Heat loss factor with hollow core ceiling

iplogger1

I was talking to a very clued in energy consultant briefly the
other day and he gave some very good advice about various
aspects of home insulation.

He raised one issue briefly which slightly went above my head.
I wonder if anyone here might be able to shed any light upon it.

Basically he asked me to check with my builder as to how the
hollow core ceiling sits on the cavity walls. He mentioned
that if the ceiling edges didn't extend to cover the outside
edge of the inner leaf wall of the cavity then there was a potential
for heat leakage. I checked this with the builder and he told
me that the hollowcore ceiling did indeed extend as far as
the outer edge of inner leaf wall. However on pondering he said
that this would be the case for front and back of the house.
At the gable ends he said it probably wasn't extending this far
out to the edge of cavity wall. Builder felt that this would be
very minimally affect U-values.

Anyone got a view on this ?

~ipl

311

I didn't have a clue what you were talking about at the start ,but when i read it completely i do.
All houses are like that as far as i've ever seen.I've lifted floors in all types of houses all over dublin.
All you could do is put fibreglass in all them spots .I presume the guy is referring to the lack of insulation that is installed behind the plasterboard ,ie the plasterboard and kinspan only meet the ceiling and start again from the floor upstairs.

De_man

iplogger1 wrote:

Basically he asked me to check with my builder as to how the
hollow core ceiling sits on the cavity walls. He mentioned
that if the ceiling edges didn't extend to cover the outside
edge of the inner leaf wall of the cavity then there was a potential
for heat leakage. I checked this with the builder and he told
me that the hollowcore ceiling did indeed extend as far as
the outer edge of inner leaf wall. However on pondering he said
that this would be the case for front and back of the house.
At the gable ends he said it probably wasn't extending this far
out to the edge of cavity wall. Builder felt that this would be
very minimally affect U-values.

Anyone got a view on this ?

~ipl

the U value change would be minimal, but from a construction & safety point of view i'd much prefer to see the hollowcore slab extend to the edge of
all the cavity walls!! and not just the front and back walls

Before you order your slabs ensure your drawings are correct and remeasure
before ordering, the concrete floor company will take the gable ends into account...and shouldn't pose a problem, one thing to consider is to try to plan ahead to where you intend to run cable pipes etc etc, the concrete slab companies' charge anywhere from 15-20euro per hole in the slab, it'll save you a lot of time and effort drilling later:D

also one thing to try to do before your hollowcore slabs are put in place is to chase the walls for electrics on the ground floor or at least ensure that a gap is in place between the hollowcore slab and wall so you can run cables easily around the house, otherwise it'll take you foreever to do the work


a pint of plain is yer only man

kadman

I definitely agree with De_man with regards to the hollowcore extending to its full extent at all the extremities where it sits on the inner leaf. It is absolutely necessary to maintain a maximum bearing for all the hollowcore installed, at the cavities the full width of the inner block. For me personally the safety issues of the hollowcore bearing , would be more important than any minimal heat leakage.

Just my 2 cents worth

kadman

De_man

hi just one last thing i must reiterate what kadman said as he's 100% on
the ball and i didn't really concentrate on the load bearing,

what type of hollowcore are you using 100/150 or 200mm and what is the max span of the slab?

i'm assuming iplogger you're at the same stage as myself, i'd bring a copy of the plans personally to the flooring company, they'll have a quick look over them and offer more practical advice.

you might have to put some or all of your downstairs walls block
on flat depending on the type & max span of the hollowcore? i personally
would want block on flat where slabs meet, block on edge is fine if only
going to be used to act as a pendulum

i'd be afraid to give you advice to which walls need to be on flat without seeing your plans get a second opinion on this one before you put
them into place, i'm assuming you're going to use blocks for the upstairs
and not "normal" studs


P.S allow a minimum of 6weeks for precast floor delivery...order it
as soon as your foundations are nearly complete, otherwise you could be waiting a while


a pint of plain is yer only man

Do-more

kadman wrote:

I definitely agree with De_man with regards to the hollowcore extending to its full extent at all the extremities where it sits on the inner leaf. It is absolutely necessary to maintain a maximum bearing for all the hollowcore installed, at the cavities the full width of the inner block.
kadman

Don't agree, the hollow core slab only needs the bear fully on it's short lenght.
The long lenght (on the gables) do not necessarilly need to bear at all. Think about it, what about all the slabs which are only supported on their short lenghts! Hollow core is designed to bear on the two ends. Most of the slabs produced wouldn't bear along their long lenght anyway due to bowing in the curing process. This is more likely to be the source of the heat loss your Energy consultant was referring to.

iplogger1

Do-more wrote:

Don't agree, the hollow core slab only needs the bear fully on it's short lenght.
The long lenght (on the gables) do not necessarilly need to bear at all. Think about it, what about all the slabs which are only supported on their short lenghts! Hollow core is designed to bear on the two ends. Most of the slabs produced wouldn't bear along their long lenght anyway due to bowing in the curing process. This is more likely to be the source of the heat loss your Energy consultant was referring to.

Assuming this is the case - is fiberglass or equivalent the way to
go in terms of filling the gaps to provide a sounder amount of
thermal insulation within the house ?

~ipl

Do-more

A few cans of expanding foam should do the trick!

De_man

fair point what do-more says the more i think about it, i'm just passing on what my architect said because we spoke at length about this very topic
(i'm on a very tight budget)

he wanted the slabs on the inner leaf of the wall due to 4 ensuites upstairs plus a lot of the plumbing will be situated on the gables, also to avoid unnecessary stresses as he reckons i'll be going to the max with my slabs on the outside , he reckons to use every bit of support that you can use

as i said before "get a second opinion on this one before you put
them into place" this is a topic which really can't be solved over the net
only direct contact with your architect/structural engineer afterall they'll have to sign off the building when it's finished



do-more What's your opinion on "connecting" slabs over block on edge?
it would save me thousands if i could "get away" with doing that

my average span is 6 metres at the moment i'm working on block on flat halfways across the entire length of the house, with one section going 7.5 metres (with 150mm slabs (shud use 200mm) but the slabs will be supported underneath in the opposite direction using block on edge

would you reckon i could get away with using block on edge instead
of block on flat? it would mean each slab would have a support of 2"

what you think, we are just finished the foundations


a pint of plain is yer only man

De_man

p.s. iplogger

i'm was assuming that the slabs are running parallel with the gable
if it's the other way 'round i.e. perpendicular to the gable walls, then you definitely do have issues with load bearing

it all depends on the design of the house


a pint of plain is yer only man

Wobs

The company you are getting your floors from should give you a plan of your house showing which walls they require on the flat, where they need RSJ's, larger lintels and pad stones. Most will give this with the quote and all should come out and do a site survey to check all htese isuues when you order your floors.

iplogger1

Wobs - yes the hollowcore guys have been out to measure and those
issues (eg, reinforcing RSJs, additional support walls,etc) have been
ironed out between the main contractor and the hollowcore guys.
I've been mainly hands off with all of that with the exception of
being kept informed that the hallway corridor and living room floor
area lost a few inches because of thickening of a support wall
needed in the partition between corridor and living room.

My interest was only sparked when I heard about there being
a potential heat loss issue which I assume has to do with
uninsulated areas at the gable ends of the house. I'm assuming
that the slabs are running parallel with gable (but I'll check this
with the builder) because I know the builder has taken account
of all the load bearing issues, etc. At the moment my focus
is on making sure I'm taking all measures to insulate the house.

~ipl

Do-more

De_man wrote:

do-more What's your opinion on "connecting" slabs over block on edge?
it would save me thousands if i could "get away" with doing that

This prompts me to ask, who was the famous Gerry? And what exactly was the disaster area he precided over that warranted his name being forever being associated with shoddy building?

Hope your question was just in jest!

De_man

Do-more wrote:

This prompts me to ask, who was the famous Gerry? And what exactly was the disaster area he precided over that warranted his name being forever being associated with shoddy building?

Hope your question was just in jest!

you've lost me i'm afraid,

clearly, you know your stuff when it comes to precast flooring i just wanted your opinion as to what the minimum requirements are for putting precast slabs on 4" blocks would it be 2" or 4" overhang that's all.

i have been told i need 4" if i can get away with 2" it would save me a lot
of time, perhaps for you a stupid question, but for me it isn't

if offence was taken, i can assure you no offence was intended


regarding the rest, i think you've mixed me up with someone else

Do-more

De-man

Sorry if my convaluted reply has lost you.

The load that you are looking at placing on a 4 inch block is very considerable and 4 inch blocks are not that strong. 2 inches is not enough to bear the load in itself. You would in all probability never be able to place two slabs butting up to each other at exactly the centre of the block anyway. So one slab could for instance bear on just an inch and a half of block. It's a recipe for disaster.

The Gerry I was referring to is the one made famous by the phrase "Gerry Built". I often ponder what the last 5 years housing stock is going to look like in 10 or 15 years time. I am frequently in newly built houses and what you can see on the surface is often very shoddy and it scares me sometimes, to think of what lies beneath!

kadman

Do-more wrote:

Don't agree, the hollow core slab only needs the bear fully on it's short lenght.
The long lenght (on the gables) do not necessarilly need to bear at all. Think about it, what about all the slabs which are only supported on their short lenghts! Hollow core is designed to bear on the two ends. Most of the slabs produced wouldn't bear along their long lenght anyway due to bowing in the curing process. This is more likely to be the source of the heat loss your Energy consultant was referring to.

I uphold your right to disagree.;)

But as a rule of thumb I would always be inclined to use maximum area of bearing , as opposed to enough. No doubt iplogger will have to get a professional direction on his query , and in the end will have to concur with his architects wishes.

kadman

Do-more

Do-more wrote:

This prompts me to ask, who was the famous Gerry? And what exactly was the disaster area he precided over that warranted his name being forever being associated with shoddy building?

Hope your question was just in jest!

I think that the phrase refers to postwar rebuilding in Germany in the late 1940's. Desperate housing shortage + poor/lack of materials, unskilled builders, etc = badly build houses, (all replaced now).

De_man

very serious gaps in my education

who can i blame:rolleyes:

Mellor

Do-more wrote:

Don't agree, the hollow core slab only needs the bear fully on it's short lenght.
The long lenght (on the gables) do not necessarilly need to bear at all. Think about it, what about all the slabs which are only supported on their short lenghts! Hollow core is designed to bear on the two ends. Most of the slabs produced wouldn't bear along their long lenght anyway due to bowing in the curing process. This is more likely to be the source of the heat loss your Energy consultant was referring to.

True they don't need gable support on gable to hold he floor. But would you suggest that you have no support? Not much tolerance there. Or prehaps any bearing is enough. But how would you build on an inch of bearing. That way probably makes it harder for building.

Do-more

Mellor wrote:

True they don't need gable support on gable to hold he floor. But would you suggest that you have no support? Not much tolerance there. Or prehaps any bearing is enough. But how would you build on an inch of bearing. That way probably makes it harder for building.

I wasn't suggesting that the slab should not extend over the inner leaf, indeed if it covers it fully, great, my point was that as the slab is bowed it won't bear along it's length.

It's probably worth pointing out here that a similar problem with heat loss can occour with timber frame buildings. Unless your stick building, the sole plate on the ground floor walls may not bear fully on the concrete slab along it's entire lenght allowing an airgap to the cavity. It's a good idea to be present when they are swinging in the ground floor walls and lay a heavy bead of silicon onto the slab just prior to the wall be swung into position. Afterwards run a bead of silicon around the inside edge of the wall where it meets the floor. In some cases you may need expanding foam to fill the gap.

Sorry to go off topic, but thought it was worth mentioning.

kadman

Do-more wrote:

I wasn't suggesting that the slab should not extend over the inner leaf, indeed if it covers it fully, great, my point was that as the slab is bowed it won't bear along it's length.

It's probably worth pointing out here that a similar problem with heat loss can occour with timber frame buildings. Unless your stick building, the sole plate on the ground floor walls may not bear fully on the concrete slab along it's entire lenght allowing an airgap to the cavity. It's a good idea to be present when they are swinging in the ground floor walls and lay a heavy bead of silicon onto the slab just prior to the wall be swung into position. Afterwards run a bead of silicon around the inside edge of the wall where it meets the floor. In some cases you may need expanding foam to fill the gap.

Sorry to go off topic, but thought it was worth mentioning.

I would not be inclined to lay a heavy silicon bed under the soleplate in a timber frame build. Any areas of no contact between the sole plate and the concrete slab , needs to have shimpacks of slate or similar placed between the soleplate bottom and concrete slab, in the centre line of any studs. If a bed of silicon was placed first, it would be extremely hard to identify the problem gaps any where. Definitely sealing after installation is an option.

But in the tf kits and builds I have seen minimal heat loss at these areas would be the least issues in some builds.

kadman

Viking House

?

Viking House

Do-more wrote:

In some cases you may need expanding foam to fill the gap.

Polyeurethene (Kingspan) reacts badly with wood, over a 15 year period in Finland it was found that the wood looses its structural strength and becomes soft. The chemicals from the Polyeurethent went into the wood and weakened the structural integrity of the Timber Frame to such an extent that they don't allow that system to be used anymore.
Not many people know about this, certainly not the people in Centuary Homes anyway.

De_man

Do-more wrote:

It's a good idea to be present when they are swinging in the ground floor walls and lay a heavy bead of silicon onto the slab just prior to the wall be swung into position.

i hadn't even thought of that, thanks very much

Do-more

Viking House wrote:

Polyeurethene (Kingspan) reacts badly with wood, over a 15 year period in Finland it was found that the wood looses its structural strength and becomes soft. The chemicals from the Polyeurethent went into the wood and weakened the structural integrity of the Timber Frame to such an extent that they don't allow that system to be used anymore.
Not many people know about this, certainly not the people in Centuary Homes anyway.

Is that all PU Viking house, i.e. foamed on site from can and pre-manufactured?

If so then all roofs that are being insulalated with PU board are liable to possible structural failure in 15 years time? Scary thought...

Thanks for the info.

Do-more

Even if it is confined to spray-on PU there must be thousands of houses which have had the underside of their roofs insulated with sprayed on PU. The insurance industry better start writing their get-out clauses now!