Hi all,

I am building a new house and have had a few restrictions placed upon me, including a really short time-frame.

The foundations must be piled and also have a suspended RC slab

With this as a starting point, I am building a timber frame house with block-work exterior (rendered) but the plot also comes with a height restriction, so trying to make sure I keep the finished floor level as low as possible.

I have had a good search through the forums and most of my question have been answered, but I need to double check I am heading in the right direction with the foundation detail before I career off in the wrong direction.

http://www.ukrobotgroup.com/images/build/Foundation%20section%20plot%208.png

Some specific issues I am struggling with:

Most of the slabs I have seen on line seem to have their top edge pretty much level with the outside ground level. If the slab is sunk a little lower than ground level, do you start to move into tanking territory? Also, a builder told me that if I raised the level of the screed above the DPC I would "be asking for trouble", not quite sure why.

At the doorways I am struggling with the detail. Front door needs to be a level threshold, but not quite sure if the detail I have drawn makes sense. Most of the doors I have been looking at seem to limit the sill to around 150mm which would put the door roughly in the front part of the 50mm cavity.

Also, most people seem to be saying that the door frame should be mounted back to the timber frame and a movement gap be left underneath it. Not quite sure how to create a gap under the sill and frame, doesn't sound right to me.

http://www.ukrobotgroup.com/images/build/Foundation%20section%20plot%208%20flush%20front%20door%20050217.png

The rear has a bi-fold door which I also want to get as flush as possible. The trouble with these is that the channels the doors run in need to be sunk down into the block-work. As my DPC is at 150mm and the finished floor level is not much higher, not sure if I'm allowed to install the channel lower than the DPC so it's top edge is flush enough. See example from Kloeber (but their design allows them to sit above the DPC)

http://www.kloeber.co.uk/shared/media/documents/87/Standard%20Track%20Flush%20Installation%20Aligned%20-%20Externally%20Opening.pdf


Thanks for any assistance.

Pete

Posted By: OracsRevengeI am building a new house and have had a few restrictions placed upon me, including a really short time-frame.

Hi Pete.

I would do my utmost to get that restriction lifted. Things don't happen to a planned timeline when you are building, unless you are a miracle worker. Plus things that need to happen to time rapidly get a lot more expensive.

The foundations must be piled and also have a suspended RC slab

Is a passive slab feasible? If so have you costed it as an alternative?

I have had a good search through the forums and most of my question have been answered, but I need to double check I am heading in the right direction with the foundation detail before I career off in the wrong direction.

You seem to have an awful lot of timber underneath the frame. What is the reason for that? It's a big thermal bridge, especially with what looks like a concrete block underneath it. Why does the internal plasterboard stop so high above the floor? Why a full block wall rather than using render boards to carry the render? Have you considered using EWI on the outside of the frame?

Slabs tend to be level with the ground to facilitate level thresholds. They tend to be higher (150 mm above ground) when there is timber construction that needs to be kept dry, although dwarf masonry walls can accomplish the same thing. If you go lower you either need to be very sure of your drainage or tank it.

At the doorways I am struggling with the detail. Front door needs to be a level threshold, but not quite sure if the detail I have drawn makes sense. Most of the doors I have been looking at seem to limit the sill to around 150mm which would put the door roughly in the front part of the 50mm cavity.

Also, most people seem to be saying that the door frame should be mounted back to the timber frame and a movement gap be left underneath it. Not quite sure how to create a gap under the sill and frame, doesn't sound right to me.

Can you stack the Marmox blocks? What is in the cavity under the threshold? You can buy extension sills or get custom ones made. We just made some in oak for some of our doors. Door frames are secured by screws through the sides into the timber frame. The sill doesn't carry any weight, except that of people standing on it as they pass through. Normally the blockwork at the threshold will be lower than the internal subfloor, I think. In our case the internal floor is 65 mm above the slab, to match a lift-and-slid stood on the slab, so there is a 40 mm or so high gap underneath the threshold of the other doors, filled with EPS.

Bi-fold doors are bad news. It's very difficult to make them airtight and to keep them airtight. I don't know, but I expect they need to be above the DPC, which may need to be stepped down at that location (or lift the internal floor level as we did).

HTH, Dave

Sorry, forgot about the cavity. There is nothing currently in the cavity.

We need to reach a bit a wall and floor U value of 0.15, so the wall is 140 timber frame and an extra 25mm insulation on the inside.

For the cavity below dpc its empty at the moment, but planning on having drain holes at the slab. As the distance to the slab is short, I believe the regs say I need a cavity tray just above dpc all the way round the building. Were you considering insulation in the cavity below dpc at. The threshold?

Thanks

I think I'd be looking for a timber frame company that shows greater experience of building highly insulated houses. Somebody who showed you a thermal bridge free design out of the starting gate would be helpful.

You can install your DPC as per that kloeber drawing; step it down outside, just make sure the external ground level is below it (it should be, as the kloeber drawing features a drain) including some way either side of the window

The co that provided my bifolds (smarts visofold 1000 profile) supplied a 225 mm cill. Bows and arrows against the lightning of my 450mm walls, so the sill finishes before the wall does, but I'm going to slope the wall away to the outside, add a membrane, have a gravel soakaway, and cantilever paving over the sill anyway, as per your drawing. Effectively the membrane extends the sill, and it will then be covered, protecting it

+1 to the comment about mounting the frames in the timber frame - not much point having a thermally broken window if it's solid stone either side of the break!

Tony - can you expand on this comment

"I would insulate ground floor with 300mm of eps as a minimum in or under the floor and mitigate thermal bridging at the sole plate (additional insulation in the cavity in that area) or it will rot prematurely due to condensation on top of its dpc."

Posted By: OracsRevengeLooking through the TRADA construction book, they give this detail

Your copy must be different to mine then, because my 5th ed copy only shows two pieces of wood stacked. The TRADA book is not state of the art (perhaps Timber can confirm this). The Passivhaus Details book has better details.

Anything based on government approved details cannot be trusted when it comes to thermal bridging, IMHO.

Additional insulation on the outside is probably the most common way around the issue.

Perhaps see what MBC or Touchwood can do (there are doubtless others as well).

This thread may be worth reading:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10226

TRADA vol2 std detail:

@tony, Yes I agree with you. I have had several 'conversations' with TRADA about this.
Nothing....they do not see the need to change/improve, and will not, until forced to. As the majority of the industry use these details as a 'fall back', we are stuck with them, until the revised Part L is trailed, with revised treatments of heat-loss through thermal bridges (Psi-values).

Is the aircrete sufficient to withstand the loads imposed by being sandwiched between marmox and studwork?

I had in mind that marmox were essentially a big EPS version of a clay brick, with solid plastic where a normal brick would have a hole right through.. As such your aircrete blocks sit atop some point loads formed by the plastic cores in the marmox..

Hi both,

The cavity is currently 50mm, if I put 50mm of eps in, it would fill the cavity which I suspect may be an issue. I can't make the cavity bigger as that would mean a re-design and re-submitting of the plans (£££).


The following text is from the Marmox site:

"How to Design a Thermally Efficient Wall-Floor Junction

Marmox Thermoblock is not an insulating building block like an aircrete block. Typically it replaces just the bottom row of blocks at floor level and in conjunction is used with aircrete blocks in the rest of the wall to stop the cold bridge at the critical wall/floor junction.

The standard block thickness in the UK as a 65mm high block, this is because this is the typical UK brick height and this thickness has been designed to give the necessary insulation required. To achieve an R Value of 2, we can also manufacture a 100mm thick version"


From their pictures, looks like the standard use is one layer of Thermoblock at the bottom, then aircrete up to the sole plate.

All this info entering my brain is wonderful, but I do worry what is being pushed out of the side into my mental recycle bin.

Thanks

Pete

Thanks, looking through it now.

I have engaged a Structural Engineer to examine the levels around the plot and determine the optimum slab level, this will then help us determine the options we have with extra insulation, etc.

Pete

He seems confident enough, and comes highly recommended.

He is looking through it all today, so hopefully will provide some advice on the foundation design and levels.

Hi Marky P,

It's a narrow plot and I am already up against the limit on the external measurement, If I alter the internal measurement to increase the cavity, the plans would need to be-redrawn. This would mean paying for the Engineering design (steels, etc) to be re-done, etc which has cost me several thousand so far.

I have the tables from the Thermoblock BRE datasheets, so will try to work it out.

Thanks

Pete

Hi,

Did the Thermoblock and Celcon cause you any issues at all during the cutting/laying, and did you suffer any Celcon cracking when the frame was bolted through to them?

Thanks