What cavity insulation for my extension?

[glangri2]

Hi All.

I am embarking on a 2 storey extension coming out 3m from the gable end of my property.
Downstairs will be a garage, and upstairs will be 2 bedrooms.

I remember from years ago that bedrooms above garages were notoriously cold, so I am planning on going over the top in terms of insulation.

In fact, I am going to build the downstairs as if it is a normal room (so that it keeps my options open for years to come, in case I go for a garage conversion.

In terms of the cavity walls though (Bricks on the outside, 100mm cavity, 100mm Aireated blocks on the inside).............

What insulation should I be going for? 100mm Full fill cavity batts, or a 50mm foil covered Rigid urethane board like a Kingspan or Celotex which will leave a 50mm air gap.

The full fill batts work out about £275, the other option is about £150 more expensive, so in the grand scheme of things the price doesn't matter.

I'll be chatting with the building inspector about it next week when he comes to inspect the brick work up to DPC, I'd just like to be armed with some knowledge from the forum before hand.

Cheers Guys

Gary

 

[freddiemercurystwin]

50mm Cel of Kings will more than satisfy the required U value and leaving a 50mm air gap in the cavity is the winner for me every time.

 

[ajrobb]

50mm Cel of Kings will more than satisfy the required U value and leaving a 50mm air gap in the cavity is the winner for me every time.

What FMT says.

My dodgy U-value calc, for what it's worth:

Untitled

• ajrobb
• 25 Mar 2010
• 1

Don't forget to insulate under the floor. The trouble is that the garage has a fire ceiling so there is no ventilation beneath the floor*. It probably means you shouldn't use mineral wool. Use 100mm Celotex etc between the joists up against the floorboards.

* You should not use a vapour proof membrane in suspended floors as it stops spills from draining. Without a VPL, mineral wool needs ventilation on the cold side to prevent condensation soaking the wool. Closed cell insulation (like Celotex) is not affected by condensation. Small gaps at the edge of the foam boards allow spills to drain/evaporate through the plaster below and allows the floor to dry.

 

[glangri2]

Thanks for the input guys. You have confirmed what I was expecting to hear, so I am happy with that.

And thanks for the U Calcs as well - I need to do that to show the inspector.

As for the insulation in the floor, TRADA timber tables say that I only need to have 145mm deep joists to get the strength, but I have planned to put in 225s so that I can get deeper insulation in.

Am I right in assuming that I can penetrate a fire ceiling for the lighting wiring, and fittings (if they are fire rated) ?
If so, could i put 150mm of Celotex in there, and leave 75mm clearance between the botton of that and the top of the plaster board, for running the cables in.

 

[ajrobb]

I'm not sure I'd bother with 150mm Celotex and extra deep joists in the floor just to accommodate it. 100mm would give a U-value below 0.23, which is better than the walls. Consider, the garage will be warmer than outside and (unless you're going for underfloor heating) the floor will be cooler than the walls and ceiling above. This means there will be significantly less temperature drop across the floor. You'd do better to squeeze a few more mm of Celotex into the wall cavity.

On the other hand, deep joists will make for better noise insulation if you convert the garage later. You could even opt for a wider joist spacing and thicker (22mm) floor boards.

 

[glangri2]

Cheers ajrobb

Put that way, it doesn't seem to make much sense going for the deep joists.

If I put shallower joists in, I can drop the ceiling joists a bit, and give a bit more head room in the loft room.

 

[alittlerespect]

Hi

Your not going to have a problem with condensation in the garage as the temperature differential is never going to be severe enough, assuming you are going to put a couple of radiators in the garage to provide an ambient temperature in winter - that is keep the garage at around 12C in winter!

As for the choice of insulation, the benefit of PIR and Polystyrene are their excellent thermal efficiencies which means you need only around half the thickness compared to more traditional insulation material but if you have the available depth you may as well go for mineral wool or fibreglass - just need to work out the cost benefit of the various materials. PIR/Poly would come into its own if space was an issue!

Regards

 

[ajrobb]

I sympathise with your quest to get a warm room over the garage. I grew up in a solid brick semi and neighbours had a room over the garage that was freezing in winter. I suspect that was mostly because these rooms had 3 solid exterior walls with a U value over 3, two steel windows, no heat coming from below and no heating to speak of. My corner box room over the unheated hall was comparatively tropical.

I suspect your extension will have a reasonably high thermal mass from the old exterior wall, which is now internal. This usually keeps it warmer at night after the heating goes off and cooler on summer afternoons. U-values around 0.25 for floor and walls are very good and the ceiling will probably be around 0.15.

Size the radiators large enough to warm the room up quickly rather than just to maintain a constant temperature and use thermostatic valves. Place them against the old exterior wall and paint the back of them black to improve heat radiation to the wall (the opposite of reflective film). If you also go for larger single-panel radiators rather than K2, this maximises the heat storage effect of the bricks. This should help compensate for the lack of heat coming up through the floor at night.

Even with 2 boilers in my garage, the ventilation needed to dry a wet car ensures that temperatures are only a few degrees above outside. (OK, so I'm the only person I know who puts their car in the garage but the electric door helps.) I'd still worry about dropping below dew point and condensation with mineral wool under the floor.

 

[noseall]

I may be a bit of a Philistine when it comes to cavity insulation, but from a bricklayers point of view, fully filling the cavity (100mm) with crown dritherm cavity batts is the simplest way.

It means you can build the masonry the correct way around i.e. the brickwork first as the affords you the setting out and gauging up.

Building the blockwork first is both alien and bonkers.

 

[mikric]

I may be a bit of a Philistine when it comes to cavity insulation, but from a bricklayers point of view, fully filling the cavity (100mm) with crown dritherm cavity batts is the simplest way.

It means you can build the masonry the correct way around i.e. the brickwork first as the affords you the setting out and gauging up.

Building the blockwork first is both alien and bonkers.

Agree in every way

 

[glangri2]

......you can build the masonry the correct way around i.e. the brickwork first as the affords you the setting out and gauging up.

Building the blockwork first is both alien and bonkers.

That is an interesting point. I had assumed that you could do the bricks first, then the blocks, then slide the insulation in afterwards.
Is that a wrong assumption, is it just fiddly, or is there a technical reason why you can't do it?

 

[freddiemercurystwin]

Thousands of houses are built every year with partially filled cavity insulation building the blockwork and then the brick. Likewise many are built nowadays with full fill. There really isn't that much to choose between the two methods these days. I just prefer a cavity.

 

[ajrobb]

......you can build the masonry the correct way around i.e. the brickwork first as the affords you the setting out and gauging up.

Building the blockwork first is both alien and bonkers.

That is an interesting point. I had assumed that you could do the bricks first, then the blocks, then slide the insulation in afterwards.
Is that a wrong assumption, is it just fiddly, or is there a technical reason why you can't do it?

I suppose the insulation board must be held snug to the block work to prevent cold drafts bypassing the insulation. It sounds like the blocks are laid with the wall ties and then the insulation clipped to the ties. The bricks are laid last but must look even and yet tie into the blocks behind the insulation.

The alternative of Crown DriTherm "Crown DriTherm full fill cavity insulation is the only insulation solution able to achieve a U-value of 0.27 (the minimum permitted in 2006’s revised Building Regulations) in a 100mm cavity in a dense block construction."

So you can save money on the insulation AND the blocks. Plus you get a higher thermal mass - interesting.

I guess it must also protect the cavity from dropped mortar.

 

[noseall]

. There really isn't that much to choose between the two methods these days. I just prefer a cavity.

From a personal point of view, there is no contest.

I build complete extensions to existing properties for private customers. There is no hiding place as far as quality is concerned.

You can precisely map out all openings and use the accurate gauging of the brickwork to find your crucial levels, (lintel height, joist, wall plate etc.) throughout the build.

The idea of building the blockwork first does not sit at all well with me and many brickies feel the same.

 

[ajrobb]

I am slightly concerned that DriTherm relies on a chemical coating (silicone) to prevent wicking. I just feel uneasy that it might be susceptible to damage from water laden with hygroscopic salts, say, and turn into a wet blanket. At least an empty cavity can recover after a flood for instance.