Sorry for this question not being very green but there are plenty of good builders on here that may be able to offer advice. I'm planning a new build (masonry) construction and I need to decide over the foundation structure. The ground structure is around 1m of clay on top of cornbrash (many layers of limestone separated by gravel or clay). It's pretty strong and the BCO thinks it is perfect for foundations.

In my mind up until now I had been assuming the foundations would be some form of insulated raft. It should be easy to provide a good insulation level and minimise thermal bridges. However, it would require the removal of a fair amount of spoil and would need quite a lot of concrete.

My architect is suggesting strip foundations with a beam and block suspended floor and airbricks to ventilate under the floor. I can see that this probably needs less concrete and, less spoil removal and its quite well with the ground make-up. However, I'm concerned about how to fit the insulation effectively under the beam and block floor and in particular about thermal bridges from the foundation structure into the internal walls of the building. (Above ground it is 215mm single skin block with external insulation.)

We could go with a suspended beam and block floor using EPS inserts instead of concrete blocks, which looks like it could get down to a U-value approaching 0.15, which is probably close enough (although I would like better). In this case, how do I minimise the thermal bridging from the foundation up into my structural wall?

The alternative would be strip foundations for the structure with some form of poured and insulated slab inside to form the floor. This appears to be slightly better than the insulated suspended floor since there is no need to ventilate the sub-floor and therefore the foundation walls will be closer to ground temperature than external air temperature thus reducing the effects of the thermal bridging. Probably means more materials and concrete than a suspended floor though.

Bottom line - what is the best way to construct a highly insulated ground floor structure, minimising thermal bridges while trying to keep the material use and spoil removal to a minimum?

Thanks Tony - knew I could rely on you! Seems my fears of beam and block for this are rational!

So, if I follow what you are suggesting then I think it looks like:

• Strip foundations.


• Concrete block to DPM.


• XPS insulation on both sides of blocks down to foundation level.




• Join with external insulation on outside


• (should minimise thermal bridging)




• Hardcore


• DPM


• Concrete pour to ground floor level


• Insulation (Optional depending on view of ground as insulator)


• UFH/Screed/Finish.

Does this sound right?

(I will go with insulation because I know that groundwater passess through the soil below the house (on top of the cornbrash layers) and will take some of the heat stored down there away every time it rains.)

Thermally, it would be ideal to join the internal foundation insulation to the slab insulation but this would mean the slab wasn't tied to the wall, which is presumably a bad idea. Could the insulation go between the hardcore and concrete slab? This would allow the slab to be tied to the wall and the insulation be connected up.

Any ideas on how to optimise this further?

Tony
Please explain why there are air tightness issues with beam and block I would have thought it would be simple enough to make sure all the gaps are filled with screed. Belt and braces would be a membrane between screed sandwitch?

hopefully you wont need any ufh or heating

Posted By: tonyhopefully you wont need any ufh or heating" alt="" src="http:///forum114/extensions/Vanillacons/smilies/standard/bigsmile.gif" >

I *need* it if my wife and family are going to move in. I hope I never have to turn it on!

Hang-on, on second thoughts maybe I'm better off leaving it out

(Apparently the average 4 year old asks 437 questions each day, or about one every 1m40s. There are only so many times that you can hear the question "Why?" in one day and retain some form of sanity.)

Posted By: joe90what about using this type of construction?

http://www.viking-house.co.uk/passive-house-foundations.html" >http://www.viking-house.co.uk/passive-house-foundations.html

Yes, the version for masonry walls ("G" Element) looks pretty good. The blockwork and external insulation look too thin, so the ringbeam would probably need to be scaled up.

Anyone know where one can get EPS 300?

Why not include a basement? Then the "ground" floor will be the ceiling of the basement and you won't have to use concrete. You could use engineered trusses, tongue and groove plywood on top and then, if you want UFH, embed the pipes in lightweight concrete on top of the plywood. Or gypcrete rather than concrete etc. If you do it right the basement will only be half buried so that you can have some windows to let in light (bottom of the window is at ground level or just below). This minimizes the amount of excavating you have to do. For the basement floor, you can either put insulation under it or not, so long as you insulate the outside of the basement wall. Once you're a couple of feet below ground level then it doesn't make much difference if the floor is insulated or not. Then there's also no worries about thermal bridges between the top of the basement wall and the structure above. Or if you're doing masonry construction anyway, go the whole hog and do the entire building using ICFs :) Though the anti-concrete people will be down on you like a ton of bricks.

Paul in Montreal.

Hi Mark. Just to let you know that Basements are now considered non-material amendments which can be added to a previosly approved scheme. So the Planners cannot relly refuse if you play your cards in the right order. Non-material amendments are also free [at the moment]

Life style and a semantic question

i would insulate under the slab as it them becomes part of the thermal mass of the house

For intermittent heating you could go above

Foe nice comfortable living under is the way to go if indeed any insulation is needed at all -- I ve included the ground under my house into the thermal mass

I have no formal heating to pay for, install or maintain and its 20.5 in here today.

Posted By: tonyLife style and a semantic question

i would insulate under the slab as it them becomes part of the thermal mass of the house

For intermittent heating you could go above

Foe nice comfortable living under is the way to go if indeed any insulation is needed at all -- I ve included the ground under my house into the thermal mass

I have no formal heating to pay for, install or maintain and its 20.5 in here today.

I basically agree with Tony, except that I(we)'re not quite brave enough to go for no heating at all. PHPP indicates that I will need some small heating for the coldest days, but Tony is clearly surviving nicely without any and we're insulating to similar standards although our house is bigger and I suspect we have more glass. I'm hoping we never need to turn it on.

Posted By: MarkBennettXPS insulation on both sides of blocks down to foundation level.

• Insulation (Optional depending on view of ground as insulator)

If the decision is minimal or no slab insulation, then no need for the XPS inboard of the foundation wall - just outboard.

Even if regarding the underfloor soil, within perimeter-insulated found walls, as useful heat store cum in-depth insulation, it's been suggested to me that it's still a good idea to have a minimal bit of under-screed insulation e.g. 25 of EPS, just to bend the temp gradient curve and maintain the floor surface closer to room temp, whether warmer or cooler, than would otherwise be the case without that insulation. Not sure if I fully understand/agree with that.

If regarding the underfloor soil, within perimeter-insulated found walls, as useful heat store, remember it's only the top/innermost 100 or so of anything, however deep and massive, that really plays a part, if heat input and output are both to/from the top/innermost face. If you want to activate all that subfloor soil depth as heat storage, you have to somehow input the heat at a lower level. That's when the thinking gets interesting!

Posted By: Viking House

Yes, the version for masonry walls ("G" Element) www.viking-house.co.uk/passive-house-foundations.html looks pretty good. The blockwork and external insulation look too thin, so the ringbeam would probably need to be scaled up. Anyone know where one can get EPS 300?

You would need some load calculations on the weight of your house to see what size ringbeam you will need but it can be widened quite easily. We built a house using this foundation system using 9 inch cavity blocks with 250mm external insulation in 2007 and it works fine. I personally think that concrete blocks steal heat that you don't have and are mostly cold to touch, that's why I prefer Quinnlite or Aircrete blocks. There are a few manufacturers of EPS 300 in the UK but the foundation system is patent pending. We usally charge €500 for the use of the patent and all the technical back up required to build your foundation, we can produce drawings for €150 extra, our information is usually sufficient for an Engineers sign off but it would be best to talk to the engineer first. The excellent prices we have negotiated from the EPS manufacturers are passed onto each client, so the €500 is easily won back.

Both foundation systems have been passed by the Passiv House Institute and it seems to be the only foundation system available that allows you to build a Super Insulated Cold Bridge free cavity wall.

The 150mm Quinnlite wall seems to be sufficent if you use ringbeams at every floor.

Thanks for the info. We may well be in touch nearer the time.

Note that the AI Passivhaus have effectively done the same thing, albeit discretely and just using standard EPS: http://www.aipassivhaus.com/index.html

One advantage of the AI approach is that they still have the full EPS thickness under the slab edge thickening, whereas in your system it thins under the ringbeam. Given that the maximum heat loss is around the perimeter of the floorslab this might make a difference, but I haven't attempted to do any sums.