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I am adding a 20 x 20 ft addition to my summer home. I am using 6 – 10″ sonatubes as my foundation. I am planning on putting a skirt made of 1/2″ cement board around the foundation. The site has a high water table, and I plan on putting sufficient vents in the cement board around the perimeter. I plan on putting vapor barrier just under the underlayment. Should I put vapor barrier over the earth floor? What would be the most effective insulation to use in the floor system? This is a cold weather area. Is there any other considerations you could suggest I implement?
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Thanks for the info Fred. So now I eliminate any foundation vents from the project, would it be beneficial to place vapor barrier on the ground AND on the "warm side" of the floor insulation? Would you insulate the cement board skirts? Thanks in advance.
Ray
*Great advise FreddyLYou forgot to tell him where to install his Radon gas detector so he can monitor his family being poisonned at night, in their beds.Hi Ray,Do install venting.Do install poly under the floor joist and insulate to the max in between. In the end you will have a healthy home and a warm floor. The structure underneath will be fresh and last longer.Gabe
*FreddyL,Can you spell mold and mildew?I can just see that crawl space inside a year as the underside rots away and the occupants start getting sick.Gabe
*Having done this I would go with Mr Martel's advise rather than Mr L's. Treat the foundation as though it were pilings left open to the air, vapor barrier, insulate and close off the floor just as what it is; both the floor and the exterior surface of the house.For esthetics you may want some filler around the pilings like latticework or some such.The end result is a smaller volume to heat/cool, less area to radiate heat, less exposure to rot and such and possible less construction cost. Note: In far north climates what I've described has been the proven construction method for many generations. There is no reason to deviate from the proven for a summer home. Now if you were trying to collect data for some experiment or trying to make a living off creating new and different theories that might be something different.
*Hi Ray,Anyone who loves golf is OK by me.The first time someone told me not to vent a crawlspace I was incredulous. It's one of the basics of building. But it is quickly becoming accepted as a fallacy. More than one engineer has told me that crawlspace venting is a now considered a bad thing.Bringing warm, high humidity summer air into a cool place will cause condensation. Two years ago I replaced many of the beams under a twenty-five-year-old house built on a ventilated crawlspace. It used to literally rain under the house in the summertime. I built several additions onto it, covered the floor with Tu-tuff and stone, and sealed the vents. It's been dry as a bone under there ever since.To me it is a question of where you want to stop the thermal envelope. If you are building on piers you can treat the area beneath the floor plane as exterior space and insulate at the floor plane, but then you will want to keep the skirt around the house as wide open as possible to keep the space underneath the house at or near ambient exterior temperature. It's when you let it get too cool under the house then allow high humidity summer air into that cool space that you have problems. If you opt to do this, leave it wide open underneath--nothing more restrictive than lattice.Or you can seal the crawlspace and bring it into the thermal envelope. This will make for a more comfortable house in the long run, but then you do have to deal with moisture and soil gasses coming out of the earth. The Tu-Tuff and stone is usually enough to take care of moisture, but if radon is a problem in your area you may want to test for it and see if you need to deal with that seperately.StevePS: here's another anectdote:My 170 yr-old house is built half over a non-vented crawlspace that's always been unvented except for two holes totaling about 10 square inches. The other half is over a full cellar that had three cellar windows/vents totaling about ten square feet. Both are over dirt floors. The original floor system over the crawlspace is perfectly sound. The floor system over the cellar was completely rotten and had to be totally rebuilt. Now I keep my cellar sealed. This is Central New York climate.PPS:What Fred B says is partly true, If you leave the crawlspace outside the thermal boundary you have smaller volume to heat, but you have greater surface exposure to exterior temperatures (400sf of floor in your case, vs. say 120 sf of additional exterior wall exposure if three sides of your 20x20 addition are exposed and two feet above ground). Winter ground temperature within the crawlspace will be much higher than ambient exterior temps, though certainly cooler than inside the house. My guess is it's about a wash BTU-wise.
*Hi Steve,If your theory were true and accurate, every porch on the continent would collapse. You don't have to be a rocket scientist for this one. Gabe
*I have a simular situation, we inherited a TINY summer cottage that was built on the ground (used bricks for the piers, which have sunk in the ground) I will replace with new piers and have removed the old floor (I now have a box and dirt floor) but the space between the (new)floor joists and ground will be less than 16".Would it be better to just pour a slab of concrete and then build the floor on top, or stone-vapor barrier-air space then the floor. The soil is very sandy and can get very wet when it rains but will dry.Thanks
*Hi Jon,Assuming that you do have a very tiny area to do and you only want to do this once you may want to use concrete.The best would be to fill and compact the center area so that you end up with a 3 to 4 inch slab of concrete for the main area, and fill the perimeter with concrete to make a small footer.Put two 1/2" rebars around the outside footing area and residential grade WWM into the slab, over poly.If you are in cold weather region, dig down a little farther and install a 2 foot strip of 2" Styrofoam SM around the slab and let the slab float.Gabe
*Thanks. The house is 13'x40' What is WWM? How deep does the footer need to be, This is in NJ shore. Since its sandy soil the frost line is deep?Thanks
*Hi Jon Welded Wire Mesh.Your tiny cottage is a little bigger than I had imagined.If the soil "drains properly", you should be able to go down only 2 feet below grade and use an additional 2" styrofoam extending AWAY from the cottage footing at least 2 feet. This will prevent the frost from getting under your footings and lifting.This would save you 2 feet of extra digging and some concrete as well. Gabe
*If I dig down 2' do I have to fill it up with stone then 4-5" concrete on top? The slab will only carry the load of the floor and contents not the walls & roof - they are on piers. Do I still need to go below grade 2'.thanks.
*Hi Jon,Having the structure on piers that you first described as sinking, I thought you had removed the entire substructure not just the center portion of the floor.I'm a little worried that I may have taken you down the garden path.What I think would suit your problem is to lift your cottage off the piers onto a skid and move it over, clear of the existing piers.Put sono tubes into the ground, past the frost (4 Ft.) and up at least 2 feet. The sono tubes normally come in 12 foot lengths so cut them in half.You will need about 6 of them. Put one on each corner and every 8 ft. in between.After you have them set in place, pour your concrete to the top at a level elevation with all the tubes.Then build a proper deck to accomodate the entire structure on top, making sure that you insulate properly over a poly.Even if your cottage is not insulated now, you may want to in the future so it is much easier and cheaper to do the deck now than wait till later and have to crawl undeneath to do the job.Once your subfloor is glued and screwed on, skid the structure back on and lower into place.Personally, I don't skid. I would bring in a crane and lift that sucker off the piers and set it aside till I was done, bring back the crane and put the cottage back on the finished deck.If you inherited this little cottage, you don't have any money in it now, so the money that you would spend to bring it up to acceptable standards in money well spent.GabeOnce your subfloor is installed
*drvnputt: Just where is this thing of yours located?My answers are in the context of cold climates where insulation is only part of the equation since soil temperatures are low and soil is often unstable. Perhaps if you are in say, Arkansas, this whole thing of energy efficiency becomes an academic exercise for the underemployed. But, if you are planning something in say, Yukon Territory, then it is something to be taken seriously and worth the brain time.
*drvnputt,
Regards,Joseph Fusco View Image
*Well like a lot of these threads it seems after the initial post everyone just swaps their own heartfelt ideas. With regard to your last post Mr. L:The type construction I described is very common in much of the North American Arctic and Sub-Arctic. These homes are built by a wide variety of people and agencies. Since this is some of the most hostile climate on the planet and they are building this way I hardly think the idea is discredited. Perhaps among the theorists, but not among those who are trying to produce cost effective, habitable homes.As for volume having nothing to do with heat loss. Poppycock. When one increases volume one increases surface area of the structure. So, not only do you then have to heat and maintain more volume you have more area to radiate heat to the outside. You also have more area to construct and maintain.As for heat transfer to the outside from drvputt's "crawlspace" vs setting on "pilings". My vision of the original question was a "crawlspace" that would extend to the surface of the ground, much like skirting around a mobile home. That being the case, depending on a variety of factors the resulting additional area of 3 or 4 feet of vertical wall would more than offset any reduction in heat transfer to the earth. Now, if you were talking about the "crawlspace" extending into the earth as might be common in a surburban home my answer might be different.Construction cost of just sheeting in under the floor would certainly be lower than constructing a proper wall the 3 or 4 feet to the ground.Also, moisture control would be much easier if placed off the ground as would the maintenance after the inevitable pests chew, burrow and other wise try to claim the home as their own.But, all that said, to keep an eye on the big picture: This is a summer home that depending on where it is may make all this efficiency stuff a non-issue.
*FredB - Could you explain your basis for the statement "When one increases volume one increases surface area of the structure. So, not only do you then have to heat and maintain more volume you have more area to radiate heat to the outside. You also have more area to construct and maintain." Looking at a 20' X 20' structure - thats 80 ft perimeter by - say - 3' high is 240 sq ft that needs to be insulated and sheathed. To do the floor, its 400 sf ft. You won't loose or gain significant heat through the ground but that's also very easy to insulate. It would appear to me that both the initial cost and life cycle cost is less by insulating a skirt around the perimeter.More, if you insulate within the joist space, the lower portion or edges of those joists will be condensing surfaces at least in the summer - when they are cooled overnight and warm, moist air flows against them in the day. Or did these plans include a mositure retarder on the outside?And what does one do about mechanicals if you don't enclose the crawlspace - or are you recommending this strategy only where there are no mechanicals?And I'm still curious why if its a good idea to insulate the floor and vent a crawl space, why isn't it also a good idea to insulate a full basement?
*Gabe,No, you don't have to be a rocket scientist for this one. Warm moist air hitting a cool surface condenses.The reason porches don't rot underneath very often is that they are essentially exterior spaces underneath. They are usually not as cool as a crawlspace or cellar. They don't have a fully-insulated house sitting on top of them to keep them cool underneath. They also have a huge area of "ventilation," so much so that it's not really ventialation in my book. It's exterior space with a decorative screening around it.If you will note, I did propose that by really opening up the skirting to the point where it is like a porch would be one alternative. It'll work if you keep the ambient temperature under the house close to the exterior ambient temperature. After that it's just a matter of which approach is the most cost effective and healthy for building and the occupants. Both can work, but at what costs?Steve
*Fred B offers good advice in that you put up lattice and consider the space under your joists as the great outdoors...Fred L also offers great advice in that you seal the great outdoors out...Gabe is typically Gabe and thinks he knows it all...If you think he does then drink his kool aid.Joe drinks the kool aid...Steve is a cool dude and I believe a serious contender for the proper "middle ground"....Me....your Gad fly.Near the stream,J
* Jack,
Joseph Fusco View Image
*Hijack,Must be tough getting those splinters out of your butt when you spend so much time sitting on that fence of your's, judging others.Fred B is okay because he says ventilate.FreddyL is okay because he says don't ventilate.I'm wrong because I'm me.Joe's wrong because every once in a while he agrees with me and vice versa.Steve is cool for whatever reason.And you're a pesty fly.Strange Jack, you not by the stream again, you're under it.Gabe
*Thanks alot for the info. Ray
*The addition is located in Central New Hampshire. Gets cold, though not as bad as the Yukon, I would think.
*Joemethinks Jackie is referring (or is that reefer-ing)toi Electric Kool-aidwhich might explain Gabe's on-again-off-again attitude. . . recurring flashbacks (to unhappy times)sweatin in Ontario-pm
*Hi Bill, glad to explain what I was talking about. This is a good way to communicate but it is tough to always write clearly.First of all energy efficiency is only one factor in a home's design. We have a tendency to concentrate on that to the exclusion of other factors like how healthy the home is and how easy it is to maintain. In this case all I was trying to say is that: "The entire volume of a house that is inside the insulation envelope must be heated/cooled." So, the easiest way to minimize energy cost is to keep the volume as small as needed for the planned purpose of the structure. Design it small and keep the roof cavity and crawlspaces out of the insulated envelope if at all feasible.We have a tendency to think in square feet when talking about a structure when thinking in terms of cubic feet is more appropriate for this purpose. Cubic feet is why the energy saving mavens advise to close off unused rooms to save heating or cooling costs.As for what to do about the heat transfer through the floor joists; since we all know heat rises it isn't as much problem as walls or ceiling. So, some just pack the floor with insulation and sheath with plywood or somesuch. Others, put a thermal break of rigid insulating board and then sheath.Mechanicals can be accomodated in the floor space, have extensions on the joists to accomodate them, put in their own insulated chases, or run through the ceilings or walls. Whatever works in that structure. That is a problem with this type structure, especially in very cold climates.
*Energy loss from any volume is proportional to the surface area, not the volume, which is why the skirt wall around the perimeter makes more sense to me. Its not the volume that cost to heat or cool, its the loss through the area surrounding it. And some 80% is by convective losses, which is why the prospect of all those holes and chases scare me away.My comment about the cold joists was regarding condensation - not heat loss. And while warm air rises because of the heavier cold air displacing it, the solid joist is conducting heat and not dependent on gravity.I do agree that reducing volume usually will reduce the area exposed for heat transfer - remembering Bucky Fuller's works at maximizing the volume within a given surface area - and that I'm dismayed by the giant mansions being built today.
*Scuba Jack attack Gabester!!!Please always answer questions on this board!!! I love the entertainment value!Near the stream, underwater swimmin with the best of my pet lobsters....Some day their gonna eat you all up!!J
*Great booze Joe...pour me one!!!Near the Fiddich stream myself,J
*I'm from Arkansas and around here we spell humidity, A-R-K-A-N-S-A-S. When I was a kid I hated going under our house because it was always so damp under there, even though it had the prescribed vents. Now I know why. And bugs seemed to love it. (Don't see many grubs, termites, etc. sunning themselves.) Had a mobile home years ago and I installed vinyl skirting all around. Used treated 2x4 on the ground with a vinyl track attached. It was really tight -- didn't leave any openings (don't know why -- I just didn't). What amazed me was that when I needed to get under there (I'd just slide a panel up and out) it was always dry! Now I know why. My take on this is that moisture in the ground tends to stay there, especially if you use a good vapor barrier over it. I do believe it is the humidity that causes the rot problems. And insulating the perimeter vs the floor is a no brainer -- the "typical" crawlspace walls will always have a fraction of the surface area of the floor. (Isn't that obvious?)As I've related elsewhere, I have a friend that built his house on a sealed crawlspace, super-insulated the foundation, and uses the entire crawlspace as the inlet plenum for his heat/air system -- no supply ducts.
*Hi Rusty,One of the dangers of discussing on the internet is the vast differences in the geographic locations of the participants.You cannot attribute the difference in conditions under the crawl space of your homestead to your trailer simply on the basis of a closed in skirt. Even with a poly on the ground, the moisture would still collect under it and the mold, mildew and bacteria would still be present.Depending on the soil makeup, water table, surface water drainage etc. what would seem to work in one place would fail miserably everywhere else.With regards to your friend, this is something that would be considered a health risk in most jurisdictions. Makes for very poor quality air with high bacteria counts.Here we spell humidity, H-U-M-I-D-I-T-Y.Gabe
*Bill: Just a quicky.It is a fact of geometry that as volume increases so does surface area for any given shape. One can reduce surface area for a given volume sometimes by changing shape. For example: With a given volume of a cube one can reduce the surface area by changing the shape to a sphere and not changing the volume. The reverse is not true.Since not many folks want to live in spheres we are left with the geometric principle for our boxes(homes) that as volume increases so does surface area.
*FredB - I'm not sure of what your point is - I don't think I said anything to the contrary. In terms of energy it's still a matter of area exposed and the temperature difference. Consider a "u" shaped house. Wall and roof off the "U" and you will have more volume with less area and less heat loss. In the crawlspace situation, the heat loss to the exterior is much more significant than to the ground thus by increasing the volume you may decrease the area exposed for heat loss.This is all pretty funny since I have long been an advocate of building off the ground on piers - but it has to be completely open under so there is no difference in temperature and humidity - a little bit of venting doesn't do it - only creates problems. And no poly or other so called vapor barriers for me either.
*I don't know, there are an awful lot of collapsing porches here in central Illinois...
*Hi Rebeccah,Interesting observation.Now, what do you attribute this to? Age? Moisture? Ventilation? Poor design? Poor construction? Poor maintenance?Join the fun,Gabe
*Hi Rebeccah,Plenty of them around here too. But the rot is usually from weather exposure from above rather than ground moisture.I like to say God invented porches to keep carpenters in work. It's tough to maintain transitional areas beacuse they are being soaked all the time. Even with the best attention to water-shedding details, the constant exposure to the elements takes its toll in the long run.Steve
*To be added to "the Best of Fred L's" Collection!!!!I wanted to mention the non-proportionality of all the volume and energy gab but damn you said it first and a whole mouthful of common scientific sensability more....Keep up the great posting Fred!And Gabe now...try not to chime in stupidly trying to tear me and Fred up for agreeing!!!!!.....Cause I'm.....Near the stream (just finishing the training on my "hunt to kill" Lobsters,)J
*Dumb and DumberGabe
*Hello folks:Having posed the original venting/insulating problem, I want to thank everyone for the lively exchange. It's been entertaining, educational, and certainly confusing. Thank God I still have plenty of time (still shingling the roof) to decide what to do in order to keep the damn thing from rotting. Think I'll have a beer and go golfing.Ray (Drvnputt)
*One factual comment: No Mr L it has not been "generations" as you allege, unless you happen to be a fruit fly. Current construction, and for good reasons.You favor no ventilation, most of the northern and severe climate folks I know favor ventilation. I have recited some of the reasons but I'm not an expert, I'm not sure anyone is. In any case the guy who asked the question now has his answer so we move on.
*A buckyball mansion?
*I have a 16' X 28' timber frame camp built in Maine with a concrete perimeter foundation to below frost with a earth floored crawl space. The building has perimeter drainage on the outside and around the interior perimeter of the foundation. I have insulated the floor with 6" of fiberglass and have put poly on the surface of the earth floor. I am getting water on the surface of the poly that appears to be condensation. What can I do, if anything to eliminate this problem? At present the joists, and sill appear to be dry and the moisture is only on the poly covered floor.
*Hi Margaret,Assuming it isn't a leak as Gabe suggested you ascertain over on the main page, what causes condensation is humid air hitting a cool surface. If you want to eliminate condensation you need to either warm the surfaces or reduce the moisture in the air.You might first try covering the plastic with small smooth stone and seal it to the sides of the crawlspace with acoustical sealant. If the plastic is not in fairly continuous contact with the dirt, ground moisture can work its way through holes and around the edges of the plastic in pretty large quantities.Is the crawlspace vented? If so you could try closing all the vents for a while and see if that helps. Often venting a cool crawlspace with warm summer air causes more problems than it solves. Try closing the vents for a few weeks and see if the condensation clears up. On one house I did some work on, just closing the vents reduced the condensation dramatically, though it did not eliminate it.Another possible solution is to insulate the perimeter wall of the crawlspace rather than the floor joists, thereby bringing the crawlspace inside of the thermal envelope. This might warm the crawlspace enough to stop the condensation.As always, try the simple and cheap solutions first. If they don't work you can move on to more elaborate measures.Good luck,Steve
*Rusty et al. In January of 1994 ASHRAE held a meeting in New Orleans, Louisiana to discuss Recommended Practices for Controlling Moisture In Crawl Spaces. The result of the meeting was a booklet of papers of the same title.Attendees were from all parts of the USA and from as far away as New Zealand and Sweden.The Swedish building scientist Samuelson noted that introducing outside air into a cool crawl space during summer, when temperatures are most conducive to mold and decay, actually raises crawl space humidity and can easily produce relative humidities of 85% to 95% even in a crawl space with dry soils or a soil cover. Of course Gabe will object to this because Samuelson didn't say where the crawls paces are, what the soil conditions are and so on ad naseum. Finally Bill Rose of University of Illinois in summarizing the 8 papers said, "There is no convincing technical basis for current code requirements for [crawl space] ventilation." Except for Gabe I know of no one who takes issue with Rose's conclusion. To put it in George Gobel's jargon, ït don't make no difference not no how where the crawl space is located. Cover the ground floor with a VDR and do not vent. GeneL.
*Anybody else notice the latest issue of JLC? (August) There is a (anti) crawlspace ventilation story that in the lead paragraph says "quality builders know that a crawlspace should be carefully sealed and insulated".The story cautions against sealing a wet crawlspace though, saying "you've got to make sure that you've eliminated the four moisture transfer mechanisms--liquid flow, capilarry action, air-transported vapor, and vapor diffusion--and let the crawlspace dry out before you insulate and seal it."For new construction they recommend letting it dry out over a winter season or using a dehumidifier to dry it before sealing.Author is Don Best. Quotes John Tooley a lot.Steve
*Steve. Your post was most timely. I haven't read the JLC article, but recommended to a questioner exactly the same advice. Don Best is the editor of Energy Design Update.GeneL.
*There you go Steve,We agree on another aspect of construction.You can't seal off a crawlspace unless it's dry to start with.The problem is most are wet from one source or another, or one season from another.If the crawlspace were guarenteed dry and free of radon gasses, I would bring it into the envelope too.But here and in more areas than not, the space is wet.Gabe
*Hi Gabe,Well whadda ya know, Gabe, consensus!But...If the crawlspace isn't dry and free of radon gases, then passive venting is not a very good solution. Radon needs to be actively vented out of the space, not just diluted with damp outdoor air, and the difference should be made up with dry, conditioned air. And any moisture sources should be eliminated or reduced. A damp cellar with open windows is a poor solution. Na ja?Steve
*Hi there Steve,Consensus on 2% of the equation is shaky but still along way to go.If you remember all of my comments about air exchangers you will note I don't subscribe to passive air exchange.With the exception of a hunt camp or two and a cottage or two, I do not build over crawlspaces. It is virtually impossible to justify the cost of a properly constructed "unuseable" crawlspace and not the cost of a fully useable basement foundation.In our neck of the woods, a crawlspace would still require a frost footing and walls around the perimeter.Most crawlspaces discussed on this board, are existing older homes that required hand digging. The interior is normally uneven and full moisture and or ponding.IF the crawlspace is level, at least 3 feet clear, properly drained with all organic material removed, with a layer of sand over a continuous poly, a full concrete or masonry wall around the entire perimeter, fully insulated with polystyrene, you MIGHT have the basis for consideration of inclusion into your envelope. BUT it would still require an additional source of dry heat on it's own zoning and the ability to ventilate or exchange the air on a regular basis.The final benchmark should be costs vs comfort.It is still less expensive to isolate then to include the crawlspace into the envelope unless you live in an arrid desert condition or on top of a dry sand hill.Gabe
*Hi Gabe,I too think that if you are going to build in our area (48" deep footers required), you might as well make a usable basement.My dealings with crawlspaces are almost exclusively in regards to older houses. It is very common around here for a house to be on a "half-cellar" the other half being over crawlspace. I would say most of the time the crawlspaces I deal with are fairly dry. More common for the cellar half of the foundation to be wet. Groundwater has a better shot at getting to the surfuce of such a deep hole.The crawlspace under the front half of my house is bone-dry, and has never had a vent larger than 6 square inches. It was never accessible for the last 130 years. The framing, 165 years old, is 100 percent sound. The cellar half, on the other hand, has had the floor joists replaced twice even though it has about 8 square feet of ventilation openings. Since I've started keeping the vents closed, it stays much drier, though I think I still will put a dehumidifier in it.Steve
*Hi Steve,Funny you should have that experience because I lived in a old stone house that had both as well. The upper crawlspace side was dry as well but it also was well above the water table and had no water pipes in it at all.The lower or full basement part was damp because is was below the high point of the water table and also had the hot water tank, water pump and all the plumbing for the upstairs. We had to use an exhaust fan and sometimes we ran the dehumidifier.However, it had 24" round cedar beams, with the top sliced off on the top for the floor planking above, so we never had to replace them.Gabe
*
I am adding a 20 x 20 ft addition to my summer home. I am using 6 - 10" sonatubes as my foundation. I am planning on putting a skirt made of 1/2" cement board around the foundation. The site has a high water table, and I plan on putting sufficient vents in the cement board around the perimeter. I plan on putting vapor barrier just under the underlayment. Should I put vapor barrier over the earth floor? What would be the most effective insulation to use in the floor system? This is a cold weather area. Is there any other considerations you could suggest I implement?