methods of sealing the ground floor slab to wall joint

SRV · 19-08-2010 12:32AM #1

Hi,
I am going to use 50mm of insulation between the ground floor slab and the inner leaf of my cavity wall. Insulation beveled at the top. Any ideas on how to create an airtight seal at this joint? Wall will be sand cement plastered.

thanks

SRV

sinnerboy · 21-08-2010 01:29AM

Take a look at page 4 here .

Key attributes are
1. insulating block below DPC in line with the floorslab
2. continue plastering down to the floor
3. Silicone seal to the base of the skirting

If you use 50mm of insulation at the floorslab edge you will end up with an exposed strip of insulation after the skirting is installed . ( Plaster 13mm , skirting 19mm will leave 18mm exposed ) . You will not be able seal the skirting base to the insulation strip . Best to use only 25mm floor slab edge insulation .

.

SRV · 22-08-2010 12:24PM

Would it be possible to use a strip of membrane which would be plastered to the wall and somehow 'bonded' to the floor SLAB............allowing the insulation to be 2 or 3 inches, beveled at the top?

Any ideas on this approach? Skirting boards warp and break the seal possibly.

thanks for your help

SRV

sinnerboy · 22-08-2010 05:37PM

Well there are tape and membranes that could be used . Using a 50mm insulation edge strip you would end up with a tape / membrane strip projecting out from under the skirting board . Beveling the insulation at the top ... a little impractical the concrete pour locally would just snap away .

Remember you will have to apply floor finishes . Tiles will not bond to an insulation strip or to a tape/membrane . Carpet edge grips will not take either .

moan 77 · 22-08-2010 06:08PM

Hi,

I have fitted 50 mm edge insulation on all walls on the ground floor, i will run tiles or timber into the wall then fit skirting over them, i dont see the big problem that all were saying that you cant do it and to only use 25mm, you wont walk that close to the wall and with 150mm in the sub floor i was not going to allow heat escape by using only 25mm on the edge so i doubled it up. right or wrong thats what i did.

sinnerboy · 23-08-2010 01:17PM

moan 77 wrote: »

i dont see the big problem .... right or wrong thats what i did.

The use of the insulating block is intended to offset the 25mm only slab edge insulation

This junction is more hassle than first meets the eye . Blockwork is pretty pourous and when you do your air tighness test this will show up . Your timber end grains will be exposed to what will be the coolest spot of the junction so I hope you get no problems with discolouration or even , in time , rot .

When drylining I agree it is a smart move to thicken up the slab edge insulation - but not otherwise .

SRV · 23-08-2010 07:39PM

Thanks Sinnerboy and Moan77,

thats a great help, and im almost sure i will go with the 25 mm strip, as there will be an insulating block as well.

regards

SRV

FergusD · 24-08-2010 07:09AM

sinnerboy wrote: »

This junction is more hassle than first meets the eye . Blockwork is pretty pourous and when you do your air tighness test this will show up . Your timber end grains will be exposed to what will be the coolest spot of the junction so I hope you get no problems with discolouration or even , in time , rot .

I'm not following this at all, can you explain it to me? The radon barrier is under the slab and comes up across this block on the inner leaf, the walls are plastered down to the radon barrier. Whether the perimeter insulation is 1, 2 or 18" what difference will this make to a). airtightness and b). your timber floor rotting?

Thanks,

Fergus (who has used a 2" perimeter strip).

sinnerboy · 24-08-2010 08:56AM

Thermal

I am assuming that moan 77 has not used an insulating block ( open to correction of course ) . So whereas the a) floor slab is edge insulated and b) the cavity is insulated .... the inner leaf descends down to the footings without a thermal break . Heat will flow relatively quickly through the block inner leaf bypassing a) and b) .

Now when one transfers from an element that is highly insulating to one that is is not you get local cooling and a risk of condensation

Air Tightness

"the walls are plastered down to the radon barrier." Well in theory yes . No platerer will wreck his trowel making a tight joint to the floor slab . No point anyway beacuse it will open up . Trace your eye around the plasterted reveal to any window opening and see the hairline crack that opens up - always .
Wait to see an air tightness test and see the smoke-gun result .

Therefore
- detail to allow a silicone joint between the skirting board and floorslab
- don't use an over thick slab edge insulation strip unless your plastered face covers it ( i.e. dry line )
- use an insulating block below dpc / in line with the floor slab

SRV · 24-08-2010 11:26AM

sinnerboy wrote: »

Thermal

I am assuming that moan 77 has not used an insulating block ( open to correction of course ) . So whereas the a) floor slab is edge insulated and b) the cavity is insulated .... the inner leaf descends down to the footings without a thermal break . Heat will flow relatively quickly through the block inner leaf bypassing a) and b) .

Now when one transfers from an element that is highly insulating to one that is is not you get local cooling and a risk of condensation

Im hoping to acheive A2 or A3 on this build and have had advice im likely to acheive A3 with my current spec.

The area im concerned with is the heat transfer from the inner leaf down to the footings.

The insulating block in line with the floor slab will help. Maybe i should use another insulating block or two? above that again. ?

Are there good returns from this extra expense?

Some of these insulating blocks are more effective than others? Whats the best?

thanks SRV

sinnerboy · 24-08-2010 01:46PM

Thermal conductivity measures the flow heat through a fabric .
Lower numbers are better

Foamglass perinsul block conductivity is 0.055

Quinn lites - there are 3 types - conductivities are 0.12 , 0.17 and 0.19 .
In breif the better the structural compessive load the worse the conductivity . The input of a structural engineer is vital .

Foamglass is realtively expensive Quinn lites are not .

So what of value ?

To answer that you to have performed comparision heat flow calculations through the detail with the various spec options .

Then you need to feed that into a building energy model ( DEAP at least or PHPP are 2 examples )

Then you will establish

1. Whether you have safely eliminated your condensations risk
2. The differences in the whole building energy demand for each spec option . You can compare this with the variuos material costs and determine payback for each option - value engineering .

To do this you need to hire a professional .

Anything else is just guesswork .

sinnerboy · 24-08-2010 01:51PM

Avoid the cold inner leaf problem like this

methods of sealing the ground floor slab to wall joint

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