over the gravel up the cement walls to act as a further vapour barrier beyond the spray foam?"
A. Five inches of closed-cell spray foam is an air barrier and a vapor barrier, so there is no need for concrete or poly.
Q. "Your thoughts, concerns and suggestions about the proposed design?"
A. In general, I don't like inaccessible crawl spaces. The success of your plan depends in part on whether you have managed to reduce moisture entry into the crawl space by providing good drainage, by managing the roof runoff with gutters and conductor pipes, and by proper exterior grading.
Martin, the question on the air register is based on your article "Building an unvented crawl space where you list in the recommendations "Install a floor register in the floor above to allow air to flow between the living area and the sealed crawl space below."
We are concerned about having the rim joists open behind the drywall so we thought the air register could be an alternative approach.
While it was not in the design, our contractor suggested we vent some conditioned air from the HVAC system into the crawl space. This was another recommendation in your crawl space article.
In terms of water management we will have substantial overhangs (4’+) on two sides of the kitchen with concrete and asphalt on the ground and the third side we will have a 2’ overhang. Gutters will be all around and grading will be sloped away from the house.
We were not thrilled with the idea of a crawl space and especially since it is not really accessible but pouring a 15’x5’x9’ concrete foundation was ruled out as too expensive and wasteful (don’t need the space) so this where we ended up.
Thank you so much for your assistance.
Greg, you note that, "It was my misunderstanding the joists would be spray foamed. They will not be insulated." Just so you understand, spray foam is insulation.
Martin, couldn't Greg install a vapor barrier to the floor of the crawlspace and apply R-20 foam to the perimeter? The new area is already connected to the existing basement, correct?
Greg,
If a 4-foot-high crawl space is not connected to a basement, it's a good idea to provide some way for conditioned air to circulate in the crawl space.
But your crawl space is only 2 inches high (after you insulate the floor of the crawl space), so there isn't much air to condition. I'd skip the recommendations for exhaust fans or supplying air from a furnace or air conditioner. As long as the new joist bays are open to the adjacent basement, the crawl space will probably stay dry enough.
So, skip the floor register in the kitchen.
Steve,
I think what Greg is saying is that there will be insulation on the crawl space floor, but there won't be any insulation between the joists. I think that he is aware that spray foam is a type of insulation.
You're right that it usually isn't necessary to install R-30 insulation on a crawl space floor, but in Toronto it can't hurt. If Greg wants to save money, the spray foam on the floor could be thinner -- or it could even be eliminated, as long as there is good insulation on the crawl space walls.
Steve - I could have been clearer " It was my misunderstanding the joists would be spray foamed - they will not be insulated."
Martin - if we skip the spray foam on the gravel I assume we will need a poly vapour barrier to prevent soil gases and any insects (ants!) coming up from the sand underneath the gravel.
Greg,
Q. "If we skip the spray foam on the gravel I assume we will need a poly vapor barrier to prevent soil gases and any insects (ants!) coming up from the sand underneath the gravel."
A. Yes, that's right. If ants are a big problem, the 5 inches of spray foam will be a better barrier than a layer of poly.
Now that we understand what is being proposed, here are my thoughts:
1) There's no particular need to use spray foam as opposed to some other type of foam, and spray foam has a bunch of disadvantages. So I'd suggest 4-5" of EPS foam boards instead. Since EPS isn't as good a vapor barrier as closed cell spray foam (ccSPF), I'd then put poly down on top of the gravel under the EPS.
Disadvantages of closed cell spray foam include:
a) It's expensive per unit R-value.
b) Unless you find someone who uses the new Lapolla 4G stuff, the bubbles in it are blown with a gas that has a huge global warming potential, more than 1000X worse than CO2. (XPS has the same problem, but EPS does not.)
c) Very very rarely, it doesn't cure right. Compared to most building processes, it's quite reliable, but unlike most building processes, when it goes wrong it seems to be a complete disaster--it emits a terrible smell that won't go away, with negative health impacts.
2) I'm a little nervous about the ants, and might put an ant barrier of some sort on top of the gravel at least for the first few feet in from the walls. Maybe insect screen, maybe aluminum flashing, or maybe more cement board. I'd be nervous about that for either kind of foam, although I've heard of problems more often with polystyrene foam than with spray foam.
I'm not worried about ventilation. And I would think that a very small low power fan in one of the connections to the basement would be plenty if it turned out that you do need it.
I like Charlie's rigid EPS suggestion. Plus, Greg could use an insect resistant product. (I used Cellofoam PermaBG+ on my project because it includes an insecticide that discourages ants and termites.)
Thanks Charlie and Steve. Great advice.
Charlie - I agree 100% on your spray foam comments. I was fully intent on not using spray foam for our renovation project but we ended up in a position where we decided it was the path of least resistance to use what was being recommended rather than what we were proposing.
Steve - I have not seen Cellofoam products in Canada. Will have to check and see if they are distributed or can be shipped to Canada.
There are other insecticide-treated EPS foams, from Atlas for example, so you might be able to find an option from a local distributor even if those particular options aren't available locally.
Greg,
I have misgivings about attaching the joists to the shifting gravel substrate with any type of foam - and effectively having this as the only barrier between the exterior and your basement if you leave the new and existing joists as a continuous space.
i'm also not sure pouring a slab of any thickness is much help. What I'd be inclined to do is de-couple the structure from the grade below much as you do when you frame a bay window. I would attach 2" of foam board to the underside of the joists and cover it with Pt plywood. I would fill the joist spaces with mineral wool batts except where there are water lines or drains. Those bays would get another two inches of foam under the pipes and be left empty. Because the addition is so small, attaching the foam and plywood soffit can be done by framing the floor upside-down, and then flipping it. We do this frequently with small outbuildings or additions. Simply nail the new ledger to the existing rim joist.
Once the addition is complete, I would make a point to keep a 3" to 4" gap between the gravel substrate and the underside clear.
Whatever path you choose, good luck with your project.